accept, accept4 - accept a connection on a socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/socket.h>
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 this call.
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 address.
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 the
call.
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),
poll(2), or
epoll(7). 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 details.
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 open file description
(see open(2)) referred to by the new file descriptor. 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 file
descriptor for the accepted socket. On error, -1 is returned,
errno is
set appropriately, and
addrlen is left unchanged.
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 and POSIX.1-2008 allow either error to be returned
for this case, and do not require these constants to have the same value,
so a portable application should check for both possibilities.
- EBADF
- sockfd is not an open file descriptor.
- ECONNABORTED
- A connection has been aborted.
- EFAULT
- The addr argument is not in a writable part of the user address
space.
- 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 on the number of open file descriptors has been
reached.
- ENFILE
- The system-wide limit on the total number of open files has been
reached.
- ENOBUFS, ENOMEM
- Not enough free memory. This often means that the memory allocation is
limited by the socket buffer limits, not by the system memory.
- ENOTSOCK
- The file descriptor sockfd does not refer to a socket.
- EOPNOTSUPP
- 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, POSIX.1-2008, SVr4, 4.4BSD (
accept() first
appeared in 4.2BSD).
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),
poll(2), or
epoll(7) 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)).
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.
In the original BSD sockets implementation (and on other older systems) the
third argument of
accept() was declared as an
int *. A
POSIX.1g draft standard wanted to change it into a
size_t *C;
later POSIX standards and glibc 2.x have
socklen_t * .
See
bind(2).
bind(2),
connect(2),
listen(2),
select(2),
socket(2),
socket(7)