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
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
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 s needs to have the O_NONBLOCK
flag set (see socket(7)).
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.
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.
The per-process limit of open file descriptors has been
The system limit on the total number of open files has been
The descriptor references a file, not a socket.
The referenced socket is not of type
accept may fail if:
The addr parameter 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
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 (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.
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)."