Ubuntu Manpage: accept, accept4 - accept a connection on a socket

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NAME

       accept, accept4 - accept a connection on a socket

SYNOPSIS

       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sys/socket.h>

       int accept4(int sockfd, struct sockaddr *addr,
                   socklen_t *addrlen, int flags);

DESCRIPTION

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) or
poll(2). 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.

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.

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 new open file description.
Using this flag saves extra calls to fcntl(2) 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) for reasons why this may
be useful.

RETURN VALUE

       On  success,  these system calls return a nonnegative integer that is a descriptor for the
       accepted socket.  On error, -1 is returned, and errno is set appropriately.

   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

EAGAIN or EWOULDBLOCK
The socket is marked nonblocking and no connections are present to be accepted.
POSIX.1-2001 allows either error to be returned for this case, and does not require
these constants to have the same value, so a portable application should check for
both possibilities.

EBADF The descriptor is invalid.

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).

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 of open file descriptors has been reached.

ENFILE The system 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 descriptor references a file, not a socket.

EOPNOTSUPP
The referenced socket is not of type SOCK_STREAM.

EPROTO Protocol error.

In addition, Linux accept() may fail if:

EPERM Firewall rules forbid connection.

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

       The accept4() system call is available starting with Linux 2.6.28;  support  in  glibc  is
       available starting with version 2.10.

CONFORMING TO

       accept(): POSIX.1-2001, SVr4, 4.4BSD, (accept() first appeared in 4.2BSD).

       accept4() is a nonstandard Linux extension.

       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().

NOTES

POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this header file is not
required on Linux. However, some historical (BSD) implementations required this header
file, and portable applications are probably wise to include it.

There may not always be a connection waiting after a SIGIO is delivered or select(2) or
poll(2) 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)).

The socklen_t type
The third argument of accept() was originally declared as an int * (and is that under
libc4 and libc5 and on many other systems like 4.x BSD, SunOS 4, SGI); a POSIX.1g draft
standard wanted to change it into a size_t *, and that is what it is for SunOS 5. Later
POSIX drafts have socklen_t *, and so do the Single UNIX Specification and glibc2.
Quoting Linus Torvalds:

"_Any_ sane library _must_ have "socklen_t" be the same size as int. Anything else breaks
any BSD socket layer stuff. POSIX initially did make it a size_t, and I (and hopefully
others, but obviously not too many) complained to them very loudly indeed. Making it a
size_t is completely broken, exactly because size_t very seldom is the same size as "int"
on 64-bit architectures, for example. And it has to be the same size as "int" because
that's what the BSD socket interface is. Anyway, the POSIX people eventually got a clue,
and created "socklen_t". They shouldn't have touched it in the first place, but once they
did they felt it had to have a named type for some unfathomable reason (probably somebody
didn't like losing face over having done the original stupid thing, so they silently just
renamed their blunder)."

EXAMPLE

       See bind(2).

COLOPHON

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