accept, accept4 - accept a connection on a socket
Standard C library (libc, -lc)
#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);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:
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.
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 file descriptor for the
accepted socket (a nonnegative integer). On error, -1 is returned,
errno is set to indicate the error, 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.
See bind(2).
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.
sockfd is not an open file descriptor.
A connection has been aborted.
The addr argument is not in a writable part of the user
address space.
The system call was interrupted by a signal that was caught before a valid connection arrived; see signal(7).
Socket is not listening for connections, or addrlen is
invalid (e.g., is negative).
(accept4()) invalid value in flags.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory.
The file descriptor sockfd does not refer to a socket.
The referenced socket is not of type SOCK_STREAM.
Firewall rules forbid connection.
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.
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().
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 * .