libc/sys/accept.2

.\"	$OpenBSD: accept.2,v 1.29 2019/05/26 09:47:28 krw Exp $
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.\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: May 26 2019 $
.Dt ACCEPT 2
.Os
.Sh NAME
.Nm accept ,
.Nm accept4
.Nd accept a connection on a socket
.Sh SYNOPSIS
.In sys/socket.h
.Ft int
.Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen"
.Ft int
.Fn accept4 "int s" "struct sockaddr *addr" "socklen_t *addrlen" "int flags"
.Sh DESCRIPTION
The argument
.Fa s
is a socket that has been created with
.Xr socket 2 ,
bound to an address with
.Xr bind 2 ,
and is listening for connections after a
.Xr listen 2 .
The
.Fn accept
call extracts the first connection request on the queue of pending
connections, creates a new socket with the same non-blocking I/O mode as
.Fa s ,
and allocates a new file descriptor for the socket with the
close-on-exec flag clear.
.Pp
The
.Fn accept4
system call is similar, however the non-blocking I/O mode of the
new socket is determined by the
.Dv SOCK_NONBLOCK
flag in the
.Fa flags
argument and the close-on-exec flag on the new file descriptor is
determined by the
.Dv SOCK_CLOEXEC
flag in the
.Fa flags
argument.
.Pp
If no pending connections are present on the queue,
and the socket is not marked as non-blocking,
.Fn 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,
.Fn accept
returns an error as described below.
The accepted socket may not be used to accept more connections.
The original socket
.Fa s
remains open.
.Pp
The argument
.Fa addr
is a result parameter that is filled in with the address of the connecting
entity as known to the communications layer.
The exact format of the
.Fa addr
parameter is determined by the domain in which the communication
is occurring.
The structure
.Li sockaddr_storage
exists for greater portability.
It is large enough to hold any of the types that may be returned in the
.Fa addr
parameter.
.Pp
The
.Fa addrlen
is a value-result parameter; it should initially contain the
amount of space pointed to by
.Fa addr ;
on return it will contain the actual length (in bytes) of the
address returned.
If
.Fa addrlen
does not point to enough space to hold the entire socket address, the
result will be truncated to the initial value of
.Fa addrlen
(in bytes).
This call is used with connection-based socket types, currently with
.Dv SOCK_STREAM .
.Pp
It is possible to
.Xr select 2
or
.Xr poll 2
a socket for the purposes of doing an
.Fn accept
by selecting it for read.
.Sh RETURN VALUES
If successful,
.Fn accept
and
.Fn accept4
return a non-negative integer, the accepted socket file descriptor.
Otherwise, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh EXAMPLES
The following code uses struct
.Li sockaddr_storage
to allocate enough space for the returned address:
.Bd -literal -offset indent
#include <sys/types.h>
#include <sys/socket.h>

struct sockaddr_storage addr;
socklen_t len = sizeof(addr);
int retcode;

retcode = accept(s, (struct sockaddr *)&addr, &len);
if (retcode == -1)
	err(1, "accept");
.Ed
.Sh ERRORS
.Fn accept
and
.Fn accept4
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The descriptor is invalid.
.It Bq Er ENOTSOCK
The descriptor doesn't reference a socket.
.It Bq Er EOPNOTSUPP
The referenced socket is not of type
.Dv SOCK_STREAM .
.It Bq Er EINTR
A signal was caught before a connection arrived.
.It Bq Er EINVAL
The referenced socket is not listening for connections (that is,
.Xr listen 2
has not yet been called).
.It Bq Er EFAULT
The
.Fa addr
or
.Fa addrlen
parameter is not in a valid part of the process address space.
.It Bq Er EWOULDBLOCK
The socket is marked non-blocking and no connections
are present to be accepted.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ECONNABORTED
A connection has been aborted.
.El
.Pp
In addition,
.Fn accept4
will fail if
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa flags
is invalid.
.El
.Sh SEE ALSO
.Xr bind 2 ,
.Xr connect 2 ,
.Xr listen 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr socket 2
.Sh STANDARDS
The
.Fn accept
function conforms to
.St -p1003.1-2008 .
The
.Fn accept4
function is expected to conform to a future revision of that standard.
.Sh HISTORY
The
.Fn accept
system call first appeared in
.Bx 4.1c
and
.Fn accept4
in
.Ox 5.7 .
.Sh CAVEATS
When
.Er EMFILE
or
.Er ENFILE
is returned,
new connections are neither dequeued nor discarded.
Thus considerable care is required in
.Xr select 2
and
.Xr poll 2
loops.