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@(#)accept.2 8.2 (Berkeley) 12/11/93
.Dd March 19, 2012 .Dt ACCEPT 2 .Os .Sh NAME .Nm accept , .Nm paccept .Nd accept a connection on a socket .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/socket.h .Ft int .Fn accept "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" .Ft int .Fn paccept "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" "const sigset_t * restrict sigmask" "int flags" .Sh DESCRIPTION The argument .Fa s is a socket that has been created with .Xr socket 2 , bound to an address with .Xr bind 2 , and is listening for connections after a .Xr listen 2 . The .Fn accept argument extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of .Fa 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, .Fn 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, .Fn accept returns an error as described below. The accepted socket may not be used to accept more connections. The original socket .Fa s remains open.
p The argument .Fa 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 .Fa addr parameter is determined by the domain in which the communication is occurring. The .Fa addrlen is a value-result parameter; it should initially contain the amount of space pointed to by .Fa 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 .Dv SOCK_STREAM .
p It is possible to .Xr select 2 or .Xr poll 2 a socket for the purposes of doing an .Fn accept by selecting or polling it for read.
p For certain protocols which require an explicit confirmation, such as .Tn ISO or .Tn DATAKIT , .Fn 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.
p One can obtain user connection request data without confirming the connection by issuing a .Xr recvmsg 2 call with an .Fa msg_iovlen of 0 and a non-zero .Fa msg_controllen , or by issuing a .Xr getsockopt 2 request. Similarly, one can provide user connection rejection information by issuing a .Xr sendmsg 2 call with providing only the control information, or by calling .Xr setsockopt 2 .
p The .Fn paccept function behaves exactly like .Fn accept , but it also allows to set the following .Fa flags on the returned file descriptor: l -tag -width SOCK_NOSIGPIPEXX -offset indent t Dv SOCK_CLOEXEC Set the close on exec property. t Dv SOCK_NONBLOCK Sets non-blocking I/O. t Dv SOCK_NOSIGPIPE Return .Er EPIPE instead of raising .Dv SIGPIPE . .El
p It can also temporarily replace the signal mask of the calling thread if .Fa sigmask is a
f non- Dv NULL pointer, then the .Fn paccept function shall replace the signal mask of the caller by the set of signals pointed to by .Fa sigmask before waiting for a connection, and shall restore the signal mask of the calling thread before returning. .Sh RETURN VALUES The .Fn accept and .Fn paccept calls return -1 on error. If they succeed, they return a non-negative integer that is a descriptor for the accepted socket. .Sh COMPATIBILITY The .Fn accept implementation makes the new file descriptor inherit file flags (like .Dv O_NONBLOCK ) from the listening socket. It's a traditional behaviour for BSD derivative systems. On the other hand, there are implementations which don't do so. Linux is an example of such implementations. Portable programs should not rely on either of the behaviours. .Sh ERRORS The .Fn accept will fail if: l -tag -width Er t Bq Er EAGAIN The socket is marked non-blocking and no connections are present to be accepted. t Bq Er EBADF The descriptor is invalid. t Bq Er ECONNABORTED A connection has been aborted. t Bq Er EFAULT The .Fa addr parameter is not in a writable part of the user address space. t Bq Er EINTR The .Fn accept call has been interrupted by a signal. t Bq Er EINVAL The socket has not been set up to accept connections (using .Xr bind 2 and .Xr listen 2 ) . t Bq Er EMFILE The per-process descriptor table is full. t Bq Er ENFILE The system file table is full. t Bq Er ENOTSOCK The descriptor references a file, not a socket. t Bq Er EOPNOTSUPP The referenced socket is not of type .Dv SOCK_STREAM . .El .Sh SEE ALSO .Xr bind 2 , .Xr connect 2 , .Xr listen 2 , .Xr poll 2 , .Xr select 2 , .Xr socket 2 .Sh HISTORY The .Fn accept function appeared in x 4.2 . The .Fn paccept function is inspired from Linux and appeared in .Nx 6.0 .