accept a connection on a
s, struct sockaddr
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
call extracts the first connection request on the queue of pending
connections, creates a new socket with the same properties 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 original
socket s remains open.
The argument addr 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 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 parameter; 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 returned. If addrlen does not
point to enough space to hold the entire socket address, the result will be
truncated to the initial value of addrlen (in bytes).
This call is used with connection-based socket types, currently with
It is possible to
poll(2) a socket for the purposes of doing an
by selecting it for read.
For certain protocols which require an explicit
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
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 providing only the control information, or by calling setsockopt(2).
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
#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");
accept() will fail if:
- The descriptor is invalid.
- The descriptor doesn't reference a socket.
- The referenced socket is not of type
- A signal was caught before a connection arrived.
- The referenced socket is not listening for connections (that is, listen(2) has not yet been called).
- 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.
- The per-process descriptor table is full.
- The system file table is full.
- A connection has been aborted.
bind(2), connect(2), listen(2), poll(2), select(2), socket(2)
accept() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
accept() system call first appeared in
ENFILE is returned, new connections are neither
dequeued nor discarded. Thus considerable care is required in