accept - accept a connection on a socket
int accept(int s, void *addr, int *addrlen);
_XOPEN_SOURCE_EXTENDED only (UNIX 98)
int accept(int s, struct sockaddr *addr, socklen_t *addrlen);
Obsolescent _XOPEN_SOURCE_EXTENDED only (UNIX 95)
int accept(int s, struct sockaddr *addr, size_t *addrlen);
The accept() system call is used with connection-based socket types,
such as SOCK_STREAM. The argument, s, is a socket descriptor created
with socket(), bound to a local address by bind(), and listening for
connections after a listen(). accept() extracts the first connection
on the queue of pending connections, creates a new socket with the
same properties as s, and returns a new file descriptor, ns, for the
If no pending connections are present on the queue and nonblocking
mode has not been enabled with the fcntl() O_NONBLOCK or O_NDELAY
flags or the ioctl() FIOSNBIO request, accept() blocks the caller
until a connection is present. O_NONBLOCK and O_NDELAY are defined in
<<<<sys/fcntl.h>>>> (see fcntl(2), fcntl(5), and socket(7)). FIOSNBIO and
the equivalent request FIONBIO are defined in <<<<sys/ioctl.h>>>>, although
use of FIONBIO is not recommended (see ioctl(2), ioctl(5), and
If the socket has nonblocking mode enabled and no pending connections
are present on the queue, accept() returns an error as described
below. The accepted socket, ns, cannot be used to accept more
connections. The original socket s remains open for incoming
connection requests. To determine whether a listening socket has
pending connection requests ready for an accept() call, use select()
The argument addr should point to a socket address structure. The
accept() call fills in this structure with the address of the
connecting entity, as known to the underlying protocol. In the case
of AF_UNIX sockets, the peer's address is filled in only if the peer
had done an explicit bind() before doing a connect(). Therefore, for
AF_UNIX sockets, in the common case, when the peer had not done an
explicit bind() before doing a connect(), the structure is filled with
a string of nulls for the address. The format of the address depends
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upon the protocol and the address-family of the socket s.
The argument addrlen is a pointer to a variable. Initially, the
variable should contain the size of the structure pointed to by addr.
On return, it contains the actual length (in bytes) of the address
returned. If the memory pointed to by addr is not large enough to
contain the entire address, only the first addrlen bytes of the
address are returned. If addr is NULL or addrlen contains 0, the
connecting entity's address will not be returned.
The fcntl() O_NONBLOCK and O_NDELAY flags and ioctl() FIOSNBIO request
are all supported. These features interact as follows:
+ If the O_NONBLOCK or O_NDELAY flag has been set, accept()
requests behave accordingly, regardless of any FIOSNBIO
+ If neither the O_NONBLOCK flag nor the O_NDELAY flag has been
set, FIOSNBIO requests control the behavior of accept().
The addr parameter to accept() returns addressing information for the
connecting entity, except for the x25ifname field of addr which
contains the name of the local X.25 interface through which the
connection request arrived. Call-acceptance can be controlled with
the ioctl() X25_CALL_ACPT_APPROVAL request (see socketx25(7)).
Upon successful completion, accept() returns a nonnegative integer
which is a descriptor for the accepted socket.
If an error occurs, accept() returns -1 and sets errno to indicate the
If accept() fails, errno is set to one of the following values:
[EAGAIN] Nonblocking I/O is enabled using O_NONBLOCK and no
connections are present to be accepted.
[EBADF] The argument, s, is not a valid file descriptor.
[ECONNABORTED] The socket is being shutdown due to a request by
software. This is usually caused by a shutdown()
[EFAULT] The addr parameter is not a valid pointer.
[EINTR] The call was interrupted by a signal before a
valid connection arrived.
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[EINVAL] The socket referenced by s is not currently a
listen socket or has been shut down with
shutdown(). A listen() must be done before an
accept() is allowed.
[EMFILE] The maximum number of file descriptors for this
process are currently open.
[ENFILE] The system's table of open files is full and no
more accept() calls can be processed at this time.
[ENOBUFS] No buffer space is available. The accept() cannot
complete. The queued socket connect request is
[ENOMEM] No memory is available. The accept() cannot
complete. The queued socket connect request is
[ENOTSOCK] The argument, s, is a valid file descriptor, but
it is not a socket.
[EOPNOTSUPP] The socket referenced by s does not support
[EWOULDBLOCK] Nonblocking I/O is enabled using O_NDELAY or
FIOSNBIO and no connections are present to be
Currently, the socklen_t and size_t types are the same size. This is
compatible with both the UNIX 95 and UNIX 98 profiles. However, in a
future release, socklen_t might be a different size. In that case,
passing a size_t pointer will evoke compile-time warnings, which must
be corrected in order for the application to behave correctly.
Applications that use socklen_t now, where appropriate, will avoid
such migration problems. On the other hand, applications that need to
be portable to the UNIX 95 profile should follow the X/Open
specification (see xopen_networking(7)).
Currently, the default behavior is the HP-UX BSD Sockets; however, it
might be changed to X/Open Sockets in a future release. At that time,
any HP-UX BSD Sockets behavior that is incompatible with X/Open
Sockets might be obsoleted. Applications that conform to the X/Open
specification now will avoid migration problems (see
The accept() system call is thread-safe. It has a cancellation point;
and it is async-cancel safe, async-signal safe, and fork-safe.
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accept() was developed by HP and the University of California,
bind(2), connect(2), listen(2), select(2), socket(2), socketx25(7),
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