Accept a connection on a socket
#include <sys/types.h> /* See NOTES */ #include <sys/socket.h> int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen); #define _GNU_SOURCE /* See feature_test_macros(7) */ #include <sys/socket.h> int accept4(int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags);
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 for the listening socket, sockfd, 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). When addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL.
The addrlen argument is a value-result argument: the caller must initialize it to contain the size (in bytes) of the structure pointed to by addr; on return it will contain the actual size of the peer address.
The returned address is truncated if the buffer provided is too small; in this case, addrlen will return a value greater than was supplied to the call.
If no pending connections are present on the queue, and the socket is not marked as nonblocking, accept() blocks the caller until a connection is present. If the socket is marked nonblocking and no pending connections are present on the queue, accept() fails with the error EAGAIN or EWOULDBLOCK.
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.
If flags is 0, then accept4() is the same as accept(). The following values can be bitwise ORed in flags to obtain different behavior:
Set the O_NONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the same result.
Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.
On success, these system calls return a nonnegative integer that is a descriptor for the accepted socket. On error, -1 is returned, and errno is set appropriately.
Linux accept() (and accept4()) passes already-pending network errors on the new socket as an error code from accept(). This behavior 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 the case of TCP/IP, these are ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.
EAGAIN or EWOULDBLOCK
The socket is marked nonblocking and no connections are present to be accepted. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
The descriptor is invalid.
A connection has been aborted.
The addr argument is not in a writable part of the user address space.
The system call was interrupted by a signal that was caught before a valid connection arrived; see signal(7).
Socket is not listening for connections, or addrlen is invalid (e.g., is negative).
(accept4()) invalid value in flags.
The per-process limit of open file descriptors has been reached.
The system limit on the total number of open files has been reached.
Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory.
The descriptor references a file, not a socket.
The referenced socket is not of type SOCK_STREAM.
In addition, 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.
The accept4() system call is available starting with Linux 2.6.28; support in glibc is available starting with version 2.10.
accept(): POSIX.1-2001, SVr4, 4.4BSD, (accept() first appeared in 4.2BSD).
accept4() is a nonstandard Linux extension.
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 behavior differs from the canonical BSD sockets implementation. Portable programs should not rely on inheritance or noninheritance of file status flags and always explicitly set all required flags on the socket returned from accept().
POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it.
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)).
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)."
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