accept, accept4 - accept a connection on a socket
#include <sys/types.h> /* See NOTES */
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
#define _GNU_SOURCE /* See feature_test_macros(7) */
int accept4(int sockfd, struct sockaddr *addr,
socklen_t *addrlen, int flags);
The accept() system call is used with connection-based socket types
(SOCK_STREAM, SOCK_SEQPACKET). It extracts the first connection
request on the queue of pending connections for the listening socket,
sockfd, creates a new connected socket, and returns a new file
descriptor referring to that socket. The newly created socket is not
in the listening state. The original socket sockfd is unaffected by
The argument sockfd is a socket that has been created with socket(2),
bound to a local address with bind(2), and is listening for connections
after a listen(2).
The argument addr is a pointer to a sockaddr structure. This structure
is filled in with the address of the peer socket, as known to the
communications layer. The exact format of the address returned addr is
determined by the socket's address family (see socket(2) and the
respective protocol man pages). When addr is NULL, nothing is filled
in; in this case, addrlen is not used, and should also be NULL.
The addrlen argument is a value-result argument: the caller must
initialize it to contain the size (in bytes) of the structure pointed
to by addr; on return it will contain the actual size of the peer
The returned address is truncated if the buffer provided is too small;
in this case, addrlen will return a value greater than was supplied to
If no pending connections are present on the queue, and the socket is
not marked as nonblocking, accept() blocks the caller until a
connection is present. If the socket is marked nonblocking and no
pending connections are present on the queue, accept() fails with the
error EAGAIN or EWOULDBLOCK.
In order to be notified of incoming connections on a socket, you can
use select(2) or poll(2). A readable event will be delivered when a
new connection is attempted and you may then call accept() to get a
socket for that connection. Alternatively, you can set the socket to
deliver SIGIO when activity occurs on a socket; see socket(7) for
For certain protocols which require an explicit confirmation, such as
DECNet, 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. Currently only
DECNet has these semantics on Linux.
If flags is 0, then accept4() is the same as accept(). The following
values can be bitwise ORed in flags to obtain different behavior:
SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the new open
file description. Using this flag saves extra calls to
fcntl(2) to achieve the same result.
SOCK_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag
in open(2) for reasons why this may be useful.
On success, these system calls return a nonnegative integer that is a
descriptor for the accepted socket. On error, -1 is returned, and
errno is set appropriately.
Linux accept() (and accept4()) passes already-pending network errors on
the new socket as an error code from accept(). This behavior differs
from other BSD socket implementations. For reliable operation the
application should detect the network errors defined for the protocol
after accept() and treat them like EAGAIN by retrying. In the case of
TCP/IP, these are ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET,
EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.
EAGAIN or EWOULDBLOCK
The socket is marked nonblocking and no connections are present
to be accepted. POSIX.1-2001 allows either error to be returned
for this case, and does not require these constants to have the
same value, so a portable application should check for both
EBADF The descriptor is invalid.
A connection has been aborted.
EFAULT The addr argument is not in a writable part of the user address
EINTR The system call was interrupted by a signal that was caught
before a valid connection arrived; see signal(7).
EINVAL Socket is not listening for connections, or addrlen is invalid
(e.g., is negative).
EINVAL (accept4()) invalid value in flags.
EMFILE The per-process limit of open file descriptors has been reached.
ENFILE The system limit on the total number of open files has been
Not enough free memory. This often means that the memory
allocation is limited by the socket buffer limits, not by the
The descriptor references a file, not a socket.
The referenced socket is not of type SOCK_STREAM.
EPROTO Protocol error.
In addition, Linux accept() may fail if:
EPERM Firewall rules forbid connection.
In addition, network errors for the new socket and as defined for the
protocol may be returned. Various Linux kernels can return other
errors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT. The
value ERESTARTSYS may be seen during a trace.
The accept4() system call is available starting with Linux 2.6.28;
support in glibc is available starting with version 2.10.
accept(): POSIX.1-2001, SVr4, 4.4BSD, (accept() first appeared in
accept4() is a nonstandard Linux extension.
On Linux, the new socket returned by accept() does not inherit file
status flags such as O_NONBLOCK and O_ASYNC from the listening socket.
This behavior differs from the canonical BSD sockets implementation.
Portable programs should not rely on inheritance or noninheritance of
file status flags and always explicitly set all required flags on the
socket returned from accept().
POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this
header file is not required on Linux. However, some historical (BSD)
implementations required this header file, and portable applications
are probably wise to include it.
There may not always be a connection waiting after a SIGIO is delivered
or select(2) or poll(2) return a readability event because the
connection might have been removed by an asynchronous network error or
another thread before accept() is called. If this happens, then the
call will block waiting for the next connection to arrive. To ensure
that accept() never blocks, the passed socket sockfd needs to have the
O_NONBLOCK flag set (see socket(7)).
The socklen_t type
The third argument of accept() was originally declared as an int * (and
is that under libc4 and libc5 and on many other systems like 4.x BSD,
SunOS 4, SGI); a POSIX.1g draft standard wanted to change it into a
size_t *, and that is what it is for SunOS 5. Later POSIX drafts have
socklen_t *, and so do the Single UNIX Specification and glibc2.
Quoting Linus Torvalds:
"_Any_ sane library _must_ have "socklen_t" be the same size as int.
Anything else breaks any BSD socket layer stuff. POSIX initially did
make it a size_t, and I (and hopefully others, but obviously not too
many) complained to them very loudly indeed. Making it a size_t is
completely broken, exactly because size_t very seldom is the same size
as "int" on 64-bit architectures, for example. And it has to be the
same size as "int" because that's what the BSD socket interface is.
Anyway, the POSIX people eventually got a clue, and created
"socklen_t". They shouldn't have touched it in the first place, but
once they did they felt it had to have a named type for some
unfathomable reason (probably somebody didn't like losing face over
having done the original stupid thing, so they silently just renamed
bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
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