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.
- Main thread connects a socket to a misbehaving remote host that won't let the connection go.
- All signals blocked.
- Read thread thread is started.
- Main unblocks and installs handler for
SIGINT. - Read thread unblocks and installs handler for
SIGUSR1. - Main thread's signal handler sends a
SIGUSR1to 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)
