accept - accept a connection on a socket
int accept(int s, struct sockaddr *addr, int *addrlen);
The argument s is a socket that has been created with
socket(2), bound to an address with bind(2), and is lis
tening for connections after a listen(2). The accept
function extracts the first connection request on the
queue of pending connections, creates a new socket with
the same properties of s, and allocates a new file
descriptor for the socket. 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 an error as described below. The socket returned
by accept may not be used to accept more connections. The
original socket s remains open.
The argument addr is a result parameter that is filled in
with the address of the connecting entity, as known to the
communications layer. The exact format of the addr param
eter is determined by the domain in which the communica
tion is occurring. addrlen is a value-result parameter:
it should initially contain the amount of space pointed to
by addr; on return it will contain the actual length (in
bytes) of the address returned. This call is used with
connection-based socket types, currently with SOCK_STREAM.
It is possible to select(2) a socket for the purposes of
doing an accept by selecting it for read.
For certain protocols which require an explicit confirma
tion, 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 you want accept to never block the listening socket
needs to have the non blocking flag set. Assuming that
there is always a connection waiting after select returned
true is not reliable, because the connection might be
removed by an asynchronous network error between the
select/poll returning and the accept call. The application
The call returns -1 on error. If it succeeds, it returns
a non-negative integer that is a descriptor for the
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 reli
able 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, EHOSTUN
REACH, EOPNOTSUPP, and ENETUNREACH.
EBADF The descriptor is invalid.
The descriptor references a file, not a socket.
The referenced socket is not of type SOCK_STREAM.
EFAULT The addr parameter is not in a writable part of
the user address space.
EAGAIN The socket is marked non-blocking and no connec
tions are present to be accepted.
EPERM Firewall rules forbid connection.
Not enough free memory.
In addition, network errors for the new socket and as
defined for the protocol may be returned.
SVr4, 4.4BSD (the accept function first appeared in BSD
bind(2), connect(2), listen(2), select(2), socket(2)