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ACCEPT(2) System Calls Manual ACCEPT(2)

accept, accept4accept a connection on a socket

#include <sys/socket.h>

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

accept4(int s, struct sockaddr *addr, socklen_t *addrlen, int flags);

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 () call extracts the first connection request on the queue of pending connections, creates a new socket with the same non-blocking I/O mode as s, and allocates a new file descriptor for the socket with the close-on-exec flag clear.

The () system call is similar, however the non-blocking I/O mode of the new socket is determined by the SOCK_NONBLOCK flag in the flags argument and the close-on-exec flag on the new file descriptor is determined by the SOCK_CLOEXEC flag in the flags argument.

If no pending connections are present on the queue, and the socket is not marked as non-blocking, () 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 structure sockaddr_storage exists for greater portability. It is large enough to hold any of the types that may be returned in the addr parameter.

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. If addrlen does not point to enough space to hold the entire socket address, the result will be truncated to the initial value of addrlen (in bytes). This call is used with connection-based socket types, currently with SOCK_STREAM.

It is possible to select(2) or poll(2) a socket for the purposes of doing an () by selecting it for read.

If successful, accept() and accept4() return a non-negative integer, the accepted socket file descriptor. Otherwise, a value of -1 is returned and errno is set to indicate the error.

The following code uses struct sockaddr_storage to allocate enough space for the returned address:

#include <sys/types.h>
#include <sys/socket.h>

struct sockaddr_storage addr;
socklen_t len = sizeof(addr);
int retcode;

retcode = accept(s, (struct sockaddr *)&addr, &len);
if (retcode == -1)
	err(1, "accept");

accept() and accept4() will fail if:

The descriptor is invalid.
The descriptor doesn't reference a socket.
The referenced socket is not of type SOCK_STREAM.
A signal was caught before a connection arrived.
The referenced socket is not listening for connections (that is, listen(2) has not yet been called).
The addr or addrlen parameter is not in a valid part of the process address space.
The socket is marked non-blocking and no connections are present to be accepted.
The per-process descriptor table is full.
The system file table is full.
A connection has been aborted.

In addition, accept4() will fail if

flags is invalid.

bind(2), connect(2), listen(2), poll(2), select(2), socket(2)

The accept() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”). The accept4() function is expected to conform to a future revision of that standard.

The accept() system call first appeared in 4.1cBSD and accept4() in OpenBSD 5.7.

When EMFILE or ENFILE is returned, new connections are neither dequeued nor discarded. Thus considerable care is required in select(2) and poll(2) loops.

September 11, 2022 OpenBSD-7.3