man-pages/man2/accept.2

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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-10-21 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998-2000 by Andi Kleen to match Linux 2.2 reality
.\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
.\" Modified 2004-06-17 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"
.TH ACCEPT 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
accept \- accept a connection on a socket
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/socket.h>
.sp
.BI "int accept(int " s ", struct sockaddr *" addr ", socklen_t *" addrlen );
.SH DESCRIPTION

The
.B accept
function is used with connection-based socket types
.RB ( SOCK_STREAM ,
.B SOCK_SEQPACKET
and
.BR SOCK_RDM ).
It extracts the first connection request on the queue of pending
connections, creates a new connected socket with mostly the same properties as
.IR s ,
and allocates a new file descriptor for the socket, which is returned.
The newly created socket is no longer in the listening state.
The original socket
.I s
is unaffected by this call. Note that any per file descriptor flags 
(everything that can be set with the 
.B F_SETFL
fcntl, like non blocking or async state) are not inherited across
an
.IR accept .
.PP
The argument
.I s
is a socket that has been created with
.BR socket (2),
bound to a local address with
.BR bind (2),
and is listening for connections after a
.BR listen (2).

The argument
.I addr
is a pointer to a sockaddr structure. This structure is filled in
with the address of the connecting entity,
as known to the communications layer.  The exact format of the
address passed in the
.I addr
parameter is determined by the socket's family (see
.BR socket (2) 
and the respective protocol man pages).
The
.I addrlen
argument is a value-result parameter: it should initially contain the
size of the structure pointed to by
.IR addr ;
on return it will contain the actual length (in bytes) of the address
returned. When 
.I addr
is NULL nothing is filled in.
.PP
If no pending
connections are present on the queue, and the socket is not marked as
non-blocking,
.B 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,
.B accept
returns EAGAIN. 
.PP
In order to be notified of incoming connections on a socket, you can use
.BR select (2)
or
.BR poll (2).
A readable event will be delivered when a new connection is attempted and you
may then call
.B accept
to get a socket for that connection.  Alternatively, you can set the socket
to deliver
.B SIGIO
when activity occurs on a socket; see
.BR socket (7)
for details.
.PP
For certain protocols which require an explicit confirmation,
such as
DECNet,
.B 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. 
.SH NOTES
There may not always be a connection waiting after a
.B SIGIO
is delivered or
.BR select (2)
or
.BR poll (2)
return a readability event because the connection might have been
removed by an asynchronous network error or another thread before
.B accept
is called.
If this happens then the call will block waiting for the next
connection to arrive.
To ensure that
.B accept
never blocks, the passed socket
.I s
needs to have the
.B O_NONBLOCK
flag set (see
.BR socket (7)).
.SH "RETURN VALUE"
On success,
.B accept
returns a non-negative integer that is a descriptor
for the accepted socket.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH "ERROR HANDLING"
Linux 
.B accept 
passes already-pending network errors on the new socket 
as an error code from 
.BR accept . 
This behaviour differs from other BSD socket
implementations. For reliable operation the application should detect
the network errors defined for the protocol after 
.B accept 
and treat
them like 
.BR EAGAIN
by retrying. In case of TCP/IP these are
.BR ENETDOWN ,
.BR EPROTO ,
.BR ENOPROTOOPT ,
.BR EHOSTDOWN ,
.BR ENONET ,
.BR EHOSTUNREACH ,
.BR EOPNOTSUPP ,
and
.BR ENETUNREACH .
.SH ERRORS
.B accept
shall fail if:
.TP
.BR EAGAIN " or " EWOULDBLOCK
The socket is marked non-blocking and no connections are
present to be accepted.
.TP
.B EBADF
The descriptor is invalid.
.TP
.B ECONNABORTED
A connection has been aborted.
.TP
.B EINTR
The system call was interrupted by a signal that was caught
before a valid connection arrived. 
.TP
.B EINVAL
Socket is not listening for connections.
.TP
.B EMFILE
The per-process limit of open file descriptors has been reached.
.TP
.B ENFILE
The system limit on the total number of open files has been reached.
.TP
.B ENOTSOCK
The descriptor references a file, not a socket.
.TP
.B EOPNOTSUPP
The referenced socket is not of type
.BR SOCK_STREAM . 
.PP
.B accept
may fail if:
.TP
.B EFAULT
The
.I addr
parameter is not in a writable part of the user address space.
.TP
.B 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.
.TP
.B EPROTO
Protocol error.
.PP
Linux
.B accept
may fail if:
.TP
.B EPERM
Firewall rules forbid connection.
.PP
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
.BR ENOSR ,
.BR ESOCKTNOSUPPORT ,
.BR EPROTONOSUPPORT ,
.BR ETIMEDOUT .
The value
.B ERESTARTSYS
may be seen during a trace.
.SH "CONFORMING TO"
SVr4, 4.4BSD (the
.B accept
function first appeared in BSD 4.2).
The BSD man page documents five possible error returns
(EBADF, ENOTSOCK, EOPNOTSUPP, EWOULDBLOCK, EFAULT).
SUSv3 documents errors EAGAIN, EBADF, ECONNABORTED, EINTR, EINVAL, EMFILE,
ENFILE, ENOBUFS, ENOMEM, ENOTSOCK, EOPNOTSUPP, EPROTO, EWOULDBLOCK. In
addition, SUSv2 documents EFAULT and ENOSR.
.LP
Linux accept does _not_ inherit socket flags like
.BR O_NONBLOCK .
This behaviour differs from other BSD socket implementations.
Portable programs should not rely on this behaviour and always set
all required flags on the socket returned from accept.
.SH NOTE
The third argument of
.B accept
was originally declared as an `int *' (and is that under libc4 and libc5
and on many other systems like BSD 4.*, SunOS 4, SGI); a POSIX 1003.1g 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:

.\" .I fails: only italicizes a single line
"_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)."

.SH "SEE ALSO"
.BR bind (2),
.BR connect (2),
.BR listen (2),
.BR select (2),
.BR socket (2)