MirBSD manpage: accept(2)

ACCEPT(2)                  BSD Programmer's Manual                   ACCEPT(2)


     accept - accept a connection on a socket


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

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


     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 accept() call extracts the first connection request on the
     queue of pending connections, creates a new socket with the same proper-
     ties of s, 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 ori-
     ginal socket s remains open.

     The argument addr is a result parameter that is filled in with the ad-
     dress 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 parame-
     ter; 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 re-
     turned. 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 accept() by selecting it for read.

     For certain protocols which 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.

     One can obtain user connection request data without confirming the con-
     nection 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 providing only the control information, or by calling


     The call returns -1 on error. If it succeeds, it returns a non-negative
     integer that is a descriptor for the accepted socket.


     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");


     The accept() will fail if:

     [EBADF]       The descriptor is invalid.

     [ENOTSOCK]    The descriptor references a file, not a socket.

     [EOPNOTSUPP]  The referenced socket is not of type SOCK_STREAM.

     [EINVAL]      The referenced socket is not listening for connections
                   (that is, listen(2) has not yet been called).

     [EFAULT]      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.

     [EMFILE]      The per-process descriptor table is full.

     [ENFILE]      The system file table is full.

                   A connection has been aborted.


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


     The accept() function appeared in 4.2BSD.

MirBSD #10-current            February 15, 1999                              1

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