ACCEPT(2) System Calls Manual ACCEPT(2)

accept — accept a connection on a socket

#include <sys/socket.h>

int
accept(int socket, struct sockaddr *restrict address, socklen_t *restrict address_len);

The argument socket 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). () extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of socket, 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 socket, remains open.

The argument address 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 address parameter is determined by the domain in which the communication is occurring. The address_len is a value-result parameter; it should initially contain the amount of space pointed to by address; 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 () by selecting it for read.

For certain protocols which require an explicit confirmation, such as ISO or DATAKIT, () 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).

The call returns -1 on error and the global variable errno is set to indicate the error. If it succeeds, it returns a non-negative integer that is a descriptor for the accepted socket.

The accept() system call will fail if:

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socket is not a valid file descriptor.
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The connection to socket has been aborted.
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The address parameter is not in a writable part of the user address space.
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The accept() system call was terminated by a signal.
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socket is unwilling to accept connections.
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The per-process descriptor table is full.
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The system file table is full.
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Insufficient memory was available to complete the operation.
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socket references a file type other than a socket.
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socket is not of type SOCK_STREAM and thus does not accept connections.
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socket is marked as non-blocking and no connections are present to be accepted.

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

The include file <sys/types.h> is necessary.

bind(2), connect(2), connectx(2), listen(2), select(2), socket(2), compat(5)

The accept() function appeared in 4.2BSD.

March 18, 2015 BSD 4.2