accept - accept a connection on a socket


#include <sys/types.h>
#include <sys/socket.h>

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);


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 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 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.


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)).


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.


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.


accept() shall fail if:
The socket is marked non-blocking and no connections are present to be accepted.
The descriptor is invalid.
A connection has been aborted.
The system call was interrupted by a signal that was caught before a valid connection arrived.
Socket is not listening for connections, or addrlen is invalid (e.g., is negative).
The per-process limit of open file descriptors has been reached.
The system limit on the total number of open files has been reached.
The descriptor references a file, not a socket.
The referenced socket is not of type SOCK_STREAM.

accept() may fail if:

The addr argument is not in a writable part of the user address space.
Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory.
Protocol error.

Linux accept() may fail if:

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.


SVr4, 4.4BSD, (accept() first appeared in 4.2BSD), POSIX.1-2001.

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().


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. 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)."


bind(2), connect(2), listen(2), select(2), socket(2)