Why doesn't Linux accept() return EINTR?

Question

Environment: a RedHat-like distro, 2.6.39 kernel, glibc 2.12.

I fully expect that if a signal was delivered while accept() was in progress, accept should fail, leaving errno==EINTR. However, mine doesn't do that, and I'm wondering why. Below are the sample program, and strace output.

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <signal.h>
#include <errno.h>
#include <arpa/inet.h>
#include <string.h>

static void sigh(int);

int main(int argc, char ** argv) {

    int s;
    struct sockaddr_in sin;

    if ((s = socket(AF_INET, SOCK_STREAM, 0))<0) {
        perror("socket");
        return 1;
    }
    memset(&sin, 0, sizeof(struct sockaddr_in));
    sin.sin_family = AF_INET;
    if (bind(s, (struct sockaddr*)&sin, sizeof(struct sockaddr_in))) {
        perror("bind"); 
        return 1;
    }
    if (listen(s, 5)) {
        perror("listen");
    }

    signal(SIGQUIT, sigh);

    while (1) {
        socklen_t sl = sizeof(struct sockaddr_in);
        int rc = accept(s, (struct sockaddr*)&sin, &sl);
        if (rc<0) {
            if (errno == EINTR) {
                printf("accept restarted\n");
                continue;
            }
            perror("accept");
            return 1;
        }
        printf("accepted fd %d\n", rc);
        close(rc);
    }

}

void sigh(int s) {

    signal(s, sigh);

    unsigned char p[100];
    int i = 0;
    while (s) {
        p[i++] = '0'+(s%10);
        s/=10;
    }
    write(1, "sig ", 4);
    for (i--; i>=0; i--) {
        write(1, &p[i], 1);
    }
    write(1, "\n", 1);

}

strace output:

execve("./accept", ["./accept"], [/* 57 vars */]) = 0
<skipped>
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
listen(3, 5)                            = 0
rt_sigaction(SIGQUIT, {0x4008c4, [QUIT], SA_RESTORER|SA_RESTART, 0x30b7e329a0}, {SIG_DFL, [], 0}, 8) = 0
accept(3, 0x7fffe3e3c500, [16])         = ? ERESTARTSYS (To be restarted)
--- SIGQUIT (Quit) @ 0 (0) ---
rt_sigaction(SIGQUIT, {0x4008c4, [QUIT], SA_RESTORER|SA_RESTART, 0x30b7e329a0}, {0x4008c4, [QUIT], SA_RESTORER|SA_RESTART, 0x30b7e329a0}, 8) = 0
write(1, "sig ", 4sig )                     = 4
write(1, "3", 13)                        = 1
write(1, "\n", 1
)                       = 1
rt_sigreturn(0x1)                       = 43
accept(3, ^C <unfinished ...>

Answer 1

Just when I was about to post this, the SA_RESTART flag in strace output caught my attention. signal(2) man page says that signal() "...calls sigaction(2) using flags that supply BSD semantics..." starting from glibc 2.x.

The SA_RESTART flag "...makes certain system calls restartable across signals...", which hides the process of restarting a call from the user. So, this is not specific to accept(), a number of other system calls are also affected, not that there is a clear list of which ones.

So, if you need to react to a signal from a thread that may be blocked on a system call, you should use sigaction() to set your signal handlers, and not signal(). Below is the modified sample program that does exactly that, for reference.

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <signal.h>
#include <errno.h>
#include <arpa/inet.h>
#include <string.h>

static void sigh(int);

static struct sigaction sa;

int main(int argc, char ** argv) {

    int s;
    struct sockaddr_in sin;

    if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        return 1;
    }
    memset(&sin, 0, sizeof(struct sockaddr_in));
    sin.sin_family = AF_INET;
    if (bind(s, (struct sockaddr*)&sin, sizeof(struct sockaddr_in))) {
        perror("bind"); 
        return 1;
    }
    if (listen(s, 5)) {
        perror("listen");
    }

    memset(&sa, 0, sizeof(struct sigaction));
    sa.sa_handler = sigh;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGQUIT, &sa, 0);

    while (1) {
        socklen_t sl = sizeof(struct sockaddr_in);
        int rc = accept(s, (struct sockaddr*)&sin, &sl);
        if (rc<0) {
            if (errno == EINTR) {
                printf("accept restarted\n");
                continue;
            }
            perror("accept");
            return 1;
        }
        printf("accepted fd %d\n", rc);
        close(rc);
    }

}

void sigh(int s) {

    sigaction(SIGQUIT, &sa, 0);

    unsigned char p[100];
    int i = 0;
    while (s) {
        p[i++] = '0'+(s%10);
        s/=10;
    }
    write(1, "sig ", 4);
    for (i--; i>=0; i--) {
        write(1, &p[i], 1);
    }
    write(1, "\n", 1);

}

And strace:

execve("./accept", ["./accept"], [/* 57 vars */]) = 0
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
listen(3, 5)                            = 0
rt_sigaction(SIGQUIT, {0x400994, [], SA_RESTORER, 0x30b7e329a0}, NULL, 8) = 0
accept(3, 0x7fffb626be90, [16])         = ? ERESTARTSYS (To be restarted)
--- SIGQUIT (Quit) @ 0 (0) ---
rt_sigaction(SIGQUIT, {0x400994, [], SA_RESTORER, 0x30b7e329a0}, NULL, 8) = 0
write(1, "sig ", 4)                  = 4
write(1, "3", 13)                        = 1
write(1, "\n", 1)                       = 1
rt_sigreturn(0x1)                       = -1 EINTR (Interrupted system call)
write(1, "accept restarted\n", 17)      = 17
accept(3,