accept —
accept
a connection on a socket
Standard C Library (libc, -lc)
#include
<sys/types.h>
#include
<sys/socket.h>
int
accept(
int s,
struct sockaddr *restrict addr,
socklen_t *addrlen);
int
accept4(
int s,
struct sockaddr *restrict addr,
socklen_t *addrlen,
int flags);
The argument
s is a socket that has been
created with
socket(3C) and bound to an address
with
bind(3C), and that is listening for
connections after a call to
listen(3C). The
accept() function extracts the first connection
on the queue of pending connections, creates a new socket with the properties
of
s, and allocates a new file descriptor,
ns, 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 as non-blocking and no pending connections
are present on the queue,
accept() returns an
error as described below. The
accept() function
uses the
netconfig(4) file to determine the
STREAMS device file name associated with
s.
This is the device on which the connect indication will be accepted. The
accepted socket,
ns, is used to read and
write data to and from the socket that connected to
ns. It is not used to accept more
connections. The original socket (
s) remains
open for accepting further connections.
The argument
addr is a result parameter that is
filled in with the address of the connecting entity as it is known to the
communications layer. The exact format of the
addr parameter is determined by the domain in
which the communication occurs.
The argument
addrlen is a value-result
parameter. Initially, it contains the amount of space pointed to by
addr; on return it contains the length in
bytes of the address returned.
The
accept() function is used with connection-based
socket types, currently with
SOCK_STREAM.
The
accept4() function allows flags that control
the behavior of a successfully accepted socket. If
flags is 0,
accept4() acts identically to
accept(). Values for
flags are constructed by a
bitwise-inclusive-OR of flags from the following list, defined in
<sys/socketvar.h>:
-
-
SOCK_CLOEXEC- The accepted socket will have the
FD_CLOEXEC flag set as if
fcntl(2) was called on it. This flag is set
before the socket is passed to the caller thus avoiding the race condition
between accept() and
fcntl(2). See
O_CLOEXEC in
open(2) for more details.
-
-
SOCK_NDELAY- The accepted socket will have the
O_NDELAY flag set as if
fcntl(2) was called on it. This sets the
socket into non-blocking mode. See
O_NDELAY in
fcntl.h(3HEAD) for more details.
-
-
SOCK_NONBLOCK- The accepted socket will have the
O_NONBLOCK flag set as if
fcntl(2) was called on it. This sets the
socket into non-blocking mode (POSIX; see
standards(5)). See
O_NONBLOCK in
fcntl.h(3HEAD) for more details.
It is possible to
select(3C) or
poll(2) a socket for the purpose of an
accept() by selecting or polling it for a read.
However, this will only indicate when a connect indication is pending; it is
still necessary to call
accept().
The
accept() function returns -1 on error. If it
succeeds, it returns a non-negative integer that is a descriptor for the
accepted socket.
accept() and
accept4()
will fail if:
-
-
- [
EBADF]
- The descriptor is invalid.
-
-
- [
ECONNABORTED]
- The remote side aborted the connection before the
accept() operation completed.
-
-
- [
EFAULT]
- The addr parameter or the
addrlen parameter is invalid.
-
-
- [
EINTR]
- The accept() attempt was
interrupted by the delivery of a signal.
-
-
- [
EMFILE]
- The per-process descriptor table is full.
-
-
- [
ENODEV]
- The protocol family and type corresponding to
s could not be found in the
netconfig file.
-
-
- [
ENOMEM]
- There was insufficient user memory available to complete
the operation.
-
-
- [
ENOSR]
- There were insufficient STREAMS resources available to
complete the operation.
-
-
- [
ENOTSOCK]
- The descriptor does not reference a socket.
-
-
- [
EOPNOTSUPP]
- The referenced socket is not of type
SOCK_STREAM.
-
-
- [
EPROTO]
- A protocol error has occurred; for example, the STREAMS
protocol stack has not been initialized or the connection has already been
released.
-
-
- [
EWOULDBLOCK]
- The socket is marked as non-blocking and no connections are
present to be accepted.
Additionally,
accept4() will fail if:
-
-
- [
EINVAL]
- The flags value is
invalid. The flags argument can only be
the bitwise inclusive-OR of
SOCK_CLOEXEC,
SOCK_NONBLOCK, and
SOCK_NDELAY.
Safe
fcntl(2),
poll(2),
bind(3C),
connect(3C),
listen(3C),
select(3C),
sockaddr(3C),
socket(3C),
fcntl.h(3HEAD),
socket.h(3HEAD),
netconfig(4),
attributes(5),
standards(5)