为什么Linux accept()不返回EINTR? [英] Why doesn't Linux accept() return EINTR?

问题描述

环境:类似RedHat的发行版，2.6.39内核，glibc 2.12.

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

我完全希望，如果在accept()进行期间传递了一个信号，accept应该会失败，从而导致errno == EINTR.但是，我的不这样做，我想知道为什么.下面是示例程序和strace输出.

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输出:

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 ...>

推荐答案

就在我要发布此帖子时，strace输出中的"SA_RESTART"标志引起了我的注意. signal(2)手册页说，signal()"...使用提供BSD语义的标志调用sigaction(2)..." 从glibc2开始.

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 glibc2.

SA_RESTART标志"...使某些系统调用可跨信号重新启动..." ，这隐藏了重新启动用户呼叫的过程.因此，这不是对accept()特定的，其他许多系统调用也会受到影响，而不是明确列出了哪些系统调用.

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.

因此，如果您需要对可能被系统调用阻塞的线程的信号做出反应，则应使用sigaction()设置信号处理程序，而不要使用signal().下面是经过修改的示例程序，正是该程序可以做到这一点，以供参考.

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);

}

和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, 

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