Check if socket is listening in C

Question

While iterating through socket file descriptors, how can I check if one of them is from a passive socket (listening for connections)?

Answer 1

This can be checked with getsockopt(SO_ACCEPTCONN). For example:

#include <sys/socket.h>

int val;
socklen_t len = sizeof(val);
if (getsockopt(fd, SOL_SOCKET, SO_ACCEPTCONN, &val, &len) == -1)
    printf("fd %d is not a socket\n", fd);
else if (val)
    printf("fd %d is a listening socket\n", fd);
else
    printf("fd %d is a non-listening socket\n", fd);