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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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PROLOG | NAME | SYNOPSIS | DESCRIPTION | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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sys_socket.h(0P) POSIX Programmer's Manual sys_socket.h(0P)
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.
sys/socket.h — main sockets header
#include <sys/socket.h>
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.
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. */
};
None.
None.
sys_types.h(0p), sys_uio.h(0p)
The System Interfaces volume of POSIX.1‐2017, accept(3p),
bind(3p), connect(3p), getpeername(3p), getsockname(3p),
getsockopt(3p), listen(3p), recv(3p), recvfrom(3p), recvmsg(3p),
send(3p), sendmsg(3p), sendto(3p), setsockopt(3p), shutdown(3p),
sockatmark(3p), socket(3p), socketpair(3p)
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(0P)
Pages that refer to this page: netdb.h(0p), netinet_in.h(0p), netinet_tcp.h(0p), sys_un.h(0p), unistd.h(0p), accept(3p), bind(3p), connect(3p), freeaddrinfo(3p), getnameinfo(3p), getpeername(3p), getsockname(3p), getsockopt(3p), listen(3p), recv(3p), recvfrom(3p), recvmsg(3p), send(3p), sendmsg(3p), sendto(3p), setsockopt(3p), shutdown(3p), sockatmark(3p), socket(3p), socketpair(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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