Accept a connection on a socket


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

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


A socket that's been created with socket().
A result parameter that's 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 connection was made.
A value-result parameter. It should initially contain the amount of space pointed to by addr; on return it contains the actual length (in bytes) of the address returned. This call is used with connection-based socket types, currently with SOCK_STREAM.



Use the -l socket option to qcc to link against this library.


The accept() function:

  1. Extracts the first connection request on the queue of pending connections.
  2. Creates a new socket with the same properties of s, where s is a socket that's been created with socket(), bound to an address with bind(), and is listening for connections after a listen().
  3. Allocates a new file descriptor for the socket.

If no pending connections are present on the queue, and the socket isn't marked as nonblocking, accept() blocks the caller until a connection is present. If the socket is marked as nonblocking 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.

If you do a select() for read on an unconnected socket (on which a listen() has been done), the select() indicates when a connect request has occurred. In this way, an accept() can be made that won't block. For more information, see select().

For certain protocols that require an explicit confirmation, 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.

You can obtain user-connection request data without confirming the connection by:

Similarly, you can provide user-connection rejection information by issuing a sendmsg() call with only the control information, or by calling setsockopt().


A descriptor for the accepted socket, or -1 if an error occurs (errno is set).


Insufficient resources to create the new socket.
Invalid descriptor s.
The accept() was terminated under software control.
The addr parameter isn't in a writable part of the user address space.
You called accept() on a socket that you hadn't called listen() on.
All file descriptors available to the process are currently open.
The referenced socket isn't a SOCK_STREAM socket.
Can't find the socket manager.
The socket is marked nonblocking and no connections are present to be accepted.


POSIX 1003.1

Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes