accept(2) - Linux manual page (original) (raw)
accept(2) System Calls Manual accept(2)
NAME top
accept, accept4 - accept a connection on a socketLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/socket.h>**
**int accept(int** _sockfd_**, struct sockaddr *_Nullable restrict** _addr_**,**
**socklen_t *_Nullable restrict** _addrlen_**);**
**#define _GNU_SOURCE** /* See feature_test_macros(7) */
**#include <sys/socket.h>**
**int accept4(int** _sockfd_**, struct sockaddr *_Nullable restrict** _addr_**,**
**socklen_t *_Nullable restrict** _addrlen_**, int** _flags_**);**DESCRIPTION top
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)](../man2/socket.2.html), bound to a local address with [bind(2)](../man2/bind.2.html), and is listening
for connections after a [listen(2)](../man2/listen.2.html).
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)](../man2/socket.2.html) 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)](../man2/select.2.html), [poll(2)](../man2/poll.2.html), or [epoll(7)](../man7/epoll.7.html). 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)](../man7/socket.7.html) 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)](../man2/open.2.html)) referred to by the new file
descriptor. Using this flag saves extra calls to [fcntl(2)](../man2/fcntl.2.html)
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)](../man2/open.2.html) for reasons why this may be useful.RETURN VALUE top
On success, these system calls return a file descriptor for the
accepted socket (a nonnegative integer). On error, -1 is
returned, _[errno](../man3/errno.3.html)_ is set to indicate the error, and _addrlen_ is left
unchanged.Error handling 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.
ERRORS top
**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)](../man7/signal.7.html).
**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**.
**EPERM** Firewall rules forbid connection.
**EPROTO** Protocol error.
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.VERSIONS top
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**().STANDARDS top
**accept**()
POSIX.1-2008.
**accept4**()
Linux.HISTORY top
**accept**()
POSIX.1-2001, SVr4, 4.4BSD (**accept**() first appeared in
4.2BSD).
**accept4**()
Linux 2.6.28, glibc 2.10.NOTES top
There may not always be a connection waiting after a **SIGIO** is
delivered or [select(2)](../man2/select.2.html), [poll(2)](../man2/poll.2.html), or [epoll(7)](../man7/epoll.7.html) 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)](../man7/socket.7.html)).
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.The socklen_t type 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 _sizet *_C; later POSIX standards and glibc 2.x have socklent * .
EXAMPLES top
See [bind(2)](../man2/bind.2.html).SEE ALSO top
[bind(2)](../man2/bind.2.html), [connect(2)](../man2/connect.2.html), [listen(2)](../man2/listen.2.html), [select(2)](../man2/select.2.html), [socket(2)](../man2/socket.2.html), [socket(7)](../man7/socket.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 accept(2)
Pages that refer to this page:bind(2), connect(2), getpeername(2), io_uring_enter2(2), io_uring_enter(2), listen(2), recv(2), seccomp_unotify(2), select(2), select_tut(2), socket(2), socketcall(2), syscalls(2), getaddrinfo(3), gethostbyname(3), getnameinfo(3), io_uring_prep_accept(3), io_uring_prep_accept_direct(3), io_uring_prep_multishot_accept(3), io_uring_prep_multishot_accept_direct(3), sockaddr(3type), capabilities(7), ddp(7), ip(7), sctp(7), signal(7), signal-safety(7), sock_diag(7), socket(7), tcp(7), unix(7)