ACCEPT(2)

NAME
accept - accept a connection on a socket

SYNOPSIS
#include <sys/types.h>

#include <sys/socket.h>

int accept(int s, struct sockaddr *addr, socklen_t *addrlen);

DESCRIPTION
The argument s is a socket that has been created with socket(2), bound to an address with bind(2), and is listening for connections after a listen(2). The accept function extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of s and allocates a new file descriptor for the socket. If no pending connections are present on the queue, and the socket is not marked as non-blocking, accept blocks the caller until a connection is present. If the socket is marked non-blocking and no pending connections are present on the queue, accept returns an error as described below. The accepted socket may not be used to accept more connections. The original socket s remains open. The argument addr is a result parameter that is filled in with the address of the connecting entity, as known to the communications layer. The exact format of the addr parameter is determined by the domain in which the communication is occurring. The addrlen is a value-result parameter; it should initially contain the amount of space pointed to by addr; on return it will contain the actual length (in bytes) of the address returned. This call is used with connection-based socket types, currently with SOCK_STREAM. It is possible to select(2) a socket for the purposes of doing an accept by selecting it for read. For certain protocols which require an explicit confirmation, such as ISO or DATAKIT, accept can be thought of as merely dequeuing the next connection request and not implying confirmation. Confirmation can be implied by a normal read or write on the new file descriptor, and rejection can be implied by closing the new socket. One can obtain user connection request data without confirming the connection by issuing a recvmsg(2) call with an msg_iovlen of 0 and a non-zero msg_controllen, or by issuing a getsockopt(2) request. Similarly, one can provide user connection rejection information by issuing a sendmsg(2) call with providing only the control information, or by calling setsockopt(2).

RETURN VALUES
The call returns -1 on error. If it succeeds, it returns a non-negative integer that is a descriptor for the accepted socket.

ERRORS
The BSD man page documents five possible error returns.

EBADF
    The descriptor is invalid.

ENOTSOCK
    The descriptor references a file, not a socket.

EOPNOTSUPP
    The referenced socket is not of type SOCK_STREAM.

EFAULT
    The addr parameter is not in a writable part of the user address space.

EWOULDBLOCK
    The socket is marked non-blocking and no connections are present to be accepted.

Various Linux kernels can return various other errors such as EMFILE, EINVAL, ENOSR, ENOBUFS, EAGAIN, EPERM, ECONNABORTED, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT, ERESTARTSYS.

CONFORMING TO
SVr4, 4.4BSD (the accept function first appeared in BSD 4.2). IRIX documents additional errors EMFILE and ENFILE. Solaris documents additional errors EINTR, ENODEV, ENOMEM, ENOSR and EPROTO.

NOTE
The third argument of accept was originally declared as an `int *' (and is that under libc4 and libc5 and on many other systems like BSD 4.*, SunOS 4, SGI); a POSIX 1003.1g draft standard wanted to change it into a `size_t *', and that is what it is for SunOS 5. Later POSIX drafts have `socklen_t *', and so do the Single Unix Specification and glibc2. Quoting Linus Torvalds: `_Any_ sane library _must_ have "socklen_t" be the same size as int. Anything else breaks any BSD socket layer stuff. POSIX initially _did_ make it a size_t, and I (and hopefully others, but obviously not too many) complained to them very loudly indeed. Making it a size_t is completely broken, exactly because size_t very seldom is the same size as "int" on 64-bit architectures, for example. And it _has_ to be the same size as "int" because that's what the BSD socket interface is. Anyway, the POSIX people eventually got a clue, and created "socklen_t". They shouldn't have touched it in the first place, but once they did they felt it had to have a named type for some unfathomable reason (probably somebody didn't like losing face over having done the original stupid thing, so they silently just renamed their blunder).'

SEE ALSO
bind(2), connect(2), listen(2), select(2), socket(2)