Question

I have a thread that sits in a blocking recv() loop and I want to terminate (assume this can’t be changed to select() or any other asynchronous approach).

I also have a signal handler that catches SIGINT and theoretically it should make recv() return with error and errno set to EINTR.

But it doesn’t, which I assume has something to do with the fact that the application is multi-threaded. There is also another thread, which is meanwhile waiting on a pthread_join() call.

What’s happening here?

EDIT:

OK, now I explicitly deliver the signal to all blocking recv() threads via pthread_kill() from the main thread (which results in the same global SIGINT signal handler installed, though multiple invocations are benign). But recv() call is still not unblocked.

EDIT:

I’ve written a code sample that reproduces the problem.

1. Main thread connects a socket to a misbehaving remote host that won’t let the connection go.
2. All signals blocked.
3. Read thread thread is started.
4. Main unblocks and installs handler for SIGINT.
5. Read thread unblocks and installs handler for SIGUSR1.
6. Main thread’s signal handler sends a SIGUSR1 to the read thread.

Interestingly, if I replace recv() with sleep() it is interrupted just fine.

PS

Alternatively you can just open a UDP socket instead of using a server.

client

#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>

static void
err(const char *msg)
{
    perror(msg);
    abort();
}

static void
blockall()
{
    sigset_t ss;
    sigfillset(&ss);
    if (pthread_sigmask(SIG_BLOCK, &ss, NULL))
        err("pthread_sigmask");
}

static void
unblock(int signum)
{
    sigset_t ss;
    sigemptyset(&ss);
    sigaddset(&ss, signum);
    if (pthread_sigmask(SIG_UNBLOCK, &ss, NULL))
        err("pthread_sigmask");
}

void
sigusr1(int signum)
{
    (void)signum;
    printf("%lu: SIGUSR1\n", pthread_self());
}

void*
read_thread(void *arg)
{
    int sock, r;
    char buf[100];

    unblock(SIGUSR1);
    signal(SIGUSR1, &sigusr1);
    sock = *(int*)arg;
    printf("Thread (self=%lu, sock=%d)\n", pthread_self(), sock);
    r = 1;
    while (r > 0)
    {
        r = recv(sock, buf, sizeof buf, 0);
        printf("recv=%d\n", r);
    }
    if (r < 0)
        perror("recv");
    return NULL;
}

int sock;
pthread_t t;

void
sigint(int signum)
{
    int r;
    (void)signum;
    printf("%lu: SIGINT\n", pthread_self());
    printf("Killing %lu\n", t);
    r = pthread_kill(t, SIGUSR1);
    if (r)
    {
        printf("%s\n", strerror(r));
        abort();
    }
}

int
main()
{
    pthread_attr_t attr;
    struct sockaddr_in addr;

    printf("main thread: %lu\n", pthread_self());
    memset(&addr, 0, sizeof addr);
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (socket < 0)
        err("socket");
    addr.sin_family = AF_INET;
    addr.sin_port = htons(8888);
    if (inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr) <= 0)
        err("inet_pton");
    if (connect(sock, (struct sockaddr *)&addr, sizeof addr))
        err("connect");

    blockall();
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    if (pthread_create(&t, &attr, &read_thread, &sock))
        err("pthread_create");
    pthread_attr_destroy(&attr);
    unblock(SIGINT);
    signal(SIGINT, &sigint);

    if (sleep(1000))
        perror("sleep");
    if (pthread_join(t, NULL))
        err("pthread_join");
    if (close(sock))
        err("close");

    return 0;
}

server

import socket
import time

s = socket.socket(socket.AF_INET)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind(('127.0.0.1',8888))
s.listen(1)
c = []
while True:
    (conn, addr) =  s.accept()
    c.append(conn)

Answer 1

Normally signals do not interrupt system calls with EINTR. Historically there were two possible signal delivery behaviors: the BSD behavior (syscalls are automatically restarted when interrupted by a signal) and the Unix System V behavior (syscalls return -1 with errno set to EINTR when interrupted by a signal). Linux (the kernel) adopted the latter, but the GNU C library developers (correctly) deemed the BSD behavior to be much more sane, and so on modern Linux systems, calling signal (which is a library function) results in the BSD behavior.

POSIX allows either behavior, so it’s advisable to always use sigaction where you can choose to set the SA_RESTART flag or omit it depending on the behavior you want. See the documentation for sigaction here:

http://www.opengroup.org/onlinepubs/9699919799/functions/sigaction.html