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man pages section 3: Basic Library Functions

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Updated: Thursday, June 13, 2019
 
 

accept4 (3C)

Name

accept, accept4 - accept a new connection on a socket

Synopsis

#include <sys/socket.h>

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

int accept4(int 
socket, struct sockaddr *restrict 
address,
     socklen_t *restrict address_len, int flags);

Description

The accept() function extracts the first connection on the queue of pending connections, creates a new socket with the same socket type protocol and address family as the specified socket, and allocates a new file descriptor for that socket.

The function takes the following arguments:

socket

Specifies a socket that was created with socket(3C), has been bound to an address with bind(3C), and has issued a successful call to listen(3C).

address

Either a null pointer, or a pointer to a sockaddr structure where the address of the connecting socket will be returned.

address_len

Points to a socklen_t which on input specifies the length of the supplied sockaddr structure, and on output specifies the length of the stored address.

If address is not a null pointer, the address of the peer for the accepted connection is stored in the sockaddr structure pointed to by address, and the length of this address is stored in the object pointed to by address_len.

If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address will be truncated.

If the protocol permits connections by unbound clients, and the peer is not bound, then the value stored in the object pointed to by address is unspecified.

If the listen queue is empty of connection requests and O_NONBLOCK is not set on the file descriptor for the socket, accept() and accept4() will block until a connection is present. If the listen(3C) queue is empty of connection requests and O_NONBLOCK is set on the file descriptor for the socket, accept() and accept4() will fail and set errno to EAGAIN or EWOULDBLOCK.

The accepted socket cannot itself accept more connections. The original socket remains open and can accept more connections.

The accept4() function behaves like accept() but the accepting socket does not inherit NDELAY and NONBLOCKING from the original socket, instead it will take the settings from the flags argument:

SOCK_NDELAY

As if O_NDELAY is set for the socket.

SOCK_NONBLOCK

As if O_NONBLOCK is set for the socket.

SOCK_NOSIGPIPE

No SIGPIPE will be sent on write when the remote connection was closed; instead EPIPE is returned.

SOCK_CLOEXEC

The socket will be closed on exec() or spawn().

SOCK_CLOFORK

The socket will not be inherited by the child on fork() or spawn().

If both SOCK_NDELAY and SOCK_NONBLOCK are set, the socket will behave as if only SOCK_NONBLOCK is set.

Usage

When a connection is available, select(3C) will indicate that the file descriptor for the socket is ready for reading.

Return Values

Upon successful completion, accept() and accept4() return the nonnegative file descriptor of the accepted socket. Otherwise, −1 is returned and errno is set to indicate the error.

Errors

The accept() and accept4() functions will fail if:

EAGAIN
EWOULDBLOCK

O_NONBLOCK is set for the socket file descriptor and no connections are present to be accepted.

EBADF

The socket argument is not a valid file descriptor.

ECONNABORTED

A connection has been aborted.

EFAULT

The address or address_len parameter can not be accessed or written.

EINTR

The accept() function was interrupted by a signal that was caught before a valid connection arrived.

EINVAL

The socket is not accepting connections.

EMFILE

OPEN_MAX file descriptors are currently open in the calling process.

ENFILE

The maximum number of file descriptors in the system are already open.

ENOTSOCK

The socket argument does not refer to a socket.

EOPNOTSUPP

The socket type of the specified socket does not support accepting connections.

The accept() and accept4() functions may fail if:

ENOBUFS

No buffer space is available.

ENOMEM

There was insufficient memory available to complete the operation.

ENOSR

There was insufficient STREAMS resources available to complete the operation.

EPROTO

A protocol error has occurred; for example, the STREAMS protocol stack has not been initialized.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Async-Signal-Safe
Standard

See Also

bind(3C), connect(3C), listen(3C), socket(3C), attributes(7), standards(7)