@Go to the ‰ŠP�”›first@�”˜, ‰ŠP�”›previous@�”˜, ‰ŠP�”›next@�”˜, ‰ŠP�”›last@�”˜ section, ‰ŠP�”›table of contents@�”˜.
Œ

ˆ‰SEC180Št�”›Sockets

@�”˜This chapter describes the GNU facilities for interprocess communication using sockets.
‰IDX913Š‰IDX914Š@�”˜A Bsocket@ is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor. But, unlike pipes, sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network. Sockets are the primary means of communicating with other machines; Htelnet@, Hrlogin@, Hftp@, Htalk@, and the other familiar network programs use sockets.
Not all operating systems support sockets. In the GNU library, the header file H`sys/socket.h'@ exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets, these functions always fail.
DIncomplete:@ We do not currently document the facilities for broadcast messages or for configuring Internet interfaces.
ˆ‰SEC181Št�”›Socket Concepts

‰IDX915Š‰IDX916Š@�”˜When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The Bcommunication style@ of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these: þÂ
• ‰IDX917Š‰IDX918Š‰IDX919ŠD�”˜What are the units of data transmission?@ Some communication styles regard the data as a sequence of bytes, with no larger structure; others group the bytes into records (which are known in this context as Bpackets@).
• ‰IDX920Š‰IDX921ŠD�”˜Can data be lost during normal operation?@ Some communication styles guarantee that all the data sent arrives in the order it was sent (barring system or network crashes); other styles occasionally lose data as a normal part of operation, and may sometimes deliver packets more than once or in the wrong order. Designing a program to use unreliable communication styles usually involves taking precautions to detect lost or misordered packets and to retransmit data as needed.
• DIs communication entirely with one partner?@ Some communication styles are like a telephone call--you make a Bconnection@ with one remote socket, and then exchange data freely. Other styles are like mailing letters--you specify a destination address for each message you send.Àð
‰IDX922Š‰IDX923Š‰IDX924Š‰IDX925Š@�”˜You must also choose a Bnamespace@ for naming the socket. A socket name ("address") is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called "domains", but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with H`PF_'@. A corresponding symbolic name starting with H`AF_'@ designates the address format for that namespace.
‰IDX926Š‰IDX927Š‰IDX928Š‰IDX929Š@�”˜Finally you must choose the Bprotocol@ to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a Bprotocol family@ because of this, which is why the namespace names start with H`PF_'@.
The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system, and you need not know about them. What you do need to know about protocols is this: þÂ
• In order to have communication between two sockets, they must specify the Bsame@ protocol.
• Each protocol is meaningful with particular style/namespace combinations and cannot be used with inappropriate combinations. For example, the TCP protocol fits only the byte stream style of communication and the Internet namespace.
• For each combination of style and namespace, there is a Bdefault protocol@ which you can request by specifying 0 as the protocol number. And that's what you should normally do--use the default.Àð
Throughout the following description at various places variables/parameters to denote sizes are required. And here the trouble starts. In the first implementations the type of these variables was simply Hint@. This type was on almost all machines of this time 32 bits wide and so a de-factor standard required 32 bit variables. This is important since references to variables of this type are passed to the kernel.
But now the POSIX people came and unified the interface with their words "all size values are of type Hsize_t@". But on 64 bit machines Hsize_t@ is 64 bits wide and so variable references are not anymore possible.
A solution provides the Unix98 specification which finally introduces a type Hsocklen_t@. This type is used in all of the cases in previously changed to use Hsize_t@. The only requirement of this type is that it is an unsigned type of at least 32 bits. Therefore, implementations which require references to 32 bit variables be passed can be as happy as implementations which right from the start of 64 bit values.
ˆ‰SEC182Št�”›Communication Styles

@�”˜The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in H`sys/socket.h'@. ‰IDX930Š

A�”˜Macro:@ int DSOCK_STREAM
Â‰IDX931Š@�”˜The HSOCK_STREAM@ style is like a pipe (see section ‰ŠP�”›Pipes and FIFOs@�”˜); it operates over a connection with a particular remote socket, and transmits data reliably as a stream of bytes.
Use of this style is covered in detail in section ‰ŠP�”›Using Sockets with Connections@�”˜.À

AMacro:@ int DSOCK_DGRAM
Â‰IDX932Š@�”˜The HSOCK_DGRAM@ style is used for sending individually-addressed packets, unreliably. It is the diametrical opposite of HSOCK_STREAM@.
Each time you write data to a socket of this kind, that data becomes one packet. Since HSOCK_DGRAM@ sockets do not have connections, you must specify the recipient address with each packet.
The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.
The typical use for HSOCK_DGRAM@ is in situations where it is acceptable to simply resend a packet if no response is seen in a reasonable amount of time.
See section ‰ŠP�”›Datagram Socket Operations@�”˜, for detailed information about how to use datagram sockets.À

AMacro:@ int DSOCK_RAW
Â‰IDX933Š@�”˜This style provides access to low-level network protocols and interfaces. Ordinary user programs usually have no need to use this style.À

ˆ‰SEC183Št�”›Socket Addresses

‰IDX934Š‰IDX935Š‰IDX936Š‰IDX937Š@�”˜The name of a socket is normally called an Baddress@. The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term "name" and sometimes using "address". You can regard these terms as synonymous where sockets are concerned.
A socket newly created with the Hsocket@ function has no address. Other processes can find it for communication only if you give it an address. We call this Bbinding@ the address to the socket, and the way to do it is with the Hbind@ function.
You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.
Occasionally a client needs to specify an address because the server discriminates based on addresses; for example, the rsh and rlogin protocols look at the client's socket address and don't bypass password checking unless it is less than HIPPORT_RESERVED@ (see section ‰ŠP�”›Internet Ports@�”˜).
The details of socket addresses vary depending on what namespace you are using. See section ‰ŠP�”›The File Namespace@�”˜, or section ‰ŠP�”›The Internet Namespace@�”˜, for specific information.
Regardless of the namespace, you use the same functions Hbind@ and Hgetsockname@ to set and examine a socket's address. These functions use a phony data type, Hstruct sockaddr *@, to accept the address. In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to Hstruct sockaddr *@ when you pass it to Hbind@.
ˆ‰SEC184ŠT�”›Address Formats

@�”˜The functions Hbind@ and Hgetsockname@ use the generic data type Hstruct sockaddr *@ to represent a pointer to a socket address. You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace.
Thus, the usual practice is to construct an address in the proper namespace-specific type, then cast a pointer to Hstruct sockaddr *@ when you call Hbind@ or Hgetsockname@.
The one piece of information that you can get from the Hstruct sockaddr@ data type is the Baddress format@ designator which tells you which data type to use to understand the address fully.
‰IDX938Š@�”˜The symbols in this section are defined in the header file H`sys/socket.h'@.

ADate Type:@ Dstruct sockaddr
Â‰IDX939Š@�”˜The Hstruct sockaddr@ type itself has the following members:
Hshort int sa_family
Ä@This is the code for the address format of this address. It identifies the format of the data which follows.
ÂHchar sa_data[14]
Ä@This is the actual socket address data, which is format-dependent. Its length also depends on the format, and may well be more than 14. The length 14 of Hsa_data@ is essentially arbitrary.Â
À

Each address format has a symbolic name which starts with H`AF_'@. Each of them corresponds to a H`PF_'@ symbol which designates the corresponding namespace. Here is a list of address format names:
HAF_FILE
Â‰IDX940Š@�”˜This designates the address format that goes with the file namespace. (HPF_FILE@ is the name of that namespace.) See section ‰ŠP�”›Details of File Namespace@�”˜, for information about this address format.
ÀHAF_UNIX
Â‰IDX941Š@�”˜This is a synonym for HAF_FILE@, for compatibility. (HPF_UNIX@ is likewise a synonym for HPF_FILE@.)
ÀHAF_INET
Â‰IDX942Š@�”˜This designates the address format that goes with the Internet namespace. (HPF_INET@ is the name of that namespace.) See section ‰ŠP�”›Internet Socket Address Formats@�”˜.
ÀHAF_INET6
Â@This is similar to HAF_INET@, but refers to the IPv6 protocol. (HPF_INET6@ is the name of the corresponding namespace.)
ÀHAF_UNSPEC
Â‰IDX943Š@�”˜This designates no particular address format. It is used only in rare cases, such as to clear out the default destination address of a "connected" datagram socket. See section ‰ŠP�”›Sending Datagrams@�”˜. The corresponding namespace designator symbol HPF_UNSPEC@ exists for completeness, but there is no reason to use it in a program.À

H`sys/socket.h'@ defines symbols starting with H`AF_'@ for many different kinds of networks, all or most of which are not actually implemented. We will document those that really work, as we receive information about how to use them.
ˆ‰SEC185ŠT�”›Setting the Address of a Socket

‰IDX944Š@�”˜Use the Hbind@ function to assign an address to a socket. The prototype for Hbind@ is in the header file H`sys/socket.h'@. For examples of use, see section ‰ŠP�”›The File Namespace@�”˜, or see section ‰ŠP�”›Internet Socket Example@�”˜.

AFunction:@ int Dbind@ B(int Bsocket@, struct sockaddr *Baddr@, socklen_t Blength@)
Â‰IDX945Š@�”˜The Hbind@ function assigns an address to the socket Bsocket@. The Baddr@ and Blength@ arguments specify the address; the detailed format of the address depends on the namespace. The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format for that namespace.
The return value is H0@ on success and H-1@ on failure. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The Bsocket@ argument is not a valid file descriptor.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ is not a socket.
ÂHEADDRNOTAVAIL
Ä@The specified address is not available on this machine.
ÂHEADDRINUSE
Ä@Some other socket is already using the specified address.
ÂHEINVAL
Ä@The socket Bsocket@ already has an address.
ÂHEACCES
Ä@You do not have permission to access the requested address. (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through HIPPORT_RESERVED@ minus one; see section ‰ŠP�”›Internet Ports@�”˜.)Â

Additional conditions may be possible depending on the particular namespace of the socket.À

ˆ‰SEC186ŠT�”›Reading the Address of a Socket

‰IDX946Š@�”˜Use the function Hgetsockname@ to examine the address of an Internet socket. The prototype for this function is in the header file H`sys/socket.h'@.

AFunction:@ int Dgetsockname@ B(int Bsocket@, struct sockaddr *Baddr@, socklen_t *Blength-ptr@)
Â‰IDX947Š@�”˜The Hgetsockname@ function returns information about the address of the socket Bsocket@ in the locations specified by the Baddr@ and Blength-ptr@ arguments. Note that the Blength-ptr@ is a pointer; you should initialize it to be the allocation size of Baddr@, and on return it contains the actual size of the address data.
The format of the address data depends on the socket namespace. The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much. The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to Hstruct sockaddr *@ to pass it to Hgetsockname@.
The return value is H0@ on success and H-1@ on error. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The Bsocket@ argument is not a valid file descriptor.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ is not a socket.
ÂHENOBUFS
Ä@There are not enough internal buffers available for the operation.Â
À

You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.
ˆ‰SEC187Št�”›The File Namespace

‰IDX948Š
@�”˜This section describes the details of the file namespace, whose symbolic name (required when you create a socket) is HPF_FILE@.
ˆ‰SEC188ŠT�”›File Namespace Concepts

@�”˜In the file namespace, socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. In order to connect to a socket, you must have read permission for it. It's common to put these files in the H`/tmp'@ directory.
One peculiarity of the file namespace is that the name is only used when opening the connection; once that is over with, the address is not meaningful and may not exist.
Another peculiarity is that you cannot connect to such a socket from another machine--not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one.
After you close a socket in the file namespace, you should delete the file name from the file system. Use Hunlink@ or Hremove@ to do this; see section ‰ŠP�”›Deleting Files@�”˜.
The file namespace supports just one protocol for any communication style; it is protocol number H0@.
ˆ‰SEC189ŠT�”›Details of File Namespace

‰IDX949Š@�”˜To create a socket in the file namespace, use the constant HPF_FILE@ as the Bnamespace@ argument to Hsocket@ or Hsocketpair@. This constant is defined in H`sys/socket.h'@.

AMacro:@ int DPF_FILE
Â‰IDX950Š@�”˜This designates the file namespace, in which socket addresses are file names, and its associated family of protocols.À

AMacro:@ int DPF_UNIX
Â‰IDX951Š@�”˜This is a synonym for HPF_FILE@, for compatibility's sake.À

The structure for specifying socket names in the file namespace is defined in the header file H`sys/un.h'@: ‰IDX952Š

A�”˜Data Type:@ Dstruct sockaddr_un
Â‰IDX953Š@�”˜This structure is used to specify file namespace socket addresses. It has the following members:
Hshort int sun_family
Ä@This identifies the address family or format of the socket address. You should store the value HAF_FILE@ to designate the file namespace. See section ‰ŠP�”›Socket Addresses@�”˜.
ÂHchar sun_path[108]
Ä@This is the file name to use. DIncomplete:@ Why is 108 a magic number? RMS suggests making this a zero-length array and tweaking the example following to use Halloca@ to allocate an appropriate amount of storage based on the length of the filename.Â
À

You should compute the Blength@ parameter for a socket address in the file namespace as the sum of the size of the Hsun_family@ component and the string length (Bnot@ the allocation size!) of the file name string.
ˆ‰SEC190ŠT�”›Example of File-Namespace Sockets

@�”˜Here is an example showing how to create and name a socket in the file namespace. #include <stddef.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>

int
make_named_socket (const char *filename)
{
struct sockaddr_un name;
int sock;
size_t size;

/* Create the socket. */

sock = socket (PF_UNIX, SOCK_DGRAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}

/* Bind a name to the socket. */

name.sun_family = AF_FILE;
strcpy (name.sun_path, filename);

/* The size of the address is
the offset of the start of the filename,
plus its length,
plus one for the terminating null byte. */
size = (offsetof (struct sockaddr_un, sun_path)
+ strlen (name.sun_path) + 1);

if (bind (sock, (struct sockaddr *) &name, size) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}

return sock;
}
ˆ‰SEC191Št�”›The Internet Namespace

‰IDX954Š
@�”˜This section describes the details the protocols and socket naming conventions used in the Internet namespace.
To create a socket in the Internet namespace, use the symbolic name HPF_INET@ of this namespace as the Bnamespace@ argument to Hsocket@ or Hsocketpair@. This macro is defined in H`sys/socket.h'@. ‰IDX955Š

A�”˜Macro:@ int DPF_INET
Â‰IDX956Š@�”˜This designates the Internet namespace and associated family of protocols.À

A socket address for the Internet namespace includes the following components: þÂ
• The address of the machine you want to connect to. Internet addresses can be specified in several ways; these are discussed in section ‰ŠP�”›Internet Socket Address Formats@�”˜, section ‰ŠP�”›Host Addresses@�”˜, and section ‰ŠP�”›Host Names@�”˜.
• A port number for that machine. See section ‰ŠP�”›Internet Ports@�”˜.Àð
You must ensure that the address and port number are represented in a canonical format called Bnetwork byte order@. See section ‰ŠP�”›Byte Order Conversion@�”˜, for information about this.
ˆ‰SEC192ŠT�”›Internet Socket Address Formats

@�”˜In the Internet namespace, for both IPv4 (HAF_INET@) and IPv6 (HAF_INET6@), a socket address consists of a host address and a port on that host. In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol.
The data types for representing socket addresses in the Internet namespace are defined in the header file H`netinet/in.h'@. ‰IDX957Š

A�”˜Data Type:@ Dstruct sockaddr_in
Â‰IDX958Š@�”˜This is the data type used to represent socket addresses in the Internet namespace. It has the following members:
Hshort int sin_family
Ä@This identifies the address family or format of the socket address. You should store the value of HAF_INET@ in this member. See section ‰ŠP�”›Socket Addresses@�”˜.
ÂHstruct in_addr sin_addr
Ä@This is the Internet address of the host machine. See section ‰ŠP�”›Host Addresses@�”˜, and section ‰ŠP�”›Host Names@�”˜, for how to get a value to store here.
ÂHunsigned short int sin_port
Ä@This is the port number. See section ‰ŠP�”›Internet Ports@�”˜.Â
À

When you call Hbind@ or Hgetsockname@, you should specify Hsizeof (struct sockaddr_in)@ as the Blength@ parameter if you are using an Internet namespace socket address.

AData Type:@ Dstruct sockaddr_in6
Â‰IDX959Š@�”˜This is the data type used to represent socket addresses in the IPv6 namespace. It has the following members:
Hshort int sin6_family
Ä@This identifies the address family or format of the socket address. You should store the value of HAF_INET6@ in this member. See section ‰ŠP�”›Socket Addresses@�”˜.
ÂHstruct in6_addr sin6_addr
Ä@This is the IPv6 address of the host machine. See section ‰ŠP�”›Host Addresses@�”˜, and section ‰ŠP�”›Host Names@�”˜, for how to get a value to store here.
ÂHuint32_t sin6_flowinfo
Ä@This is a currently unimplemented field.
ÂHuint16_t sin6_port
Ä@This is the port number. See section ‰ŠP�”›Internet Ports@�”˜. Â
À

ˆ‰SEC193ŠT�”›Host Addresses

@�”˜Each computer on the Internet has one or more BInternet addresses@, numbers which identify that computer among all those on the Internet. Users typically write IPv4 numeric host addresses as sequences of four numbers, separated by periods, as in H`128.52.46.32'@, and IPv6 numeric host addresses as sequences of up to eight numbers seperated by colons, as in H`5f03:1200:836f:c100::1'@.
Each computer also has one or more Bhost names@, which are strings of words separated by periods, as in H`churchy.gnu.ai.mit.edu'@.
Programs that let the user specify a host typically accept both numeric addresses and host names. But the program needs a numeric address to open a connection; to use a host name, you must convert it to the numeric address it stands for.
ˆ‰SEC194ŠT�”›Internet Host Addresses

‰IDX960Š‰IDX961Š
‰IDX962Š‰IDX963Š@�”˜An Internet host address is a number containing four bytes of data. These are divided into two parts, a Bnetwork number@ and a Blocal network address number@ within that network. The network number consists of the first one, two or three bytes; the rest of the bytes are the local address.
Network numbers are registered with the Network Information Center (NIC), and are divided into three classes--A, B, and C. The local network address numbers of individual machines are registered with the administrator of the particular network.
Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specifies a network. The remaining bytes of the Internet address specify the address within that network.
The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network.
The Class A network 127 is reserved for loopback; you can always use the Internet address H`127.0.0.1'@ to refer to the host machine.
Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address.
‰IDX964Š‰IDX965Š@�”˜There are four forms of the Bstandard numbers-and-dots notation@ for Internet addresses:
JaH.JbH.JcH.Jd
Â@This specifies all four bytes of the address individually.
ÀJaH.JbH.Jc
Â@The last part of the address, Bc@, is interpreted as a 2-byte quantity. This is useful for specifying host addresses in a Class B network with network address number JaH.Jb@.
ÀJaH.Jb
Â@The last part of the address, Bc@, is interpreted as a 3-byte quantity. This is useful for specifying host addresses in a Class A network with network address number Ba@.
ÀJa
Â@If only one part is given, this corresponds directly to the host address number.À

Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading H`0x'@ or H`0X'@ implies hexadecimal radix; a leading H`0'@ implies octal; and otherwise decimal radix is assumed.
ˆ‰SEC195ŠT�”›Host Address Data Type

@�”˜Internet host addresses are represented in some contexts as integers (type Hunsigned long int@). In other contexts, the integer is packaged inside a structure of type Hstruct in_addr@. It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure.
The following basic definitions for Internet addresses appear in the header file H`netinet/in.h'@: ‰IDX966Š

A�”˜Data Type:@ Dstruct in_addr
Â‰IDX967Š@�”˜This data type is used in certain contexts to contain an Internet host address. It has just one field, named Hs_addr@, which records the host address number as an Hunsigned long int@.À

AMacro:@ unsigned int DINADDR_LOOPBACK
Â‰IDX968Š@�”˜You can use this constant to stand for "the address of this machine," instead of finding its actual address. It is the Internet address H`127.0.0.1'@, which is usually called H`localhost'@. This special constant saves you the trouble of looking up the address of your own machine. Also, the system usually implements HINADDR_LOOPBACK@ specially, avoiding any network traffic for the case of one machine talking to itself.À

AMacro:@ unsigned int DINADDR_ANY
Â‰IDX969Š@�”˜You can use this constant to stand for "any incoming address," when binding to an address. See section ‰ŠP�”›Setting the Address of a Socket@�”˜. This is the usual address to give in the Hsin_addr@ member of Hstruct sockaddr_in@ when you want to accept Internet connections.À

AMacro:@ unsigned int DINADDR_BROADCAST
Â‰IDX970Š@�”˜This constant is the address you use to send a broadcast message.À

AMacro:@ unsigned int DINADDR_NONE
Â‰IDX971Š@�”˜This constant is returned by some functions to indicate an error.À

AData Type:@ Dstruct in6_addr
Â‰IDX972Š@�”˜This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways.À

AConstant:@ struct in6_addr Din6addr_loopback.
Â‰IDX973Š@�”˜This constant is the IPv6 address H`::1'@, the loopback address. See above for a description of what this means. The macro HIN6ADDR_LOOPBACK_INIT@ is provided to allow you to initialise your own variables to this value.À

AConstant:@ struct in6_addr Din6addr_any
Â‰IDX974Š@�”˜This constant is the IPv6 address H`::'@, the unspecified address. See above for a description of what this means. The macro HIN6ADDR_ANY_INIT@ is provided to allow you to initialise your own variables to this value.À

ˆ‰SEC196ŠT�”›Host Address Functions

‰IDX975Š@�”˜These additional functions for manipulating Internet addresses are declared in H`arpa/inet.h'@. They represent Internet addresses in network byte order; they represent network numbers and local-address-within-network numbers in host byte order. See section ‰ŠP�”›Byte Order Conversion@�”˜, for an explanation of network and host byte order.

AFunction:@ int Dinet_aton@ B(const char *Bname@, struct in_addr *Baddr@)
Â‰IDX976Š@�”˜This function converts the Internet host address Bname@ from the standard numbers-and-dots notation into binary data and stores it in the Hstruct in_addr@ that Baddr@ points to. Hinet_aton@ returns nonzero if the address is valid, zero if not.À

AFunction:@ unsigned long int Dinet_addr@ B(const char *Bname@)
Â‰IDX977Š@�”˜This function converts the Internet host address Bname@ from the standard numbers-and-dots notation into binary data. If the input is not valid, Hinet_addr@ returns HINADDR_NONE@. This is an obsolete interface to Hinet_aton@, described immediately above; it is obsolete because HINADDR_NONE@ is a valid address (255.255.255.255), and Hinet_aton@ provides a cleaner way to indicate error return.À

AFunction:@ unsigned long int Dinet_network@ B(const char *Bname@)
Â‰IDX978Š@�”˜This function extracts the network number from the address Bname@, given in the standard numbers-and-dots notation. If the input is not valid, Hinet_network@ returns H-1@.À

AFunction:@ char * Dinet_ntoa@ B(struct in_addr Baddr@)
Â‰IDX979Š@�”˜This function converts the Internet host address Baddr@ to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.
In multi-threaded programs each thread has an own statically-allocated buffer. But still subsequent calls of Hinet_ntoa@ in the same thread will overwrite the result of the last call.À

AFunction:@ struct in_addr Dinet_makeaddr@ B(int Bnet@, int Blocal@)
Â‰IDX980Š@�”˜This function makes an Internet host address by combining the network number Bnet@ with the local-address-within-network number Blocal@.À

AFunction:@ int Dinet_lnaof@ B(struct in_addr Baddr@)
Â‰IDX981Š@�”˜This function returns the local-address-within-network part of the Internet host address Baddr@.À

AFunction:@ int Dinet_netof@ B(struct in_addr Baddr@)
Â‰IDX982Š@�”˜This function returns the network number part of the Internet host address Baddr@.À

AFunction:@ int Dinet_pton@ B(int Baf@, const char *Bcp@, void *Bbuf@)
Â‰IDX983Š@�”˜This function converts an Internet address (either IPv4 or IPv6) from presentation (textual) to network (binary) format. Baf@ should be either HAF_INET@ or HAF_INET6@, as appropriate for the type of address being converted. Bcp@ is a pointer to the input string, and Bbuf@ is a pointer to a buffer for the result. It is the caller's responsibility to make sure the buffer is large enough.À

AFunction:@ char * Dinet_ntop@ B(int Baf@, const void *Bcp@, char *Bbuf@, size_t Blen@)
Â‰IDX984Š@�”˜This function converts an Internet address (either IPv4 or IPv6) from network (binary) to presentation (textual) form. Baf@ should be either HAF_INET@ or HAF_INET6@, as appropriate. Bcp@ is a pointer to the address to be converted. Bbuf@ should be a pointer to a buffer to hold the result, and Blen@ is the length of this buffer. The return value from the function will be this buffer address.À

ˆ‰SEC197ŠT�”›Host Names

‰IDX985Š‰IDX986Š‰IDX987Š
@�”˜Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address H`128.52.46.32'@ is also known as H`churchy.gnu.ai.mit.edu'@; and other machines in the H`gnu.ai.mit.edu'@ domain can refer to it simply as H`churchy'@.
‰IDX988Š‰IDX989Š@�”˜Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file H`/etc/hosts'@ or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in H`netdb.h'@. They are BSD features, defined unconditionally if you include H`netdb.h'@.

AData Type:@ Dstruct hostent
Â‰IDX990Š@�”˜This data type is used to represent an entry in the hosts database. It has the following members:
Hchar *h_name
Ä@This is the "official" name of the host.
ÂHchar **h_aliases
Ä@These are alternative names for the host, represented as a null-terminated vector of strings.
ÂHint h_addrtype
Ä@This is the host address type; in practice, its value is always either HAF_INET@ or HAF_INET6@, with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the data base as well as Internet addresses; if this were done, you might find a value in this field other than HAF_INET@ or HAF_INET6@. See section ‰ŠP�”›Socket Addresses@�”˜.
ÂHint h_length
Ä@This is the length, in bytes, of each address.
ÂHchar **h_addr_list
Ä@This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer.
ÂHchar *h_addr
Ä@This is a synonym for Hh_addr_list[0]@; in other words, it is the first host address.Â
À

As far as the host database is concerned, each address is just a block of memory Hh_length@ bytes long. But in other contexts there is an implicit assumption that you can convert this to a Hstruct in_addr@ or an Hunsigned long int@. Host addresses in a Hstruct hostent@ structure are always given in network byte order; see section ‰ŠP�”›Byte Order Conversion@�”˜.
You can use Hgethostbyname@, Hgethostbyname2@ or Hgethostbyaddr@ to search the hosts database for information about a particular host. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. You can also use Hgetaddrinfo@ and Hgetnameinfo@ to obtain this information.

AFunction:@ struct hostent * Dgethostbyname@ B(const char *Bname@)
Â‰IDX991Š@�”˜The Hgethostbyname@ function returns information about the host named Bname@. If the lookup fails, it returns a null pointer.À

AFunction:@ struct hostent * Dgethostbyname2@ B(const char *Bname@, int Baf@)
Â‰IDX992Š@�”˜The Hgethostbyname2@ function is like Hgethostbyname@, but allows the caller to specify the desired address family (e.g. HAF_INET@ or HAF_INET6@) for the result.À

AFunction:@ struct hostent * Dgethostbyaddr@ B(const char *Baddr@, int Blength@, int Bformat@)
Â‰IDX993Š@�”˜The Hgethostbyaddr@ function returns information about the host with Internet address Baddr@. The Blength@ argument is the size (in bytes) of the address at Baddr@. Bformat@ specifies the address format; for an Internet address, specify a value of HAF_INET@.
If the lookup fails, Hgethostbyaddr@ returns a null pointer.À

‰IDX994Š@�”˜If the name lookup by Hgethostbyname@ or Hgethostbyaddr@ fails, you can find out the reason by looking at the value of the variable Hh_errno@. (It would be cleaner design for these functions to set Herrno@, but use of Hh_errno@ is compatible with other systems.) Before using Hh_errno@, you must declare it like this: extern int h_errno;
Here are the error codes that you may find in Hh_errno@:
HHOST_NOT_FOUND
Â‰IDX995Š@�”˜No such host is known in the data base.
ÀHTRY_AGAIN
Â‰IDX996Š@�”˜This condition happens when the name server could not be contacted. If you try again later, you may succeed then.
ÀHNO_RECOVERY
Â‰IDX997Š@�”˜A non-recoverable error occurred.
ÀHNO_ADDRESS
Â‰IDX998Š@�”˜The host database contains an entry for the name, but it doesn't have an associated Internet address.À

You can also scan the entire hosts database one entry at a time using Hsethostent@, Hgethostent@, and Hendhostent@. Be careful in using these functions, because they are not reentrant.

AFunction:@ void Dsethostent@ B(int Bstayopen@)
Â‰IDX999Š@�”˜This function opens the hosts database to begin scanning it. You can then call Hgethostent@ to read the entries.
If the Bstayopen@ argument is nonzero, this sets a flag so that subsequent calls to Hgethostbyname@ or Hgethostbyaddr@ will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.À

AFunction:@ struct hostent * Dgethostent@ B()
Â‰IDX1000Š@�”˜This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries.À

AFunction:@ void Dendhostent@ B()
Â‰IDX1001Š@�”˜This function closes the hosts database.À

ˆ‰SEC198ŠT�”›Internet Ports

‰IDX1002Š
@�”˜A socket address in the Internet namespace consists of a machine's Internet address plus a Bport number@ which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535.
Port numbers less than HIPPORT_RESERVED@ are reserved for standard servers, such as Hfinger@ and Htelnet@. There is a database that keeps track of these, and you can use the Hgetservbyname@ function to map a service name onto a port number; see section ‰ŠP�”›The Services Database@�”˜.
If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it. Use a number greater than HIPPORT_USERRESERVED@; such numbers are reserved for servers and won't ever be generated automatically by the system. Avoiding conflicts with servers being run by other users is up to you.
When you use a socket without specifying its address, the system generates a port number for it. This number is between HIPPORT_RESERVED@ and HIPPORT_USERRESERVED@.
On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number). You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it. Normally, the system won't let you do it; Hbind@ normally insists on distinct port numbers. To reuse a port number, you must set the socket option HSO_REUSEADDR@. See section ‰ŠP�”›Socket-Level Options@�”˜.
‰IDX1003Š@�”˜These macros are defined in the header file H`netinet/in.h'@.

AMacro:@ int DIPPORT_RESERVED
Â‰IDX1004Š@�”˜Port numbers less than HIPPORT_RESERVED@ are reserved for superuser use.À

AMacro:@ int DIPPORT_USERRESERVED
Â‰IDX1005Š@�”˜Port numbers greater than or equal to HIPPORT_USERRESERVED@ are reserved for explicit use; they will never be allocated automatically.À

ˆ‰SEC199ŠT�”›The Services Database

‰IDX1006Š‰IDX1007Š‰IDX1008Š
‰IDX1009Š@�”˜The database that keeps track of "well-known" services is usually either the file H`/etc/services'@ or an equivalent from a name server. You can use these utilities, declared in H`netdb.h'@, to access the services database. ‰IDX1010Š

A�”˜Data Type:@ Dstruct servent
Â‰IDX1011Š@�”˜This data type holds information about entries from the services database. It has the following members:
Hchar *s_name
Ä@This is the "official" name of the service.
ÂHchar **s_aliases
Ä@These are alternate names for the service, represented as an array of strings. A null pointer terminates the array.
ÂHint s_port
Ä@This is the port number for the service. Port numbers are given in network byte order; see section ‰ŠP�”›Byte Order Conversion@�”˜.
ÂHchar *s_proto
Ä@This is the name of the protocol to use with this service. See section ‰ŠP�”›Protocols Database@�”˜.Â
À

To get information about a particular service, use the Hgetservbyname@ or Hgetservbyport@ functions. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

AFunction:@ struct servent * Dgetservbyname@ B(const char *Bname@, const char *Bproto@)
Â‰IDX1012Š@�”˜The Hgetservbyname@ function returns information about the service named Bname@ using protocol Bproto@. If it can't find such a service, it returns a null pointer.
This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see section ‰ŠP�”›Listening for Connections@�”˜).À

AFunction:@ struct servent * Dgetservbyport@ B(int Bport@, const char *Bproto@)
Â‰IDX1013Š@�”˜The Hgetservbyport@ function returns information about the service at port Bport@ using protocol Bproto@. If it can't find such a service, it returns a null pointer.À

You can also scan the services database using Hsetservent@, Hgetservent@, and Hendservent@. Be careful in using these functions, because they are not reentrant.

AFunction:@ void Dsetservent@ B(int Bstayopen@)
Â‰IDX1014Š@�”˜This function opens the services database to begin scanning it.
If the Bstayopen@ argument is nonzero, this sets a flag so that subsequent calls to Hgetservbyname@ or Hgetservbyport@ will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.À

AFunction:@ struct servent * Dgetservent@ B(void)
Â‰IDX1015Š@�”˜This function returns the next entry in the services database. If there are no more entries, it returns a null pointer.À

AFunction:@ void Dendservent@ B(void)
Â‰IDX1016Š@�”˜This function closes the services database.À

ˆ‰SEC200ŠT�”›Byte Order Conversion

‰IDX1017Š‰IDX1018Š
‰IDX1019Š‰IDX1020Š@�”˜Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called "big-endian" order), and others put it last ("little-endian" order).
‰IDX1021Š@�”˜So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as the Bnetwork byte order@.
When establishing an Internet socket connection, you must make sure that the data in the Hsin_port@ and Hsin_addr@ members of the Hsockaddr_in@ structure are represented in the network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don't do this, your program may fail when running on or talking to other kinds of machines.
If you use Hgetservbyname@ and Hgethostbyname@ or Hinet_addr@ to get the port number and host address, the values are already in the network byte order, and you can copy them directly into the Hsockaddr_in@ structure.
Otherwise, you have to convert the values explicitly. Use Hhtons@ and Hntohs@ to convert values for the Hsin_port@ member. Use Hhtonl@ and Hntohl@ to convert values for the Hsin_addr@ member. (Remember, Hstruct in_addr@ is equivalent to Hunsigned long int@.) These functions are declared in H`netinet/in.h'@. ‰IDX1022Š

A�”˜Function:@ unsigned short int Dhtons@ B(unsigned short int Bhostshort@)
Â‰IDX1023Š@�”˜This function converts the Hshort@ integer Bhostshort@ from host byte order to network byte order.À

AFunction:@ unsigned short int Dntohs@ B(unsigned short int Bnetshort@)
Â‰IDX1024Š@�”˜This function converts the Hshort@ integer Bnetshort@ from network byte order to host byte order.À

AFunction:@ unsigned long int Dhtonl@ B(unsigned long int Bhostlong@)
Â‰IDX1025Š@�”˜This function converts the Hlong@ integer Bhostlong@ from host byte order to network byte order.À

AFunction:@ unsigned long int Dntohl@ B(unsigned long int Bnetlong@)
Â‰IDX1026Š@�”˜This function converts the Hlong@ integer Bnetlong@ from network byte order to host byte order.À

ˆ‰SEC201ŠT�”›Protocols Database

‰IDX1027Š
@�”˜The communications protocol used with a socket controls low-level details of how data is exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly.
‰IDX1028Š@�”˜The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP ("transmission control protocol"). For datagram communication, the default is UDP ("user datagram protocol"). For reliable datagram communication, the default is RDP ("reliable datagram protocol"). You should nearly always use the default.
‰IDX1029Š@�”˜Internet protocols are generally specified by a name instead of a number. The network protocols that a host knows about are stored in a database. This is usually either derived from the file H`/etc/protocols'@, or it may be an equivalent provided by a name server. You look up the protocol number associated with a named protocol in the database using the Hgetprotobyname@ function.
Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in H`netdb.h'@. ‰IDX1030Š

A�”˜Data Type:@ Dstruct protoent
Â‰IDX1031Š@�”˜This data type is used to represent entries in the network protocols database. It has the following members:
Hchar *p_name
Ä@This is the official name of the protocol.
ÂHchar **p_aliases
Ä@These are alternate names for the protocol, specified as an array of strings. The last element of the array is a null pointer.
ÂHint p_proto
Ä@This is the protocol number (in host byte order); use this member as the Bprotocol@ argument to Hsocket@.Â
À

You can use Hgetprotobyname@ and Hgetprotobynumber@ to search the protocols database for a specific protocol. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

AFunction:@ struct protoent * Dgetprotobyname@ B(const char *Bname@)
Â‰IDX1032Š@�”˜The Hgetprotobyname@ function returns information about the network protocol named Bname@. If there is no such protocol, it returns a null pointer.À

AFunction:@ struct protoent * Dgetprotobynumber@ B(int Bprotocol@)
Â‰IDX1033Š@�”˜The Hgetprotobynumber@ function returns information about the network protocol with number Bprotocol@. If there is no such protocol, it returns a null pointer.À

You can also scan the whole protocols database one protocol at a time by using Hsetprotoent@, Hgetprotoent@, and Hendprotoent@. Be careful in using these functions, because they are not reentrant.

AFunction:@ void Dsetprotoent@ B(int Bstayopen@)
Â‰IDX1034Š@�”˜This function opens the protocols database to begin scanning it.
If the Bstayopen@ argument is nonzero, this sets a flag so that subsequent calls to Hgetprotobyname@ or Hgetprotobynumber@ will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.À

AFunction:@ struct protoent * Dgetprotoent@ B(void)
Â‰IDX1035Š@�”˜This function returns the next entry in the protocols database. It returns a null pointer if there are no more entries.À

AFunction:@ void Dendprotoent@ B(void)
Â‰IDX1036Š@�”˜This function closes the protocols database.À

ˆ‰SEC202ŠT�”›Internet Socket Example

@�”˜Here is an example showing how to create and name a socket in the Internet namespace. The newly created socket exists on the machine that the program is running on. Rather than finding and using the machine's Internet address, this example specifies HINADDR_ANY@ as the host address; the system replaces that with the machine's actual address. #include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>

int
make_socket (unsigned short int port)
{
int sock;
struct sockaddr_in name;

/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}

/* Give the socket a name. */
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}

return sock;
}
Here is another example, showing how you can fill in a Hsockaddr_in@ structure, given a host name string and a port number: #include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

void
init_sockaddr (struct sockaddr_in *name,
const char *hostname,
unsigned short int port)
{
struct hostent *hostinfo;

name->sin_family = AF_INET;
name->sin_port = htons (port);
hostinfo = gethostbyname (hostname);
if (hostinfo == NULL)
{
fprintf (stderr, "Unknown host %s.\n", hostname);
exit (EXIT_FAILURE);
}
name->sin_addr = *(struct in_addr *) hostinfo->h_addr;
}
ˆ‰SEC203Št�”›Other Namespaces

‰IDX1037Š‰IDX1038Š‰IDX1039Š‰IDX1040Š‰IDX1041Š@�”˜Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used. HPF_NS@ refers to the Xerox Network Software protocols. HPF_ISO@ stands for Open Systems Interconnect. HPF_CCITT@ refers to protocols from CCITT. H`socket.h'@ defines these symbols and others naming protocols not actually implemented.
HPF_IMPLINK@ is used for communicating between hosts and Internet Message Processors. For information on this, and on HPF_ROUTE@, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future).
ˆ‰SEC204Št�”›Opening and Closing Sockets

@�”˜This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles.
ˆ‰SEC205ŠT�”›Creating a Socket

‰IDX1042Š‰IDX1043Š‰IDX1044Š
@�”˜The primitive for creating a socket is the Hsocket@ function, declared in H`sys/socket.h'@. ‰IDX1045Š

A�”˜Function:@ int Dsocket@ B(int Bnamespace@, int Bstyle@, int Bprotocol@)
Â‰IDX1046Š@�”˜This function creates a socket and specifies communication style Bstyle@, which should be one of the socket styles listed in section ‰ŠP�”›Communication Styles@�”˜. The Bnamespace@ argument specifies the namespace; it must be HPF_FILE@ (see section ‰ŠP�”›The File Namespace@�”˜) or HPF_INET@ (see section ‰ŠP�”›The Internet Namespace@�”˜). Bprotocol@ designates the specific protocol (see section ‰ŠP�”›Socket Concepts@�”˜); zero is usually right for Bprotocol@.
The return value from Hsocket@ is the file descriptor for the new socket, or H-1@ in case of error. The following Herrno@ error conditions are defined for this function:
HEPROTONOSUPPORT
Ä@The Bprotocol@ or Bstyle@ is not supported by the Bnamespace@ specified.
ÂHEMFILE
Ä@The process already has too many file descriptors open.
ÂHENFILE
Ä@The system already has too many file descriptors open.
ÂHEACCESS
Ä@The process does not have privilege to create a socket of the specified Bstyle@ or Bprotocol@.
ÂHENOBUFS
Ä@The system ran out of internal buffer space.Â

The file descriptor returned by the Hsocket@ function supports both read and write operations. But, like pipes, sockets do not support file positioning operations.À

For examples of how to call the Hsocket@ function, see section ‰ŠP�”›The File Namespace@�”˜, or section ‰ŠP�”›Internet Socket Example@�”˜.
ˆ‰SEC206ŠT�”›Closing a Socket

‰IDX1047Š‰IDX1048Š‰IDX1049Š‰IDX1050Š
@�”˜When you are finished using a socket, you can simply close its file descriptor with Hclose@; see section ‰ŠP�”›Opening and Closing Files@�”˜. If there is still data waiting to be transmitted over the connection, normally Hclose@ tries to complete this transmission. You can control this behavior using the HSO_LINGER@ socket option to specify a timeout period; see section ‰ŠP�”›Socket Options@�”˜.
‰IDX1051Š@�”˜You can also shut down only reception or only transmission on a connection by calling Hshutdown@, which is declared in H`sys/socket.h'@.

AFunction:@ int Dshutdown@ B(int Bsocket@, int Bhow@)
Â‰IDX1052Š@�”˜The Hshutdown@ function shuts down the connection of socket Bsocket@. The argument Bhow@ specifies what action to perform:
H0
Ä@Stop receiving data for this socket. If further data arrives, reject it.
ÂH1
Ä@Stop trying to transmit data from this socket. Discard any data waiting to be sent. Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost.
ÂH2
Ä@Stop both reception and transmission.Â

The return value is H0@ on success and H-1@ on failure. The following Herrno@ error conditions are defined for this function:
HEBADF
ÄBsocket@ is not a valid file descriptor.
ÂHENOTSOCK
ÄBsocket@ is not a socket.
ÂHENOTCONN
ÄBsocket@ is not connected.Â
À

ˆ‰SEC207ŠT�”›Socket Pairs

‰IDX1053Š‰IDX1054Š‰IDX1055Š
‰IDX1056Š@�”˜A Bsocket pair@ consists of a pair of connected (but unnamed) sockets. It is very similar to a pipe and is used in much the same way. Socket pairs are created with the Hsocketpair@ function, declared in H`sys/socket.h'@. A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (see section ‰ŠP�”›Pipes and FIFOs@�”˜).

AFunction:@ int Dsocketpair@ B(int Bnamespace@, int Bstyle@, int Bprotocol@, int BfiledesH[2]@)
Â‰IDX1057Š@�”˜This function creates a socket pair, returning the file descriptors in JfiledesH[0]@ and JfiledesH[1]@. The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end.
The Bnamespace@, Bstyle@, and Bprotocol@ arguments are interpreted as for the Hsocket@ function. Bstyle@ should be one of the communication styles listed in section ‰ŠP�”›Communication Styles@�”˜. The Bnamespace@ argument specifies the namespace, which must be HAF_FILE@ (see section ‰ŠP�”›The File Namespace@�”˜); Bprotocol@ specifies the communications protocol, but zero is the only meaningful value.
If Bstyle@ specifies a connectionless communication style, then the two sockets you get are not Bconnected@, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.
The Hsocketpair@ function returns H0@ on success and H-1@ on failure. The following Herrno@ error conditions are defined for this function:
HEMFILE
Ä@The process has too many file descriptors open.
ÂHEAFNOSUPPORT
Ä@The specified namespace is not supported.
ÂHEPROTONOSUPPORT
Ä@The specified protocol is not supported.
ÂHEOPNOTSUPP
Ä@The specified protocol does not support the creation of socket pairs.Â
À

ˆ‰SEC208Št�”›Using Sockets with Connections

‰IDX1058Š‰IDX1059Š‰IDX1060Š@�”˜The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the Bclient@) acts to request a connection, while the other side (the Bserver@) makes a socket and waits for the connection request. þÂ
• section ‰ŠP�”›Making a Connection@�”˜, describes what the client program must do to initiate a connection with a server.
• section ‰ŠP�”›Listening for Connections@�”˜, and section ‰ŠP�”›Accepting Connections@�”˜, describe what the server program must do to wait for and act upon connection requests from clients.
• section ‰ŠP�”›Transferring Data@�”˜, describes how data is transferred through the connected socket.Àð
ˆ‰SEC209ŠT�”›Making a Connection

‰IDX1061Š‰IDX1062Š‰IDX1063Š‰IDX1064Š
@�”˜In making a connection, the client makes a connection while the server waits for and accepts the connection. Here we discuss what the client program must do, using the Hconnect@ function, which is declared in H`sys/socket.h'@.

AFunction:@ int Dconnect@ B(int Bsocket@, struct sockaddr *Baddr@, socklen_t Blength@)
Â‰IDX1065Š@�”˜The Hconnect@ function initiates a connection from the socket with file descriptor Bsocket@ to the socket whose address is specified by the Baddr@ and Blength@ arguments. (This socket is typically on another machine, and it must be already set up as a server.) See section ‰ŠP�”›Socket Addresses@�”˜, for information about how these arguments are interpreted.
Normally, Hconnect@ waits until the server responds to the request before it returns. You can set nonblocking mode on the socket Bsocket@ to make Hconnect@ return immediately without waiting for the response. See section ‰ŠP�”›File Status Flags@�”˜, for information about nonblocking mode.
The normal return value from Hconnect@ is H0@. If an error occurs, Hconnect@ returns H-1@. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The socket Bsocket@ is not a valid file descriptor.
ÂHENOTSOCK
Ä@File descriptor Bsocket@ is not a socket.
ÂHEADDRNOTAVAIL
Ä@The specified address is not available on the remote machine.
ÂHEAFNOSUPPORT
Ä@The namespace of the Baddr@ is not supported by this socket.
ÂHEISCONN
Ä@The socket Bsocket@ is already connected.
ÂHETIMEDOUT
Ä@The attempt to establish the connection timed out.
ÂHECONNREFUSED
Ä@The server has actively refused to establish the connection.
ÂHENETUNREACH
Ä@The network of the given Baddr@ isn't reachable from this host.
ÂHEADDRINUSE
Ä@The socket address of the given Baddr@ is already in use.
ÂHEINPROGRESS
Ä@The socket Bsocket@ is non-blocking and the connection could not be established immediately. You can determine when the connection is completely established with Hselect@; see section ‰ŠP�”›Waiting for Input or Output@�”˜. Another Hconnect@ call on the same socket, before the connection is completely established, will fail with HEALREADY@.
ÂHEALREADY
Ä@The socket Bsocket@ is non-blocking and already has a pending connection in progress (see HEINPROGRESS@ above).Â
À

ˆ‰SEC210ŠT�”›Listening for Connections

‰IDX1066Š‰IDX1067Š‰IDX1068Š
@�”˜Now let us consider what the server process must do to accept connections on a socket. First it must use the Hlisten@ function to enable connection requests on the socket, and then accept each incoming connection with a call to Haccept@ (see section ‰ŠP�”›Accepting Connections@�”˜). Once connection requests are enabled on a server socket, the Hselect@ function reports when the socket has a connection ready to be accepted (see section ‰ŠP�”›Waiting for Input or Output@�”˜).
The Hlisten@ function is not allowed for sockets using connectionless communication styles.
You can write a network server that does not even start running until a connection to it is requested. See section ‰ŠX�”›inetdP Servers@�”˜.
In the Internet namespace, there are no special protection mechanisms for controlling access to connect to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.
In the File namespace, the ordinary file protection bits control who has access to connect to the socket.

AFunction:@ int Dlisten@ B(int Bsocket@, unsigned int Bn@)
Â‰IDX1069Š@�”˜The Hlisten@ function enables the socket Bsocket@ to accept connections, thus making it a server socket.
The argument Bn@ specifies the length of the queue for pending connections. When the queue fills, new clients attempting to connect fail with HECONNREFUSED@ until the server calls Haccept@ to accept a connection from the queue.
The Hlisten@ function returns H0@ on success and H-1@ on failure. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The argument Bsocket@ is not a valid file descriptor.
ÂHENOTSOCK
Ä@The argument Bsocket@ is not a socket.
ÂHEOPNOTSUPP
Ä@The socket Bsocket@ does not support this operation.Â
À

ˆ‰SEC211ŠT�”›Accepting Connections

‰IDX1070Š‰IDX1071Š
@�”˜When a server receives a connection request, it can complete the connection by accepting the request. Use the function Haccept@ to do this.
A socket that has been established as a server can accept connection requests from multiple clients. The server's original socket Bdoes not become part@ of the connection; instead, Haccept@ makes a new socket which participates in the connection. Haccept@ returns the descriptor for this socket. The server's original socket remains available for listening for further connection requests.
The number of pending connection requests on a server socket is finite. If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with a HECONNREFUSED@ error. You can specify the maximum length of this queue as an argument to the Hlisten@ function, although the system may also impose its own internal limit on the length of this queue.

AFunction:@ int Daccept@ B(int Bsocket@, struct sockaddr *Baddr@, socklen_t *Blength-ptr@)
Â‰IDX1072Š@�”˜This function is used to accept a connection request on the server socket Bsocket@.
The Haccept@ function waits if there are no connections pending, unless the socket Bsocket@ has nonblocking mode set. (You can use Hselect@ to wait for a pending connection, with a nonblocking socket.) See section ‰ŠP�”›File Status Flags@�”˜, for information about nonblocking mode.
The Baddr@ and Blength-ptr@ arguments are used to return information about the name of the client socket that initiated the connection. See section ‰ŠP�”›Socket Addresses@�”˜, for information about the format of the information.
Accepting a connection does not make Bsocket@ part of the connection. Instead, it creates a new socket which becomes connected. The normal return value of Haccept@ is the file descriptor for the new socket.
After Haccept@, the original socket Bsocket@ remains open and unconnected, and continues listening until you close it. You can accept further connections with Bsocket@ by calling Haccept@ again.
If an error occurs, Haccept@ returns H-1@. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The Bsocket@ argument is not a valid file descriptor.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ argument is not a socket.
ÂHEOPNOTSUPP
Ä@The descriptor Bsocket@ does not support this operation.
ÂHEWOULDBLOCK
ÄBsocket@ has nonblocking mode set, and there are no pending connections immediately available.Â
À

The Haccept@ function is not allowed for sockets using connectionless communication styles.
ˆ‰SEC212ŠT�”›Who is Connected to Me?

A�”˜Function:@ int Dgetpeername@ B(int Bsocket@, struct sockaddr *Baddr@, size_t *Blength-ptr@)
Â‰IDX1073Š@�”˜The Hgetpeername@ function returns the address of the socket that Bsocket@ is connected to; it stores the address in the memory space specified by Baddr@ and Blength-ptr@. It stores the length of the address in H*Jlength-ptr@.
See section ‰ŠP�”›Socket Addresses@�”˜, for information about the format of the address. In some operating systems, Hgetpeername@ works only for sockets in the Internet domain.
The return value is H0@ on success and H-1@ on error. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The argument Bsocket@ is not a valid file descriptor.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ is not a socket.
ÂHENOTCONN
Ä@The socket Bsocket@ is not connected.
ÂHENOBUFS
Ä@There are not enough internal buffers available.Â
À

ˆ‰SEC213ŠT�”›Transferring Data

‰IDX1074Š‰IDX1075Š
@�”˜Once a socket has been connected to a peer, you can use the ordinary Hread@ and Hwrite@ operations (see section ‰ŠP�”›Input and Output Primitives@�”˜) to transfer data. A socket is a two-way communications channel, so read and write operations can be performed at either end.
There are also some I/O modes that are specific to socket operations. In order to specify these modes, you must use the Hrecv@ and Hsend@ functions instead of the more generic Hread@ and Hwrite@ functions. The Hrecv@ and Hsend@ functions take an additional argument which you can use to specify various flags to control the special I/O modes. For example, you can specify the HMSG_OOB@ flag to read or write out-of-band data, the HMSG_PEEK@ flag to peek at input, or the HMSG_DONTROUTE@ flag to control inclusion of routing information on output.
ˆ‰SEC214ŠT�”›Sending Data

‰IDX1076Š@�”˜The Hsend@ function is declared in the header file H`sys/socket.h'@. If your Bflags@ argument is zero, you can just as well use Hwrite@ instead of Hsend@; see section ‰ŠP�”›Input and Output Primitives@�”˜. If the socket was connected but the connection has broken, you get a HSIGPIPE@ signal for any use of Hsend@ or Hwrite@ (see section ‰ŠP�”›Miscellaneous Signals@�”˜).

AFunction:@ int Dsend@ B(int Bsocket@, void *Bbuffer@, size_t Bsize@, int Bflags@)
Â‰IDX1077Š@�”˜The Hsend@ function is like Hwrite@, but with the additional flags Bflags@. The possible values of Bflags@ are described in section ‰ŠP�”›Socket Data Options@�”˜.
This function returns the number of bytes transmitted, or H-1@ on failure. If the socket is nonblocking, then Hsend@ (like Hwrite@) can return after sending just part of the data. See section ‰ŠP�”›File Status Flags@�”˜, for information about nonblocking mode.
Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.
The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The Bsocket@ argument is not a valid file descriptor.
ÂHEINTR
Ä@The operation was interrupted by a signal before any data was sent. See section ‰ŠP�”›Primitives Interrupted by Signals@�”˜.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ is not a socket.
ÂHEMSGSIZE
Ä@The socket type requires that the message be sent atomically, but the message is too large for this to be possible.
ÂHEWOULDBLOCK
Ä@Nonblocking mode has been set on the socket, and the write operation would block. (Normally Hsend@ blocks until the operation can be completed.)
ÂHENOBUFS
Ä@There is not enough internal buffer space available.
ÂHENOTCONN
Ä@You never connected this socket.
ÂHEPIPE
Ä@This socket was connected but the connection is now broken. In this case, Hsend@ generates a HSIGPIPE@ signal first; if that signal is ignored or blocked, or if its handler returns, then Hsend@ fails with HEPIPE@.Â
À

ˆ‰SEC215ŠT�”›Receiving Data

‰IDX1078Š@�”˜The Hrecv@ function is declared in the header file H`sys/socket.h'@. If your Bflags@ argument is zero, you can just as well use Hread@ instead of Hrecv@; see section ‰ŠP�”›Input and Output Primitives@�”˜.

AFunction:@ int Drecv@ B(int Bsocket@, void *Bbuffer@, size_t Bsize@, int Bflags@)
Â‰IDX1079Š@�”˜The Hrecv@ function is like Hread@, but with the additional flags Bflags@. The possible values of Bflags@ are described In section ‰ŠP�”›Socket Data Options@�”˜.
If nonblocking mode is set for Bsocket@, and no data is available to be read, Hrecv@ fails immediately rather than waiting. See section ‰ŠP�”›File Status Flags@�”˜, for information about nonblocking mode.
This function returns the number of bytes received, or H-1@ on failure. The following Herrno@ error conditions are defined for this function:
HEBADF
Ä@The Bsocket@ argument is not a valid file descriptor.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ is not a socket.
ÂHEWOULDBLOCK
Ä@Nonblocking mode has been set on the socket, and the read operation would block. (Normally, Hrecv@ blocks until there is input available to be read.)
ÂHEINTR
Ä@The operation was interrupted by a signal before any data was read. See section ‰ŠP�”›Primitives Interrupted by Signals@�”˜.
ÂHENOTCONN
Ä@You never connected this socket.Â
À

ˆ‰SEC216ŠT�”›Socket Data Options

‰IDX1080Š@�”˜The Bflags@ argument to Hsend@ and Hrecv@ is a bit mask. You can bitwise-OR the values of the following macros together to obtain a value for this argument. All are defined in the header file H`sys/socket.h'@.

AMacro:@ int DMSG_OOB
Â‰IDX1081Š@�”˜Send or receive out-of-band data. See section ‰ŠP�”›Out-of-Band Data@�”˜.À

AMacro:@ int DMSG_PEEK
Â‰IDX1082Š@�”˜Look at the data but don't remove it from the input queue. This is only meaningful with input functions such as Hrecv@, not with Hsend@.À

AMacro:@ int DMSG_DONTROUTE
Â‰IDX1083Š@�”˜Don't include routing information in the message. This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs. We don't try to explain it here.À

ˆ‰SEC217ŠT�”›Byte Stream Socket Example

@�”˜Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits. #include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define PORT 5555
#define MESSAGE "Yow!!! Are we having fun yet?!?"
#define SERVERHOST "churchy.gnu.ai.mit.edu"

void
write_to_server (int filedes)
{
int nbytes;

nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
if (nbytes < 0)
{
perror ("write");
exit (EXIT_FAILURE);
}
}

int
main (void)
{
extern void init_sockaddr (struct sockaddr_in *name,
const char *hostname,
unsigned short int port);
int sock;
struct sockaddr_in servername;

/* Create the socket. */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket (client)");
exit (EXIT_FAILURE);
}

/* Connect to the server. */
init_sockaddr (&servername, SERVERHOST, PORT);
if (0 > connect (sock,
(struct sockaddr *) &servername,
sizeof (servername)))
{
perror ("connect (client)");
exit (EXIT_FAILURE);
}

/* Send data to the server. */
write_to_server (sock);
close (sock);
exit (EXIT_SUCCESS);
}
ˆ‰SEC218ŠT�”›Byte Stream Connection Server Example

@�”˜The server end is much more complicated. Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling Hread@ or Hrecv@. Instead, the right thing to do is to use Hselect@ (see section ‰ŠP�”›Waiting for Input or Output@�”˜) to wait for input on all of the open sockets. This also allows the server to deal with additional connection requests.
This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).
This program uses Hmake_socket@ and Hinit_sockaddr@ to set up the socket address; see section ‰ŠP�”›Internet Socket Example@�”˜. #include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define PORT 5555
#define MAXMSG 512

int
read_from_client (int filedes)
{
char buffer[MAXMSG];
int nbytes;

nbytes = read (filedes, buffer, MAXMSG);
if (nbytes < 0)
{
/* Read error. */
perror ("read");
exit (EXIT_FAILURE);
}
else if (nbytes == 0)
/* End-of-file. */
return -1;
else
{
/* Data read. */
fprintf (stderr, "Server: got message: `%s'\n", buffer);
return 0;
}
}

int
main (void)
{
extern int make_socket (unsigned short int port);
int sock;
fd_set active_fd_set, read_fd_set;
int i;
struct sockaddr_in clientname;
size_t size;

/* Create the socket and set it up to accept connections. */
sock = make_socket (PORT);
if (listen (sock, 1) < 0)
{
perror ("listen");
exit (EXIT_FAILURE);
}

/* Initialize the set of active sockets. */
FD_ZERO (&active_fd_set);
FD_SET (sock, &active_fd_set);

while (1)
{
/* Block until input arrives on one or more active sockets. */
read_fd_set = active_fd_set;
if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
{
perror ("select");
exit (EXIT_FAILURE);
}

/* Service all the sockets with input pending. */
for (i = 0; i < FD_SETSIZE; ++i)
if (FD_ISSET (i, &read_fd_set))
{
if (i == sock)
{
/* Connection request on original socket. */
int new;
size = sizeof (clientname);
new = accept (sock,
(struct sockaddr *) &clientname,
&size);
if (new < 0)
{
perror ("accept");
exit (EXIT_FAILURE);
}
fprintf (stderr,
"Server: connect from host %s, port %hd.\n",
inet_ntoa (clientname.sin_addr),
ntohs (clientname.sin_port));
FD_SET (new, &active_fd_set);
}
else
{
/* Data arriving on an already-connected socket. */
if (read_from_client (i) < 0)
{
close (i);
FD_CLR (i, &active_fd_set);
}
}
}
}
}
ˆ‰SEC219ŠT�”›Out-of-Band Data

‰IDX1084Š‰IDX1085Š@�”˜Streams with connections permit Bout-of-band@ data that is delivered with higher priority than ordinary data. Typically the reason for sending out-of-band data is to send notice of an exceptional condition. The way to send out-of-band data is using Hsend@, specifying the flag HMSG_OOB@ (see section ‰ŠP�”›Sending Data@�”˜).
Out-of-band data is received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use Hrecv@ with the HMSG_OOB@ flag (see section ‰ŠP�”›Receiving Data@�”˜). Ordinary read operations do not read out-of-band data; they read only the ordinary data.
‰IDX1086Š@�”˜When a socket finds that out-of-band data is on its way, it sends a HSIGURG@ signal to the owner process or process group of the socket. You can specify the owner using the HF_SETOWN@ command to the Hfcntl@ function; see section ‰ŠP�”›Interrupt-Driven Input@�”˜. You must also establish a handler for this signal, as described in section ‰ŠP�”›Signal Handling@�”˜, in order to take appropriate action such as reading the out-of-band data.
Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the Hselect@ function; it can wait for an exceptional condition on the socket. See section ‰ŠP�”›Waiting for Input or Output@�”˜, for more information about Hselect@.
Notification of out-of-band data (whether with HSIGURG@ or with Hselect@) indicates that out-of-band data is on the way; the data may not actually arrive until later. If you try to read the out-of-band data before it arrives, Hrecv@ fails with an HEWOULDBLOCK@ error.
Sending out-of-band data automatically places a "mark" in the stream of ordinary data, showing where in the sequence the out-of-band data "would have been". This is useful when the meaning of out-of-band data is "cancel everything sent so far". Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark: success = ioctl (socket, SIOCATMARK, &result);
Here's a function to discard any ordinary data preceding the out-of-band mark: int
discard_until_mark (int socket)
{
while (1)
{
/* This is not an arbitrary limit; any size will do. */
char buffer[1024];
int result, success;

/* If we have reached the mark, return. */
success = ioctl (socket, SIOCATMARK, &result);
if (success < 0)
perror ("ioctl");
if (result)
return;

/* Otherwise, read a bunch of ordinary data and discard it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");
}
}
If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data. If you try to read out-of-band data and get an HEWOULDBLOCK@ error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room. Here is an example: struct buffer
{
char *buffer;
int size;
struct buffer *next;
};

/* Read the out-of-band data from SOCKET and return it
as a `struct buffer', which records the address of the data
and its size.

It may be necessary to read some ordinary data
in order to make room for the out-of-band data.
If so, the ordinary data is saved as a chain of buffers
found in the `next' field of the value. */

struct buffer *
read_oob (int socket)
{
struct buffer *tail = 0;
struct buffer *list = 0;

while (1)
{
/* This is an arbitrary limit.
Does anyone know how to do this without a limit? */
char *buffer = (char *) xmalloc (1024);
struct buffer *link;
int success;
int result;

/* Try again to read the out-of-band data. */
success = recv (socket, buffer, sizeof buffer, MSG_OOB);
if (success >= 0)
{
/* We got it, so return it. */
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buffer = buffer;
link->size = success;
link->next = list;
return link;
}

/* If we fail, see if we are at the mark. */
success = ioctl (socket, SIOCATMARK, &result);
if (success < 0)
perror ("ioctl");
if (result)
{
/* At the mark; skipping past more ordinary data cannot help.
So just wait a while. */
sleep (1);
continue;
}

/* Otherwise, read a bunch of ordinary data and save it.
This is guaranteed not to read past the mark
if it starts before the mark. */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");

/* Save this data in the buffer list. */
{
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buffer = buffer;
link->size = success;

/* Add the new link to the end of the list. */
if (tail)
tail->next = link;
else
list = link;
tail = link;
}
}
}
ˆ‰SEC220Št�”›Datagram Socket Operations

‰IDX1087Š@�”˜This section describes how to use communication styles that don't use connections (styles HSOCK_DGRAM@ and HSOCK_RDM@). Using these styles, you group data into packets and each packet is an independent communication. You specify the destination for each packet individually.
Datagram packets are like letters: you send each one independently, with its own destination address, and they may arrive in the wrong order or not at all.
The Hlisten@ and Haccept@ functions are not allowed for sockets using connectionless communication styles.
ˆ‰SEC221ŠT�”›Sending Datagrams

‰IDX1088Š‰IDX1089Š‰IDX1090Š
‰IDX1091Š@�”˜The normal way of sending data on a datagram socket is by using the Hsendto@ function, declared in H`sys/socket.h'@.
You can call Hconnect@ on a datagram socket, but this only specifies a default destination for further data transmission on the socket. When a socket has a default destination, then you can use Hsend@ (see section ‰ŠP�”›Sending Data@�”˜) or even Hwrite@ (see section ‰ŠP�”›Input and Output Primitives@�”˜) to send a packet there. You can cancel the default destination by calling Hconnect@ using an address format of HAF_UNSPEC@ in the Baddr@ argument. See section ‰ŠP�”›Making a Connection@�”˜, for more information about the Hconnect@ function.

AFunction:@ int Dsendto@ B(int Bsocket@, void *Bbuffer@. size_t Bsize@, int Bflags@, struct sockaddr *Baddr@, socklen_t Blength@)
Â‰IDX1092Š@�”˜The Hsendto@ function transmits the data in the Bbuffer@ through the socket Bsocket@ to the destination address specified by the Baddr@ and Blength@ arguments. The Bsize@ argument specifies the number of bytes to be transmitted.
The Bflags@ are interpreted the same way as for Hsend@; see section ‰ŠP�”›Socket Data Options@�”˜.
The return value and error conditions are also the same as for Hsend@, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no one at the specified address to receive it, and the operating system on your machine usually does not know this.
It is also possible for one call to Hsendto@ to report an error due to a problem related to a previous call.À

ˆ‰SEC222ŠT�”›Receiving Datagrams

‰IDX1093Š
@�”˜The Hrecvfrom@ function reads a packet from a datagram socket and also tells you where it was sent from. This function is declared in H`sys/socket.h'@.

AFunction:@ int Drecvfrom@ B(int Bsocket@, void *Bbuffer@, size_t Bsize@, int Bflags@, struct sockaddr *Baddr@, socklen_t *Blength-ptr@)
Â‰IDX1094Š@�”˜The Hrecvfrom@ function reads one packet from the socket Bsocket@ into the buffer Bbuffer@. The Bsize@ argument specifies the maximum number of bytes to be read.
If the packet is longer than Bsize@ bytes, then you get the first Bsize@ bytes of the packet, and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect.
The Baddr@ and Blength-ptr@ arguments are used to return the address where the packet came from. See section ‰ŠP�”›Socket Addresses@�”˜. For a socket in the file domain, the address information won't be meaningful, since you can't read the address of such a socket (see section ‰ŠP�”›The File Namespace@�”˜). You can specify a null pointer as the Baddr@ argument if you are not interested in this information.
The Bflags@ are interpreted the same way as for Hrecv@ (see section ‰ŠP�”›Socket Data Options@�”˜). The return value and error conditions are also the same as for Hrecv@.À

You can use plain Hrecv@ (see section ‰ŠP�”›Receiving Data@�”˜) instead of Hrecvfrom@ if you know don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike). Even Hread@ can be used if you don't want to specify Bflags@ (see section ‰ŠP�”›Input and Output Primitives@�”˜).
ˆ‰SEC223ŠT�”›Datagram Socket Example

@�”˜Here is a set of example programs that send messages over a datagram stream in the file namespace. Both the client and server programs use the Hmake_named_socket@ function that was presented in section ‰ŠP�”›The File Namespace@�”˜, to create and name their sockets.
First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously, this isn't a particularly useful program, but it does show the general ideas involved. #include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SERVER "/tmp/serversocket"
#define MAXMSG 512

int
main (void)
{
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;

/* Make the socket, then loop endlessly. */

sock = make_named_socket (SERVER);
while (1)
{
/* Wait for a datagram. */
size = sizeof (name);
nbytes = recvfrom (sock, message, MAXMSG, 0,
(struct sockaddr *) & name, &size);
if (nbytes < 0)
{
perror ("recfrom (server)");
exit (EXIT_FAILURE);
}

/* Give a diagnostic message. */
fprintf (stderr, "Server: got message: %s\n", message);

/* Bounce the message back to the sender. */
nbytes = sendto (sock, message, nbytes, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (server)");
exit (EXIT_FAILURE);
}
}
}
ˆ‰SEC224ŠT�”›Example of Reading Datagrams

@�”˜Here is the client program corresponding to the server above.
It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client. #include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SERVER "/tmp/serversocket"
#define CLIENT "/tmp/mysocket"
#define MAXMSG 512
#define MESSAGE "Yow!!! Are we having fun yet?!?"

int
main (void)
{
extern int make_named_socket (const char *name);
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;

/* Make the socket. */
sock = make_named_socket (CLIENT);

/* Initialize the server socket address. */
name.sun_family = AF_UNIX;
strcpy (name.sun_path, SERVER);
size = strlen (name.sun_path) + sizeof (name.sun_family);

/* Send the datagram. */
nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (client)");
exit (EXIT_FAILURE);
}

/* Wait for a reply. */
nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
if (nbytes < 0)
{
perror ("recfrom (client)");
exit (EXIT_FAILURE);
}

/* Print a diagnostic message. */
fprintf (stderr, "Client: got message: %s\n", message);

/* Clean up. */
remove (CLIENT);
close (sock);
}
Keep in mind that datagram socket communications are unreliable. In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back. It's up to the user running the program to kill it and restart it, if desired. A more automatic solution could be to use Hselect@ (see section ‰ŠP�”›Waiting for Input or Output@�”˜) to establish a timeout period for the reply, and in case of timeout either resend the message or shut down the socket and exit.
ˆ‰SEC225Št�”›The |inetdt Daemon

@�”˜We've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it.
Another way to provide service for an Internet port is to let the daemon program Hinetd@ do the listening. Hinetd@ is a program that runs all the time and waits (using Hselect@) for messages on a specified set of ports. When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program. You specify the ports and their programs in the file H`/etc/inetd.conf'@.
ˆ‰SEC226Š\�”›inetdT Servers

@�”˜Writing a server program to be run by Hinetd@ is very simple. Each time someone requests a connection to the appropriate port, a new server process starts. The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process. So the server program can begin reading and writing data right away. Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by Hinetd@.
You can also use Hinetd@ for servers that use connectionless communication styles. For these servers, Hinetd@ does not try to accept a connection, since no connection is possible. It just starts the server program, which can read the incoming datagram packet from descriptor 0. The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit. You must specify which of these two techniques the server uses, when you configure Hinetd@.
ˆ‰SEC227ŠT�”›Configuring \inetd

@�”˜The file H`/etc/inetd.conf'@ tells Hinetd@ which ports to listen to and what server programs to run for them. Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace. Lines that start with H`#'@ are comments.
Here are two standard entries in H`/etc/inetd.conf'@: ftp stream tcp nowait root /libexec/ftpd ftpd
talk dgram udp wait root /libexec/talkd talkd
An entry has this format: Bservice@ Bstyle@ Bprotocol@ Bwait@ Busername@ Bprogram@ Barguments@
The Bservice@ field says which service this program provides. It should be the name of a service defined in H`/etc/services'@. Hinetd@ uses Bservice@ to decide which port to listen on for this entry.
The fields Bstyle@ and Bprotocol@ specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with H`SOCK_'@ deleted--for example, H`stream'@ or H`dgram'@. Bprotocol@ should be one of the protocols listed in H`/etc/protocols'@. The typical protocol names are H`tcp'@ for byte stream connections and H`udp'@ for unreliable datagrams.
The Bwait@ field should be either H`wait'@ or H`nowait'@. Use H`wait'@ if Bstyle@ is a connectionless style and the server, once started, handles multiple requests, as many as come in. Use H`nowait'@ if Hinetd@ should start a new process for each message or request that comes in. If Bstyle@ uses connections, then Bwait@ Dmust@ be H`nowait'@.
Buser@ is the user name that the server should run as. Hinetd@ runs as root, so it can set the user ID of its children arbitrarily. It's best to avoid using H`root'@ for Buser@ if you can; but some servers, such as Telnet and FTP, read a username and password themselves. These servers need to be root initially so they can log in as commanded by the data coming over the network.
Bprogram@ together with Barguments@ specifies the command to run to start the server. Bprogram@ should be an absolute file name specifying the executable file to run. Barguments@ consists of any number of whitespace-separated words, which become the command-line arguments of Bprogram@. The first word in Barguments@ is argument zero, which should by convention be the program name itself (sans directories).
If you edit H`/etc/inetd.conf'@, you can tell Hinetd@ to reread the file and obey its new contents by sending the Hinetd@ process the HSIGHUP@ signal. You'll have to use Hps@ to determine the process ID of the Hinetd@ process, as it is not fixed.
ˆ‰SEC228Št�”›Socket Options

‰IDX1095Š
@�”˜This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.
‰IDX1096Š‰IDX1097Š@�”˜When you are manipulating a socket option, you must specify which Blevel@ the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.
ˆ‰SEC229ŠT�”›Socket Option Functions

‰IDX1098Š@�”˜Here are the functions for examining and modifying socket options. They are declared in H`sys/socket.h'@.

AFunction:@ int Dgetsockopt@ B(int Bsocket@, int Blevel@, int Boptname@, void *Boptval@, socklen_t *Boptlen-ptr@)
Â‰IDX1099Š@�”˜The Hgetsockopt@ function gets information about the value of option Boptname@ at level Blevel@ for socket Bsocket@.
The option value is stored in a buffer that Boptval@ points to. Before the call, you should supply in H*Joptlen-ptr@ the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer.
Most options interpret the Boptval@ buffer as a single Hint@ value.
The actual return value of Hgetsockopt@ is H0@ on success and H-1@ on failure. The following Herrno@ error conditions are defined:
HEBADF
Ä@The Bsocket@ argument is not a valid file descriptor.
ÂHENOTSOCK
Ä@The descriptor Bsocket@ is not a socket.
ÂHENOPROTOOPT
Ä@The Boptname@ doesn't make sense for the given Blevel@.Â
À

AFunction:@ int Dsetsockopt@ B(int Bsocket@, int Blevel@, int Boptname@, void *Boptval@, socklen_t Boptlen@)
Â‰IDX1100Š@�”˜This function is used to set the socket option Boptname@ at level Blevel@ for socket Bsocket@. The value of the option is passed in the buffer Boptval@, which has size Boptlen@.
The return value and error codes for Hsetsockopt@ are the same as for Hgetsockopt@.À

ˆ‰SEC230ŠT�”›Socket-Level Options

A�”˜Constant:@ int DSOL_SOCKET
Â‰IDX1101Š@�”˜Use this constant as the Blevel@ argument to Hgetsockopt@ or Hsetsockopt@ to manipulate the socket-level options described in this section.À

‰IDX1102Š@�”˜Here is a table of socket-level option names; all are defined in the header file H`sys/socket.h'@.
HSO_DEBUG
Â@This option toggles recording of debugging information in the underlying protocol modules. The value has type Hint@; a nonzero value means "yes".
ÀHSO_REUSEADDR
Â@This option controls whether Hbind@ (see section ‰ŠP�”›Setting the Address of a Socket@�”˜) should permit reuse of local addresses for this socket. If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet. The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same socket number. The value has type Hint@; a nonzero value means "yes".
ÀHSO_KEEPALIVE
Â@This option controls whether the underlying protocol should periodically transmit messages on a connected socket. If the peer fails to respond to these messages, the connection is considered broken. The value has type Hint@; a nonzero value means "yes".
ÀHSO_DONTROUTE
Â@This option controls whether outgoing messages bypass the normal message routing facilities. If set, messages are sent directly to the network interface instead. The value has type Hint@; a nonzero value means "yes".
ÀHSO_LINGER
Â@This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see section ‰ŠP�”›Closing a Socket@�”˜. The value has type Hstruct linger@.
AData Type:@ Dstruct linger
Ä‰IDX1103Š@�”˜This structure type has the following members:
Hint l_onoff
Æ@This field is interpreted as a boolean. If nonzero, Hclose@ blocks until the data is transmitted or the timeout period has expired.
ÄHint l_linger
Æ@This specifies the timeout period, in seconds.Ä
Â

ÀHSO_BROADCAST
Â@This option controls whether datagrams may be broadcast from the socket. The value has type Hint@; a nonzero value means "yes".
ÀHSO_OOBINLINE
Â@If this option is set, out-of-band data received on the socket is placed in the normal input queue. This permits it to be read using Hread@ or Hrecv@ without specifying the HMSG_OOB@ flag. See section ‰ŠP�”›Out-of-Band Data@�”˜. The value has type Hint@; a nonzero value means "yes".
ÀHSO_SNDBUF
Â@This option gets or sets the size of the output buffer. The value is a Hsize_t@, which is the size in bytes.
ÀHSO_RCVBUF
Â@This option gets or sets the size of the input buffer. The value is a Hsize_t@, which is the size in bytes.
ÀHSO_STYLE
Â
ÀHSO_TYPE
Â@This option can be used with Hgetsockopt@ only. It is used to get the socket's communication style. HSO_TYPE@ is the historical name, and HSO_STYLE@ is the preferred name in GNU. The value has type Hint@ and its value designates a communication style; see section ‰ŠP�”›Communication Styles@�”˜.
ÀHSO_ERROR
Â@This option can be used with Hgetsockopt@ only. It is used to reset the error status of the socket. The value is an Hint@, which represents the previous error status.À

ˆ‰SEC231Št�”›Networks Database

‰IDX1104Š‰IDX1105Š‰IDX1106Š
‰IDX1107Š‰IDX1108Š@�”˜Many systems come with a database that records a list of networks known to the system developer. This is usually kept either in the file H`/etc/networks'@ or in an equivalent from a name server. This data base is useful for routing programs such as Hroute@, but it is not useful for programs that simply communicate over the network. We provide functions to access this data base, which are declared in H`netdb.h'@.

AData Type:@ Dstruct netent
Â‰IDX1109Š@�”˜This data type is used to represent information about entries in the networks database. It has the following members:
Hchar *n_name
Ä@This is the "official" name of the network.
ÂHchar **n_aliases
Ä@These are alternative names for the network, represented as a vector of strings. A null pointer terminates the array.
ÂHint n_addrtype
Ä@This is the type of the network number; this is always equal to HAF_INET@ for Internet networks.
ÂHunsigned long int n_net
Ä@This is the network number. Network numbers are returned in host byte order; see section ‰ŠP�”›Byte Order Conversion@�”˜.Â
À

Use the Hgetnetbyname@ or Hgetnetbyaddr@ functions to search the networks database for information about a specific network. The information is returned in a statically-allocated structure; you must copy the information if you need to save it.

AFunction:@ struct netent * Dgetnetbyname@ B(const char *Bname@)
Â‰IDX1110Š@�”˜The Hgetnetbyname@ function returns information about the network named Bname@. It returns a null pointer if there is no such network.À

AFunction:@ struct netent * Dgetnetbyaddr@ B(long Bnet@, int Btype@)
Â‰IDX1111Š@�”˜The Hgetnetbyaddr@ function returns information about the network of type Btype@ with number Bnet@. You should specify a value of HAF_INET@ for the Btype@ argument for Internet networks.
Hgetnetbyaddr@ returns a null pointer if there is no such network.À

You can also scan the networks database using Hsetnetent@, Hgetnetent@, and Hendnetent@. Be careful in using these functions, because they are not reentrant.

AFunction:@ void Dsetnetent@ B(int Bstayopen@)
Â‰IDX1112Š@�”˜This function opens and rewinds the networks database.
If the Bstayopen@ argument is nonzero, this sets a flag so that subsequent calls to Hgetnetbyname@ or Hgetnetbyaddr@ will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.À

AFunction:@ struct netent * Dgetnetent@ B(void)
Â‰IDX1113Š@�”˜This function returns the next entry in the networks database. It returns a null pointer if there are no more entries.À

AFunction:@ void Dendnetent@ B(void)
Â‰IDX1114Š@�”˜This function closes the networks database.À

Œ
Go to the ‰ŠP�”›first@�”˜, ‰ŠP�”›previous@�”˜, ‰ŠP�”›next@�”˜, ‰ŠP�”›last@�”˜ section, ‰ŠP�”›table of contents@�”˜.