8.1. accept()

Accept an incoming connection on a listening socket


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

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


Once you've gone through the trouble of getting a SOCK_STREAM socket and setting it up for incoming connections with listen(), then you call accept() to actually get yourself a new socket descriptor to use for subsequent communication with the newly connected client.

The old socket that you are using for listening is still there, and will be used for further accept() calls as they come in.


The listen()ing socket descriptor.


This is filled in with the address of the site that's connecting to you.


This is filled in with the sizeof() the structure returned in the addr parameter. You can safely ignore it if you assume you're getting a struct sockaddr_in back, which you know you are, because that's the type you passed in for addr.

accept() will normally block, and you can use select() to peek on the listening socket descriptor ahead of time to see if it's "ready to read". If so, then there's a new connection waiting to be accept()ed! Yay! Alternatively, you could set the O_NONBLOCK flag on the listening socket using fcntl(), and then it will never block, choosing instead to return -1 with errno set to EWOULDBLOCK.

The socket descriptor returned by accept() is a bona fide socket descriptor, open and connected to the remote host. You have to close() it when you're done with it.

Return Value

accept() returns the newly connected socket descriptor, or -1 on error, with errno set appropriately.


int s, s2;
struct sockaddr_in myaddr, remoteaddr;
socklen_t remoteaddr_len;

myaddr.sin_family = AF_INET;
myaddr.sin_port = htons(3490); // clients connect to this port
myaddr.sin_addr.s_addr = INADDR_ANY; // autoselect IP address

s = socket(PF_INET, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)myaddr, sizeof(myaddr));

listen(s, 10); // set s up to be a server (listening) socket

for(;;) {
    s2 = accept(s, &remoteaddr, &remoteaddr_len);

    // now you can send() and recv() with the
    // connected client via socket s2

See Also

socket(), listen(), struct sockaddr_in