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accept() - Unix, Linux System Call


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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). It extracts the first connection request on the queue of pending connections, creates a new connected 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 communications layer. The exact format of the address returnedaddr 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 connection 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 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 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 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:

TagDescription
EAGAINorEWOULDBLOCK 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 typeSOCK_STREAM.

accept() may fail if:

TagDescription
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 allocation is limited by the socket bufferlimits, not by the system memory.
EPROTO Protocol error.

Linux accept() may fail if:

TagDescription
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).

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.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.

SEE ALSO



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