accept
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 s, struct sockaddr *addr, socklen_t *addrlen);
DESCRIPTION
The accept function is used with connection-based socket types
(SOCK_STREAM, SOCK_SEQPACKET and 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 s, and allocates a
new file descriptor for the socket, which is returned. The newly cre-
ated socket is no longer in the listening state. The original socket
s is unaffected by this call. Note that any per file descriptor flags
(everything that can be set with the F_SETFL fcntl, like non blocking
or async state) are not inherited across an accept.
The argument s 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 connecting entity, as known to
the communications layer. The exact format of the address passed in
the addr parameter is determined by the socket’s family (see socket(2)
and the respective protocol man pages). The addrlen argument is a
value-result parameter: 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 returns 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
DECNet has these semantics on Linux.
NOTES
There may not always be a connection waiting after a SIGIO is deliv-
ered 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 s needs to have the O_NONBLOCK
flag set (see socket(7)).
RETURN VALUE
The call returns -1 on error. If it succeeds, it returns a non-nega-
tive integer that is a descriptor for the accepted socket.
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, EOPNOT-
SUPP, 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.
ENOTSOCK
The descriptor references a file, not a socket.
EOPNOTSUPP
The referenced socket is not of type SOCK_STREAM.
EINTR The system call was interrupted by a signal that was caught
before a valid connection arrived.
ECONNABORTED
A connection has been aborted.
EINVAL Socket is not listening for connections.
EMFILE The per-process limit of open file descriptors has been
reached.
ENFILE The system maximum for file descriptors has been reached.
accept may fail if:
EFAULT The addr parameter 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 sys-
tem 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 (the accept function first appeared in BSD 4.2). The BSD
man page documents five possible error returns (EBADF, ENOTSOCK, EOP-
NOTSUPP, EWOULDBLOCK, EFAULT). SUSv3 documents errors EAGAIN, EBADF,
ECONNABORTED, EINTR, EINVAL, EMFILE, ENFILE, ENOBUFS, ENOMEM, ENOT-
SOCK, EOPNOTSUPP, EPROTO, EWOULDBLOCK. In addition, SUSv2 documents
EFAULT and ENOSR.
Linux accept does _not_ inherit socket flags like 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.
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 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: _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 archi-
tectures, 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).
SEE ALSO
bind(2), connect(2), listen(2), select(2), socket(2)
Linux 2.2 Page 2002-04-23 ACCEPT(2)
