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ACCEPT
Section: Linux Programmer's Manual (2)
Updated: 2004-06-17
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NAME
accept - 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);
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).
The
addrlen
argument is a value-result argument: it should initially contain the
size of the structure pointed to by
addr;
on return it will contain the actual length (in bytes) of the address
returned.
When
addr
is NULL nothing is filled in.
If no pending
connections are present on the queue, and the socket is not marked as
non-blocking,
accept()
blocks the caller until a connection is present.
If the socket is marked
non-blocking and no pending connections are present on the queue,
accept()
fails with the error
EAGAIN.
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.
RETURN VALUE
On success,
accept()
returns a non-negative integer that is a descriptor
for the accepted socket.
On error, -1 is returned, and
errno
is set appropriately.
Error Handling
Linux
accept()
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 case of TCP/IP these are
ENETDOWN,
EPROTO,
ENOPROTOOPT,
EHOSTDOWN,
ENONET,
EHOSTUNREACH,
EOPNOTSUPP,
and
ENETUNREACH.
ERRORS
accept()
shall fail if:
- EAGAIN or EWOULDBLOCK
-
The socket is marked non-blocking and no connections are
present to be accepted.
- EBADF
-
The descriptor is invalid.
- ECONNABORTED
-
A connection has been aborted.
- 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).
- 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.
- ENOTSOCK
-
The descriptor references a file, not a socket.
- EOPNOTSUPP
-
The referenced socket is not of type
SOCK_STREAM.
accept()
may fail if:
- EFAULT
-
The
addr
argument is not in a writable part of the user address space.
- 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.
- EPROTO
-
Protocol error.
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.
CONFORMING TO
SVr4, 4.4BSD,
(accept()
first appeared in 4.2BSD), POSIX.1-2001.
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 non-inheritance
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).
SEE ALSO
bind(2),
connect(2),
listen(2),
select(2),
socket(2)
COLOPHON
This page is part of release 3.05 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- Error Handling
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- The socklen_t type
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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