NAME

sys/socket.h - Main Sockets Header

SYNOPSIS

#include <sys/socket.h>

DESCRIPTION

<sys/socket.h> makes available a type, socklen_t, which is an opaque integral type of length of at least 32 bits¹.

The <sys/socket.h> header defines the unsigned integral type sa_family_t.

The <sys/socket.h> header defines the sockaddr structure that includes at least the following members:

sa_family_t    sa_family      address family
char           sa_data[]      socket address (variable-length data)

The <sys/socket.h> header defines the sockaddr_storage structure. This structure must 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 contains field ss_family which is of type sa_family_t. When a sockaddr_storage structure is cast as a sockaddr structure, the ss_family field of the sockaddr_storage structure maps onto the sa_familyfieldofthe sockaddr structure. When a sockaddr_storage structure is cast as a protocol-specific address structure, the ss_family field maps onto a field of that structure that is of type sa_family_t and that identifies the protocol's address family.

Note:

Like all notes in this document, this note is non-normative. The sockaddr_storage structure solves the problem of declaring storage for automatic variables which is large enough and aligned enough for storing 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.

An example implementation design of such a data structure would be as follows:

/* 
 * Desired design of maximum size and alignment
 */
#define _SS_MAXSIZE 128 /* Implementation specific max size */ 
#define _SS_ALIGNSIZE (sizeof (int64_t)) 
               /* Implementation specific 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 specific */
    char _ss_pad1[_SS_PAD1SIZE];
              /* 6 byte pad, this is to make implementation */
              /* specific 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 */
}; 

The above example implementation illustrates a data structure which will align on a 64-bit boundary. An implementation-specific field "_ss_align" along "_ss_pad1" is used to force a 64-bit alignment which covers proper alignment good enough for needs of sockaddr_in6 (IPv6), sockaddr_in (IPv4) address data structures. The size of padding fields _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 specific (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 specific definitions and structure field names above start with an underscore to denote implementation private namespace. Portable code is not expected to access or reference those fields or constants.

The <sys/socket.h> header defines the msghdr structure that includes 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 iovec structure is defined through typedef as described in <sys/uio.h>.

The <sys/socket.h> header defines the cmsghdr structure that includes 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

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 will be legal values for the level argument to the getsockopt() and setsockopt() functions. The system documentation should specify the cmsg_type definitions for the supported protocols.

Ancillary data is also possible at the socket level. The <sys/socket.h> header defines the following macro 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 defines 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 returns 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 returns 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 returns 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 defines the linger structure that includes 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 defines the following macros, with distinct integral values:

SOCK_DGRAM

Datagram socket

SOCK_STREAM

Byte-stream socket

SOCK_SEQPACKET

Sequenced-packet socket

The <sys/socket.h> header defines the following macro 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 defines the following macros, with distinct integral values, for use as the option_name argument in getsockopt() or setsockopt() calls:

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 defines the following macros, with distinct integral values, for use as the valid values for the msg_flags field in the msghdr structure, or the flags parameter in recvfrom(), recvmsg(), sendto() or sendmsg() 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_PEEK

Leave received data in queue.

MSG_TRUNC

Normal data truncated.

MSG_WAITALL

Wait for complete message.

The <sys/socket.h> header defines the following macros, with distinct integral values:

AF_UNIX

UNIX domain sockets

AF_UNSPEC

Unspecified

AF_INET

Internet domain sockets for use with IPv4 addresses |

AF_INET6

Internet domain sockets for use with IPv6 addresses

The <sys/socket.h> header defines the following macros, with distinct integral values:

SHUT_RD

Disables further receive operations.

SHUT_WR

Disables further send operations.

SHUT_RDWR

Disables further send and receive operations.

The following are declared as functions, and may also be defined as macros:

int     accept(int socket, struct sockaddr *address,
             socklen_t *address_len);
int     bind(int socket, const struct sockaddr *address,
             socklen_t address_len);
int     connect(int socket, const struct sockaddr *address,
             socklen_t address_len);
int     getpeername(int socket, struct sockaddr *address,
             socklen_t *address_len);
int     getsockname(int socket, struct sockaddr *address,
             socklen_t *address_len);
int     getsockopt(int socket, int level, int option_name,
             void *option_value, socklen_t *option_len);
int     listen(int socket, int backlog);
ssize_t recv(int socket, void *buffer, size_t length, int flags);
ssize_t recvfrom(int socket, void *buffer, size_t length,
             int flags, struct sockaddr *address, socklen_t *address_len);
ssize_t recvmsg(int socket, struct msghdr *message, int flags);
ssize_t send(int socket, const void *message, size_t length, int flags);
ssize_t sendmsg(int socket, const struct msghdr *message, int flags);
ssize_t sendto(int socket, const void *message, size_t length, int flags,
             const struct sockaddr *dest_addr, socklen_t dest_len);
int     setsockopt(int socket, int level, int option_name,
             const void *option_value, socklen_t option_len);
int     shutdown(int socket, int how);
int     socket(int domain, int type, int protocol);
int     socketpair(int domain, int type, int protocol,
             int socket_vector[2]);

Inclusion of <sys/socket.h> may also make visible all symbols from <sys/uio.h>.

SEE ALSO

accept(), bind(), connect(), getpeername(), getsockname(), getsockopt(), listen(), recv(), recvfrom(), recvmsg(), send(), sendmsg(), sendto(), setsockopt(), shutdown(), socket(), socketpair(), <sys/uio.h>.

CHANGE HISTORY

First released in Issue 4.

Footnotes

1.

To forestall portability problems, it is recommended that applications should not use values larger than 2³² - 1.