ACCEPT

Section: Linux Programmer's Manual (2)
Updated: 2004-06-17
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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 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 connection 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 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 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

On success, accept returns a non-negative integer that is a descriptor 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 ENETDOWN, 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.
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 parameter is not in a writable part of the user address space.
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.
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, 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.

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 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)."