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 converted to a pointer to a sockaddr structure, or vice versa, the ss_family member of the sockaddr_storage structure shall map onto the sa_family member of the sockaddr structure. When a pointer to a sockaddr_storage structure is converted to a pointer to a protocol-specific address structure, or vice versa, the ss_family member shall map onto a member of that structure that is of type sa_family_t that identifies the protocol's address family. When a pointer to a sockaddr structure is converted to a pointer to a protocol-specific address structure, or vice versa, the sa_family member shall map onto a member of that structure that is of type sa_family_t that identifies the protocol's address family. Additionally, the structures shall be defined in such a way that the compiler treats an access to the stored value of the sa_family_t member of any of these structures, via an lvalue expression whose type involves any other one of these structures, as permissible, despite the more restrictive expression rules on stored value access as stated in the ISO C standard. Similarly, when a pointer to a sockaddr_storage or sockaddr structure is converted to a pointer to a protocol-specific address structure, the compiler shall treat an access (using this converted pointer) to the stored value of any member of the protocol-specific structure as permissible. The application shall ensure that the protocol-specific address structure corresponds to the family indicated by the member with type sa_family_t of that structure and the pointed-to object has sufficient memory for addressing all members of the protocol-specific structure.

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 reduce 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 the second argument points to the last cmsghdr and data array pair in the ancillary data. If the ancillary data contains another cmsghdr structure after this one but the cmsg_len value in that structure is such that the data array following that structure would extend beyond the end of the ancillary data, it is unspecified whether this macro returns a pointer to that cmsghdr structure or returns a null pointer.

If the first argument is a pointer to a msghdr structure and the second argument is a null pointer, this macro shall be equivalent to CMSG_FIRSTHDR(mhdr).

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 either there is no ancillary data associated with the msghdr structure (msg_controllen is zero) or there is insufficient room in the ancillary data for a complete cmsghdr structure (msg_controllen is non-zero but less than sizeof(struct cmsghdr)).

CMSG_SPACE(length)

If the argument has a type such that its value can be assigned to an object of type socklen_t, this macro shall return the space required by an ancillary data object of the specified length and its cmsghdr structure, including any padding needed to satisfy alignment requirements. This macro can be used, for example, to allocate space dynamically for the ancillary data. This macro should not be used to initialize the cmsg_len member of a cmsghdr structure. If the argument is an integer constant expression, this macro shall expand to an integer constant expression.

CMSG_LEN(length)

If the argument has a type such that its value can be assigned to an object of type socklen_t, this macro shall return the value to store in the cmsg_len member of the cmsghdr structure for an ancillary data object of the specified length, taking into account any padding needed to satisfy alignment requirements. If the argument is an integer constant expression, this macro shall expand to an integer constant expression.

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 socket types (see XSH 2.10.6 Socket Types) as symbolic constants with distinct values:

SOCK_DGRAM

Datagram socket.

SOCK_RAW

^[RS] Raw Protocol Interface.

SOCK_SEQPACKET

Sequenced-packet socket.

SOCK_STREAM

Byte-stream socket.

Implementations may provide additional socket types.

The header shall define the following socket creation flags, for use in socket(), socketpair(), and accept4(). These flags shall be symbolic constants with values that are bitwise distinct from each other and from all SOCK_* constants representing socket types:

SOCK_NONBLOCK

Create a socket file descriptor with the O_NONBLOCK flag atomically set on the new open file description.

SOCK_CLOEXEC

Create a socket file descriptor with the FD_CLOEXEC flag atomically set on that file descriptor.

SOCK_CLOFORK

Create a socket file descriptor with the FD_CLOFORK flag atomically set on that file descriptor.

Implementations may provide additional socket creation flags.

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 XSH 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_DOMAIN

Socket domain.

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_PROTOCOL

Socket protocol.

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_CMSG_CLOEXEC

Atomically set the FD_CLOEXEC flag on any file descriptors created via SCM_RIGHTS during recvmsg().

MSG_CMSG_CLOFORK

Atomically set the FD_CLOFORK flag on any file descriptors created via SCM_RIGHTS during recvmsg().

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

^[IP6] 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     accept4(int, struct sockaddr *restrict, socklen_t *restrict,
        int);
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>.

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. Note that this example only deals with size and alignment; see RATIONALE for additional issues related to these structures.

/*
 *  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. */
};

Portable applications need to account for the alternative behaviors of the CMSG_NXTHDR macro as follows:

• When constructing ancillary data in a msghdr structure, ensure that all locations within the ancillary data that might be returned by CMSG_NXTHDR contain a cmsg_len value of zero (typically this is achieved by using memset() to initialize the entire msg_control buffer to null bytes before populating the first cmsghdr structure).

• When extracting ancillary data from a received msghdr structure, check that the data array following the last cmsghdr structure does not extend beyond the end of the ancillary data.

RATIONALE

Note that defining the sockaddr_storage and sockaddr structures using only mechanisms defined in early editions of the ISO C standard may produce aliasing diagnostics when applications use casting between pointers to the various socket address structures. Because of the large body of existing code utilizing sockets in a way that could trigger undefined behavior due to strict aliasing rules, this standard mandates that these structures can alias each other for accessing the sa_family_t member of the structures (or other members for protocol-specific structure references), so as to preserve well-defined semantics. An implementation's header files may need to use anonymous unions, or even an implementation-specific extension, to comply with the requirements of this standard.

FUTURE DIRECTIONS

None.

SEE ALSO

<sys/types.h>, <sys/uio.h>

XSH accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, recvfrom, recvmsg, send, sendmsg, sendto, setsockopt, shutdown, sockatmark, socket, socketpair

CHANGE HISTORY

First released in Issue 6. Derived from the XNS, Issue 5.2 specification.

The restrict keyword is added to the prototypes for accept(), getpeername(), getsockname(), getsockopt(), and recvfrom().

Issue 7

SD5-XBD-ERN-56 is applied, adding a reference to <sys/types.h> for the ssize_t type.

SD5-XBD-ERN-62 is applied.

The MSG_NOSIGNAL symbolic constant is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

This reference page is clarified with respect to macros and symbolic constants, and a declaration for the size_t type is added.

POSIX.1-2008, Technical Corrigendum 1, XBD/TC1-2008/0067 [355] is applied.

POSIX.1-2008, Technical Corrigendum 2, XBD/TC2-2008/0077 [934] is applied.

Issue 8

Austin Group Defects 411 and 1318 are applied, adding SOCK_NONBLOCK, SOCK_CLOEXEC, SOCK_CLOFORK, MSG_CMSG_CLOEXEC, MSG_CMSG_CLOFORK, and accept4().

Austin Group Defect 840 is applied, adding SO_DOMAIN and SO_PROTOCOL.

Austin Group Defect 978 is applied, adding CMSG_SPACE and CMSG_LEN, and clarifying the behavior of CMSG_NXTHDR when the second argument is a null pointer.

Austin Group Defect 1056 is applied, clarifying the conditions under which CMSG_NXTHDR and CMSG_FIRSTHDR return a null pointer, and adding a new paragraph to APPLICATION USAGE.

Austin Group Defect 1641 is applied, clarifying the requirements for conversions between pointers to sockaddr_storage and storage structures and between pointers to those structures and pointers to protocol-specific address structures.