'\" et
.TH sys_socket.h "0P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH 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.

.SH NAME
sys/socket.h
\(em main sockets header
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.fi
.SH DESCRIPTION
The
.IR <sys/socket.h> 
header shall define the
.BR socklen_t
type, which is an integer type of width of at least 32 bits;
see APPLICATION USAGE.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR sa_family_t
unsigned integer type.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR sockaddr
structure, which shall include at least the following members:
.sp
.RS 4
.nf
\fB
sa_family_t  sa_family  \fRAddress family.\fR
char         sa_data[]  \fRSocket address (variable-length data).\fR
.fi \fR
.P
.RE
.P
The
.BR sockaddr
structure is used to define a socket address which is used
in the
\fIbind\fR(),
\fIconnect\fR(),
\fIgetpeername\fR(),
\fIgetsockname\fR(),
\fIrecvfrom\fR(),
and
\fIsendto\fR()
functions.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR sockaddr_storage
structure, which shall be:
.IP " *" 4
Large enough to accommodate all supported protocol-specific address
structures
.IP " *" 4
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
.P
The
.BR sockaddr_storage
structure shall include at least the following members:
.sp
.RS 4
.nf
\fB
sa_family_t   ss_family
.fi \fR
.P
.RE
.P
When a pointer to a
.BR sockaddr_storage
structure is cast as a pointer to a
.BR sockaddr
structure, the
.IR ss_family
field of the
.BR sockaddr_storage
structure shall map onto the
.IR sa_family
field of the
.BR sockaddr
structure. When a pointer to a
.BR sockaddr_storage
structure is cast as a pointer to a protocol-specific address structure,
the
.IR ss_family
field shall map onto
a field of that structure that is of type
.BR sa_family_t
and that identifies the protocol's address family.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR msghdr
structure, which shall include at least the following members:
.sp
.RS 4
.nf
\fB
void          *msg_name        \fROptional address.\fR
socklen_t      msg_namelen     \fRSize of address.\fR
struct iovec  *msg_iov         \fRScatter/gather array.\fR
int            msg_iovlen      \fRMembers in \fImsg_iov\fR.\fR
void          *msg_control     \fRAncillary data; see below.\fR
socklen_t      msg_controllen  \fRAncillary data buffer \fIlen\fR.\fR
int            msg_flags       \fRFlags on received message.\fR
.fi \fR
.P
.RE
.P
The
.BR msghdr
structure is used to minimize the number of directly supplied
parameters to the
\fIrecvmsg\fR()
and
\fIsendmsg\fR()
functions. This structure is used as a
.IR value \(hy\c
.IR result
parameter in the
\fIrecvmsg\fR()
function and
.IR value
only for the
\fIsendmsg\fR()
function.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR iovec
structure as described in
.IR <sys/uio.h> .
.P
The
.IR <sys/socket.h> 
header shall define the
.BR cmsghdr
structure, which shall include at least the following members:
.sp
.RS 4
.nf
\fB
socklen_t  cmsg_len    \fRData byte count, including the \fBcmsghdr\fR.\fR
int        cmsg_level  \fROriginating protocol.\fR
int        cmsg_type   \fRProtocol-specific type.\fR
.fi \fR
.P
.RE
.P
The
.BR cmsghdr
structure is used for storage of ancillary data object information.
.P
Ancillary data consists of a sequence of pairs, each consisting of a
.BR cmsghdr
structure followed by a data array. The data array contains the
ancillary data message, and the
.BR cmsghdr
structure contains descriptive information that allows an application
to correctly parse the data.
.P
The values for
.IR cmsg_level
shall be legal values for the
.IR level
argument to the
\fIgetsockopt\fR()
and
\fIsetsockopt\fR()
functions. The system documentation shall specify the
.IR cmsg_type
definitions for the supported protocols.
.P
Ancillary data is also possible at the socket level. The
.IR <sys/socket.h> 
header shall define the following symbolic constant for use as the
.IR cmsg_type
value when
.IR cmsg_level
is SOL_SOCKET:
.IP SCM_RIGHTS 14
Indicates that the data array contains the access rights to be sent or
received.
.P
The
.IR <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:
.IP "CMSG_DATA(\fIcmsg\fP)" 6
.br
If the argument is a pointer to a
.BR cmsghdr
structure, this macro shall return an unsigned character pointer
to the data array associated with the
.BR cmsghdr
structure.
.IP "CMSG_NXTHDR(\fImhdr,cmsg\fP)" 6
.br
If the first argument is a pointer to a
.BR msghdr
structure and the second argument is a pointer to a
.BR cmsghdr
structure in the ancillary data pointed to by the
.IR msg_control
field of that
.BR msghdr
structure, this macro shall return a pointer to the next
.BR cmsghdr
structure, or a null pointer if this structure is the last
.BR cmsghdr
in the ancillary data.
.IP "CMSG_FIRSTHDR(\fImhdr\fP)" 6
.br
If the argument is a pointer to a
.BR msghdr
structure, this macro shall return a pointer to the first
.BR cmsghdr
structure in the ancillary data associated with this
.BR msghdr
structure, or a null pointer if there is no ancillary data associated
with the
.BR msghdr
structure.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR linger
structure, which shall include at least the following members:
.sp
.RS 4
.nf
\fB
int  l_onoff   \fRIndicates whether linger option is enabled.\fR
int  l_linger  \fRLinger time, in seconds.\fR
.fi \fR
.P
.RE
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constants with distinct values:
.IP SOCK_DGRAM 14
Datagram socket.
.IP SOCK_RAW 14
Raw Protocol Interface.
.IP SOCK_SEQPACKET 14
Sequenced-packet socket.
.IP SOCK_STREAM 14
Byte-stream socket.
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constant for use as the
.IR level
argument of
\fIsetsockopt\fR()
and
\fIgetsockopt\fR().
.IP SOL_SOCKET 14
Options to be accessed at socket level, not protocol level.
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constants with distinct values
for use as the
.IR option_name
argument in
\fIgetsockopt\fR()
or
\fIsetsockopt\fR()
calls (see the System Interfaces volume of POSIX.1\(hy2008,
.IR "Section 2.10.16" ", " "Use of Options"):
.IP SO_ACCEPTCONN 14
Socket is accepting connections.
.IP SO_BROADCAST 14
Transmission of broadcast messages is supported.
.IP SO_DEBUG 14
Debugging information is being recorded.
.IP SO_DONTROUTE 14
Bypass normal routing.
.IP SO_ERROR 14
Socket error status.
.IP SO_KEEPALIVE 14
Connections are kept alive with periodic messages.
.IP SO_LINGER 14
Socket lingers on close.
.IP SO_OOBINLINE 14
Out-of-band data is transmitted in line.
.IP SO_RCVBUF 14
Receive buffer size.
.IP SO_RCVLOWAT 14
Receive ``low water mark''.
.IP SO_RCVTIMEO 14
Receive timeout.
.IP SO_REUSEADDR 14
Reuse of local addresses is supported.
.IP SO_SNDBUF 14
Send buffer size.
.IP SO_SNDLOWAT 14
Send ``low water mark''.
.IP SO_SNDTIMEO 14
Send timeout.
.IP SO_TYPE 14
Socket type.
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constant for use as the maximum
.IR backlog
queue length which may be specified by the
.IR backlog
field of the
\fIlisten\fR()
function:
.IP SOMAXCONN 14
The maximum
.IR backlog
queue length.
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constants with distinct values
for use as the valid values for the
.IR msg_flags
field in the
.BR msghdr
structure, or the
.IR flags
parameter in
\fIrecv\fR(),
\fIrecvfrom\fR(),
\fIrecvmsg\fR(),
\fIsend\fR(),
\fIsendmsg\fR(),
or
\fIsendto\fR()
calls:
.IP MSG_CTRUNC 14
Control data truncated.
.IP MSG_DONTROUTE 14
Send without using routing tables.
.IP MSG_EOR 14
Terminates a record (if supported by the protocol).
.IP MSG_OOB 14
Out-of-band data.
.IP MSG_NOSIGNAL 14
No SIGPIPE generated when an attempt to send is made on a
stream-oriented socket that is no longer connected.
.IP MSG_PEEK 14
Leave received data in queue.
.IP MSG_TRUNC 14
Normal data truncated.
.IP MSG_WAITALL 14
Attempt to fill the read buffer.
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constants with distinct values:
.IP AF_INET 14
Internet domain sockets for use with IPv4 addresses.
.IP AF_INET6 14
Internet domain sockets for use with IPv6 addresses.
.IP AF_UNIX 14
UNIX domain sockets.
.IP AF_UNSPEC 14
Unspecified.
.P
The
.IR <sys/socket.h> 
header shall define the following symbolic constants with distinct values:
.IP SHUT_RD 14
Disables further receive operations.
.IP SHUT_RDWR 14
Disables further send and receive operations.
.IP SHUT_WR 14
Disables further send operations.
.P
The
.IR <sys/socket.h> 
header shall define the
.BR size_t
and
.BR ssize_t
types as described in
.IR <sys/types.h> .
.P
The following shall be declared as functions and may also be defined as
macros. Function prototypes shall be provided.
.sp
.RS 4
.nf
\fB
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]);
.fi \fR
.P
.RE
.P
Inclusion of
.IR <sys/socket.h> 
may also make visible all symbols from
.IR <sys/uio.h> .
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
To forestall portability problems, it is recommended that applications
not use values larger than 2\u\s-331\s+3\d \(mi1 for the
.BR socklen_t
type.
.P
The
.BR 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
\fIgetpeername\fR(),
and determine the address family by accessing the received content
after the call.
.P
The example below illustrates a data structure which aligns on a 64-bit
boundary. An implementation-defined field
.IR _ss_align
following
.IR _ss_pad1
is used to force a 64-bit alignment which covers proper alignment good
enough for needs of at least
.BR sockaddr_in6
(IPv6) and
.BR sockaddr_in
(IPv4) address data structures. The size of padding field
.IR _ss_pad1
depends on the chosen alignment boundary. The size of padding field
.IR _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.
.sp
.RS 4
.nf
\fB
/*
 *  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. */
.P
/*
 *  Definitions used for sockaddr_storage structure paddings design.
 */
#define _SS_PAD1SIZE (_SS_ALIGNSIZE \(mi sizeof(sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE \(mi (sizeof(sa_family_t)+ \e
                      _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. */
};
.fi \fR
.P
.RE
.SH "RATIONALE"
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fB<sys_types.h>\fP",
.IR "\fB<sys_uio.h>\fP"
.P
The System Interfaces volume of POSIX.1\(hy2008,
.IR "\fIaccept\fR\^(\|)",
.IR "\fIbind\fR\^(\|)",
.IR "\fIconnect\fR\^(\|)",
.IR "\fIgetpeername\fR\^(\|)",
.IR "\fIgetsockname\fR\^(\|)",
.IR "\fIgetsockopt\fR\^(\|)",
.IR "\fIlisten\fR\^(\|)",
.IR "\fIrecv\fR\^(\|)",
.IR "\fIrecvfrom\fR\^(\|)",
.IR "\fIrecvmsg\fR\^(\|)",
.IR "\fIsend\fR\^(\|)",
.IR "\fIsendmsg\fR\^(\|)",
.IR "\fIsendto\fR\^(\|)",
.IR "\fIsetsockopt\fR\^(\|)",
.IR "\fIshutdown\fR\^(\|)",
.IR "\fIsockatmark\fR\^(\|)",
.IR "\fIsocket\fR\^(\|)",
.IR "\fIsocketpair\fR\^(\|)"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .