EAGAIN error for accept on blocking socket

Question

I am using a blocking socket to accept connection. I rarely get this error which makes it hard to debug. The accept returns with EAGAIN error. How can that be for a blocking socket?

Answer 1

If the socket has a receive timeout set (with the SO_RCVTIMEO socket option), then accept will return EAGAIN when the timeout expires.

This code will demonstrate it (and also let you investigate the effect of delivering a signal):

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/time.h>

#define TESTEXIT(s, f) if (s < 0) { perror(f); exit(1); }

void handler(int x)
{
    return;
}

int main()
{
    int s;
    int r;
    struct sockaddr_in sin;
    socklen_t sin_len = sizeof sin;
    struct timeval timeo = { .tv_sec = 5, .tv_usec = 0 };

    signal(SIGUSR1, handler);

    s = socket(PF_INET, SOCK_STREAM, 0);
    TESTEXIT(s, "socket");

    r = listen(s, 10);
    TESTEXIT(r, "listen");

    r = setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, &timeo, sizeof timeo);
    TESTEXIT(r, "setsockopt");

    r = accept(s, (struct sockaddr *)&sin, &sin_len);
    TESTEXIT(r, "accept");

    return 0;
}