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ACCEPT(2) Linux Programmer's Manual ACCEPT(2)
NAME
accept - accept a connection on a socket
SYNOPSIS
#include <sys/types.h>
#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 and SOCK_RDM). It extracts the first con-
nection request on the queue of pending connections, creates a new con-
nected 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 com-
munications 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 connec-
tion 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 connec-
tion 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 DEC-
Net has these semantics on Linux.
NOTES
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 connec-
tion 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)).
RETURN VALUE
On success, accept() returns a non-negative integer that is a descrip-
tor 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 behaviour 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 ENET
DOWN, 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.
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 allo-
cation 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). 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.
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 behaviour 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().
NOTE
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 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:
"_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 unfath-
omable reason (probably somebody didn't like losing face over having
done the original stupid thing, so they silently just renamed their
blunder)."
SEE ALSO
bind(2), connect(2), listen(2), select(2), socket(2)
Linux 2.6.7 2004-06-17 ACCEPT(2)
Man(1) output converted with
man2html and wrapped by fishsponge
This page was generated on Tue Feb 13 02:17:37 GMT 2007
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