accept(2) — Linux manual page


accept(2)                  System Calls Manual                 accept(2)

NAME         top

       accept, accept4 - accept a connection on a socket

LIBRARY         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), 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

              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.

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 is set to indicate the error, and addrlen is left

   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

ERRORS         top

              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.

              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.

       ENOMEM 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.

       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
       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

STANDARDS         top



HISTORY         top

              POSIX.1-2001, SVr4, 4.4BSD (accept() first appeared in

              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), 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.

   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 size_t *C;
       later POSIX standards and glibc 2.x have socklen_t * .

EXAMPLES         top

       See bind(2).

SEE ALSO         top

       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                      accept(2)

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