--- name: "accept" section: "2freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/accept/2freebsd" --- # accept(2freebsd) accept a connection on a socket ## NAME **accept**, **accept4** – accept a connection on a socket ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** **#include \** *int*\ **accept**(*int s*, *struct sockaddr \* restrict addr*, *socklen\_t \* restrict addrlen*); *int*\ **accept4**(*int s*, *struct sockaddr \* restrict addr*, *socklen\_t \* restrict addrlen*, *int flags*); ## DESCRIPTION The argument *s* is a socket that has been created with [socket(2)](/socket/2.md), bound to an address with [bind(2)](/bind/2.md), and is listening for connections after a [listen(2)](/listen/2.md). The **accept**() system call extracts the first connection request on the queue of pending connections, creates a new socket, and allocates a new file descriptor for the socket which inherits the state of the O\_NONBLOCK and O\_ASYNC properties and the destination of SIGIO and SIGURG signals from the original socket *s*. The **accept4**() system call is similar, but the O\_NONBLOCK property of the new socket is instead determined by the SOCK\_NONBLOCK flag in the *flags* argument, the O\_ASYNC property is cleared, the signal destination is cleared and the close-on-exec flag on the new file descriptor can be set via the SOCK\_CLOEXEC flag in the *flags* argument. If no pending connections are present on the queue, and the original socket is not marked as non-blocking, **accept**() blocks the caller until a connection is present. If the original 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 argument that is filled-in with the address of the connecting entity, as known to the communications layer. The exact format of the *addr* argument is determined by the domain in which the communication is occurring. A null pointer may be specified for *addr* if the address information is not desired; in this case, *addrlen* is not used and should also be null. Otherwise, the *addrlen* argument is a value-result argument; 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)](/select/2.md) 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 dequeueing 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. For some applications, performance may be enhanced by using an [accept\_filter(9)](/accept_filter/9.md) to pre-process incoming connections. When using **accept**(), portable programs should not rely on the O\_NONBLOCK and O\_ASYNC properties and the signal destination being inherited, but should set them explicitly using [fcntl(2)](/fcntl/2.md); **accept4**() sets these properties consistently, but may not be fully portable across UNIX platforms. ## RETURN VALUES These calls return -1 on error. If they succeed, they return a non-negative integer that is a descriptor for the accepted socket. ## ERRORS The **accept**() and **accept4**() system calls will fail if: - \[EBADF\] The descriptor is invalid. - \[EINTR\] The **accept**() operation was interrupted. - \[EMFILE\] The per-process descriptor table is full. - \[ENFILE\] The system file table is full. - \[ENOTSOCK\] The descriptor references a file, not a socket. - \[EINVAL\] [listen(2)](/listen/2.md) has not been called on the socket descriptor. - \[EFAULT\] The *addr* argument is not in a writable part of the user address space. - \[EWOULDBLOCK\] or \[EAGAIN\] The socket is marked non-blocking and no connections are present to be accepted. - \[ECONNABORTED\] A connection arrived, but it was closed while waiting on the listen queue. The **accept4**() system call will also fail if: - \[EINVAL\] The *flags* argument is invalid. ## SEE ALSO [bind(2)](/bind/2.md), [connect(2)](/connect/2.md), [getpeername(2)](/getpeername/2.md), [getsockname(2)](/getsockname/2.md), [listen(2)](/listen/2.md), [select(2)](/select/2.md), [socket(2)](/socket/2.md), [accept\_filter(9)](/accept_filter/9.md) ## HISTORY The **accept**() system call appeared in 4.2BSD. The **accept4**() system call appeared in FreeBSD 10.0.