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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
sys/socket.h — main sockets header
SYNOPSIS
#include <sys/socket.h>
DESCRIPTION
The <sys/socket.h> header shall define the socklen_t type, which is an integer type of width of at least
32 bits; see APPLICATION USAGE.
The <sys/socket.h> header shall define the sa_family_t unsigned integer type.
The <sys/socket.h> header shall define the sockaddr structure, which shall include at least the following
members:
sa_family_t sa_family Address family.
char sa_data[] Socket address (variable-length data).
The sockaddr structure is used to define a socket address which is used in the bind(), connect(),
getpeername(), getsockname(), recvfrom(), and sendto() functions.
The <sys/socket.h> header shall define the sockaddr_storage structure, which shall be:
* Large enough to accommodate all supported protocol-specific address structures
* Aligned at an appropriate boundary so that pointers to it can be cast as pointers to protocol-
specific address structures and used to access the fields of those structures without alignment
problems
The sockaddr_storage structure shall include at least the following members:
sa_family_t ss_family
When a pointer to a sockaddr_storage structure is cast as a pointer to a sockaddr structure, the
ss_family field of the sockaddr_storage structure shall map onto the sa_family field of the sockaddr
structure. When a pointer to a sockaddr_storage structure is cast as a pointer to a protocol-specific
address structure, the ss_family field shall map onto a field of that structure that is of type
sa_family_t and that identifies the protocol's address family.
The <sys/socket.h> header shall define the msghdr structure, which shall include at least the following
members:
void *msg_name Optional address.
socklen_t msg_namelen Size of address.
struct iovec *msg_iov Scatter/gather array.
int msg_iovlen Members in msg_iov.
void *msg_control Ancillary data; see below.
socklen_t msg_controllen Ancillary data buffer len.
int msg_flags Flags on received message.
The msghdr structure is used to minimize the number of directly supplied parameters to the recvmsg() and
sendmsg() functions. This structure is used as a value‐result parameter in the recvmsg() function and
value only for the sendmsg() function.
The <sys/socket.h> header shall define the iovec structure as described in <sys/uio.h>.
The <sys/socket.h> header shall define the cmsghdr structure, which shall include at least the following
members:
socklen_t cmsg_len Data byte count, including the cmsghdr.
int cmsg_level Originating protocol.
int cmsg_type Protocol-specific type.
The cmsghdr structure is used for storage of ancillary data object information.
Ancillary data consists of a sequence of pairs, each consisting of a cmsghdr structure followed by a data
array. The data array contains the ancillary data message, and the cmsghdr structure contains descriptive
information that allows an application to correctly parse the data.
The values for cmsg_level shall be legal values for the level argument to the getsockopt() and
setsockopt() functions. The system documentation shall specify the cmsg_type definitions for the
supported protocols.
Ancillary data is also possible at the socket level. The <sys/socket.h> header shall define the following
symbolic constant for use as the cmsg_type value when cmsg_level is SOL_SOCKET:
SCM_RIGHTS Indicates that the data array contains the access rights to be sent or received.
The <sys/socket.h> header shall define the following macros to gain access to the data arrays in the
ancillary data associated with a message header:
CMSG_DATA(cmsg)
If the argument is a pointer to a cmsghdr structure, this macro shall return an unsigned character
pointer to the data array associated with the cmsghdr structure.
CMSG_NXTHDR(mhdr,cmsg)
If the first argument is a pointer to a msghdr structure and the second argument is a pointer to a
cmsghdr structure in the ancillary data pointed to by the msg_control field of that msghdr
structure, this macro shall return a pointer to the next cmsghdr structure, or a null pointer if
this structure is the last cmsghdr in the ancillary data.
CMSG_FIRSTHDR(mhdr)
If the argument is a pointer to a msghdr structure, this macro shall return a pointer to the first
cmsghdr structure in the ancillary data associated with this msghdr structure, or a null pointer if
there is no ancillary data associated with the msghdr structure.
The <sys/socket.h> header shall define the linger structure, which shall include at least the following
members:
int l_onoff Indicates whether linger option is enabled.
int l_linger Linger time, in seconds.
The <sys/socket.h> header shall define the following symbolic constants with distinct values:
SOCK_DGRAM Datagram socket.
SOCK_RAW Raw Protocol Interface.
SOCK_SEQPACKET
Sequenced-packet socket.
SOCK_STREAM Byte-stream socket.
The <sys/socket.h> header shall define the following symbolic constant for use as the level argument of
setsockopt() and getsockopt().
SOL_SOCKET Options to be accessed at socket level, not protocol level.
The <sys/socket.h> header shall define the following symbolic constants with distinct values for use as
the option_name argument in getsockopt() or setsockopt() calls (see the System Interfaces volume of
POSIX.1‐2017, Section 2.10.16, Use of Options):
SO_ACCEPTCONN Socket is accepting connections.
SO_BROADCAST Transmission of broadcast messages is supported.
SO_DEBUG Debugging information is being recorded.
SO_DONTROUTE Bypass normal routing.
SO_ERROR Socket error status.
SO_KEEPALIVE Connections are kept alive with periodic messages.
SO_LINGER Socket lingers on close.
SO_OOBINLINE Out-of-band data is transmitted in line.
SO_RCVBUF Receive buffer size.
SO_RCVLOWAT Receive ``low water mark''.
SO_RCVTIMEO Receive timeout.
SO_REUSEADDR Reuse of local addresses is supported.
SO_SNDBUF Send buffer size.
SO_SNDLOWAT Send ``low water mark''.
SO_SNDTIMEO Send timeout.
SO_TYPE Socket type.
The <sys/socket.h> header shall define the following symbolic constant for use as the maximum backlog
queue length which may be specified by the backlog field of the listen() function:
SOMAXCONN The maximum backlog queue length.
The <sys/socket.h> header shall define the following symbolic constants with distinct values for use as
the valid values for the msg_flags field in the msghdr structure, or the flags parameter in recv(),
recvfrom(), recvmsg(), send(), sendmsg(), or sendto() calls:
MSG_CTRUNC Control data truncated.
MSG_DONTROUTE Send without using routing tables.
MSG_EOR Terminates a record (if supported by the protocol).
MSG_OOB Out-of-band data.
MSG_NOSIGNAL No SIGPIPE generated when an attempt to send is made on a stream-oriented socket that is no
longer connected.
MSG_PEEK Leave received data in queue.
MSG_TRUNC Normal data truncated.
MSG_WAITALL Attempt to fill the read buffer.
The <sys/socket.h> header shall define the following symbolic constants with distinct values:
AF_INET Internet domain sockets for use with IPv4 addresses.
AF_INET6 Internet domain sockets for use with IPv6 addresses.
AF_UNIX UNIX domain sockets.
AF_UNSPEC Unspecified.
The value of AF_UNSPEC shall be 0.
The <sys/socket.h> header shall define the following symbolic constants with distinct values:
SHUT_RD Disables further receive operations.
SHUT_RDWR Disables further send and receive operations.
SHUT_WR Disables further send operations.
The <sys/socket.h> header shall define the size_t and ssize_t types as described in <sys/types.h>.
The following shall be declared as functions and may also be defined as macros. Function prototypes shall
be provided.
int accept(int, struct sockaddr *restrict, socklen_t *restrict);
int bind(int, const struct sockaddr *, socklen_t);
int connect(int, const struct sockaddr *, socklen_t);
int getpeername(int, struct sockaddr *restrict, socklen_t *restrict);
int getsockname(int, struct sockaddr *restrict, socklen_t *restrict);
int getsockopt(int, int, int, void *restrict, socklen_t *restrict);
int listen(int, int);
ssize_t recv(int, void *, size_t, int);
ssize_t recvfrom(int, void *restrict, size_t, int,
struct sockaddr *restrict, socklen_t *restrict);
ssize_t recvmsg(int, struct msghdr *, int);
ssize_t send(int, const void *, size_t, int);
ssize_t sendmsg(int, const struct msghdr *, int);
ssize_t sendto(int, const void *, size_t, int, const struct sockaddr *,
socklen_t);
int setsockopt(int, int, int, const void *, socklen_t);
int shutdown(int, int);
int sockatmark(int);
int socket(int, int, int);
int socketpair(int, int, int, int [2]);
Inclusion of <sys/socket.h> may also make visible all symbols from <sys/uio.h>.
The following sections are informative.
APPLICATION USAGE
To forestall portability problems, it is recommended that applications not use values larger than 231 -1
for the socklen_t type.
The sockaddr_storage structure solves the problem of declaring storage for automatic variables which is
both large enough and aligned enough for storing the socket address data structure of any family. For
example, code with a file descriptor and without the context of the address family can pass a pointer to
a variable of this type, where a pointer to a socket address structure is expected in calls such as
getpeername(), and determine the address family by accessing the received content after the call.
The example below illustrates a data structure which aligns on a 64-bit boundary. An implementation-
defined field _ss_align following _ss_pad1 is used to force a 64-bit alignment which covers proper
alignment good enough for needs of at least sockaddr_in6 (IPv6) and sockaddr_in (IPv4) address data
structures. The size of padding field _ss_pad1 depends on the chosen alignment boundary. The size of
padding field _ss_pad2 depends on the value of overall size chosen for the total size of the structure.
This size and alignment are represented in the above example by implementation-defined (not required)
constants _SS_MAXSIZE (chosen value 128) and _SS_ALIGNMENT (with chosen value 8). Constants _SS_PAD1SIZE
(derived value 6) and _SS_PAD2SIZE (derived value 112) are also for illustration and not required. The
implementation-defined definitions and structure field names above start with an <underscore> to denote
implementation private name space. Portable code is not expected to access or reference those fields or
constants.
/*
* Desired design of maximum size and alignment.
*/
#define _SS_MAXSIZE 128
/* Implementation-defined maximum size. */
#define _SS_ALIGNSIZE (sizeof(int64_t))
/* Implementation-defined desired alignment. */
/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE (_SS_ALIGNSIZE - sizeof(sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof(sa_family_t)+ \
_SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
sa_family_t ss_family; /* Address family. */
/*
* Following fields are implementation-defined.
*/
char _ss_pad1[_SS_PAD1SIZE];
/* 6-byte pad; this is to make implementation-defined
pad up to alignment field that follows explicit in
the data structure. */
int64_t _ss_align; /* Field to force desired structure
storage alignment. */
char _ss_pad2[_SS_PAD2SIZE];
/* 112-byte pad to achieve desired size,
_SS_MAXSIZE value minus size of ss_family
__ss_pad1, __ss_align fields is 112. */
};
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
<sys_types.h>, <sys_uio.h>
The System Interfaces volume of POSIX.1‐2017, accept(), bind(), connect(), getpeername(), getsockname(),
getsockopt(), listen(), recv(), recvfrom(), recvmsg(), send(), sendmsg(), sendto(), setsockopt(),
shutdown(), sockatmark(), socket(), socketpair()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 sys_socket.h(7POSIX)