Technical Reference: Communications, Volume 2

accept Subroutine

Purpose

Accepts a connection on a socket to create a new socket.

Syntax

#include <sys/socket.h>

int accept ( Socket, Address, AddressLength) int Socket; struct sockaddr *Address; socklen_t *AddressLength;

The accept subroutine extracts the first connection on the queue of pending connections, creates a new socket with the same properties as the specified socket, and allocates a new file descriptor for that socket.

If the listen queue is empty of connection requests, the accept subroutine:

Blocks a calling socket of the blocking type until a connection is present.
Returns an EWOULDBLOCK error code for sockets marked nonblocking.

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

The accept subroutine is used with SOCK_STREAM and SOCK_CONN_DGRAM socket types.

For SOCK_CONN_DGRAM socket type and ATM protocol, a socket is not ready to transmit/receive data until SO_ATM_ACCEPT socket option is called. This allows notification of an incoming connection to the application, followed by modification of appropriate parameters and then indicate that a connection can become fully operational.

Parameters

Socket	Specifies a socket created with the socket subroutine that is bound to an address with the bind subroutine and has issued a successful call to the listen subroutine.
Address	Specifies 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 occurs.
AddressLength	Specifies a parameter that initially contains the amount of space pointed to by the Address parameter. Upon return, the parameter contains the actual length (in bytes) of the address returned. The accept subroutine is used with SOCK_STREAM socket types.

Return Values

Upon successful completion, the accept subroutine returns the nonnegative socket descriptor of the accepted socket.

If the accept subroutine is unsuccessful, the subroutine handler performs the following functions:

Returns a value of -1 to the calling program.
Moves an error code, indicating the specific error, into the ernno global variable.

Error Codes

The accept subroutine is unsuccessful if one or more of the following is true:

EBADF	The Socket parameter is not valid.
ENOTSOCK	The Socket parameter refers to a file, not a socket.
EOPNOTSUPP	The referenced socket is not of type SOCK_STREAM.
EFAULT	The Address parameter is not in a writable part of the user address space.
EWOULDBLOCK	The socket is marked as nonblocking, and no connections are present to be accepted.
ENETDOWN	The network with which the socket is associated is down.
ENOTCONN	The socket is not in the connected state.

Examples

As illustrated in this program fragment, once a socket is marked as listening, a server process can accept a connection:

struct sockaddr_in from;
.
.
.
fromlen = sizeof(from);
newsock = accept(socket, (struct sockaddr*)&from, &fromlen);

Implementation Specifics

The accept subroutine is part of Base Operating System (BOS) Runtime.

The socket applications can be compiled with COMPAT_43 defined. This will make the sockaddr structure BSD 4.3 compatible. For more details refer to socket.h.

Related Information

The connect subroutine, getsockname subroutine, listen subroutine, socket subroutine.

Accepting UNIX Stream Connections Example Program, Binding Names to Sockets, Sockets Overview, Understanding Socket Connections, and Understanding Socket Creation in AIX 5L Version 5.1 Communications Programming Concepts.