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.
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 type
SOCK_STREAM.
accept() may fail if:
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 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.
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)."
