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
created 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 a 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 con
necting 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 connection is present. If the socket
is marked non-blocking and no pending connections are pre
sent 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 con
nection. 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 confirma
tion, 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.
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 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 wait
ing 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-negative 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 reli
able 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, EHOSTUN
REACH, EOPNOTSUPP, and ENETUNREACH.
ERRORS
EAGAIN or EWOULDBLOCK
The socket is marked non-blocking and no connec
tions 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.
EFAULT The addr parameter is not in a writable part of the
user address space.
EPERM Firewall rules forbid connection.
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, but this is not
100% consistent.
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 EMFILE, EINVAL,
ENOSR, ENOBUFS, EPERM, ECONNABORTED, ESOCKTNOSUPPORT,
EPROTONOSUPPORT, ETIMEDOUT, ERESTARTSYS.
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, EOPNOTSUPP, EWOULDBLOCK,
EFAULT). SUSv2 documents errors EAGAIN, EBADF,
ECONNABORTED, EFAULT, EINTR, EINVAL, EMFILE, ENFILE,
ENOBUFS, ENOMEM, ENOSR, ENOTSOCK, EOPNOTSUPP, EPROTO,
EWOULDBLOCK.
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 obvi
ously 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).
SEE ALSO
bind(2), connect(2), listen(2), select(2), socket(2)
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