popen()/fgets() 间歇性返回不完整的输出

我遇到了一个奇怪的问题popen and fgetsLinux系统上的库函数。

演示该问题的简短程序如下：

1. 安装信号处理程序SIGUSR1.
2. 创建辅助线程来重复发送SIGUSR1到主线程。
3. 在主线程中，通过以下方式重复执行一个非常简单的 shell 命令popen()，通过获取输出fgets()，并检查输出是否符合预期长度。

输出意外地间歇性被截断。为什么？

命令行调用示例：

$ gcc -Wall test.c -lpthread && ./a.out 
iteration 0
iteration 1
iteration 2
iteration 3
iteration 4
iteration 5
unexpected length: 0

我的机器的详细信息（该程序也将编译并运行这个在线C编译器 http://coliru.stacked-crooked.com/):

$ cat /etc/redhat-release
CentOS release 6.5 (Final)

$ uname -a
Linux localhost.localdomain 2.6.32-431.17.1.el6.x86_64 #1 SMP Wed May 7 23:32:49 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

# gcc 4.4.7
$ gcc --version
gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

# glibc 2.12
$ ldd --version
ldd (GNU libc) 2.12
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

该程序：

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <pthread.h>
#include <errno.h>

void dummy_signal_handler(int signal);
void* signal_spam_task(void* arg);
void echo_and_verify_output();
char* fgets_with_retry(char *buffer, int size, FILE *stream);

static pthread_t main_thread;

/**
 * Prints an error message and exits if the output is truncated, which happens
 * about 5% of the time.
 *
 * Installing the signal handler with the SA_RESTART flag, blocking SIGUSR1
 * during the call to fgets(), or sleeping for a few milliseconds after the
 * call to popen() will completely prevent truncation.
 */
int main(int argc, char **argv) {

    // install signal handler for SIGUSR1
    struct sigaction sa, osa;
    sa.sa_handler = dummy_signal_handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    sigaction(SIGUSR1, &sa, &osa);

    // create a secondary thread to repeatedly send SIGUSR1 to main thread
    main_thread = pthread_self();
    pthread_t spam_thread;
    pthread_create(&spam_thread, NULL, signal_spam_task, NULL);

    // repeatedly execute simple shell command until output is unexpected
    unsigned int i = 0;
    for (;;) {
        printf("iteration %u\n", i++);
        echo_and_verify_output();
    }

    return 0;
}

void dummy_signal_handler(int signal) {}

void* signal_spam_task(void* arg) {
    for (;;)
        pthread_kill(main_thread, SIGUSR1);
    return NULL;
}

void echo_and_verify_output() {

    // run simple command
    FILE* stream = popen("echo -n hello", "r");
    if (!stream)
        exit(1);

    // count the number of characters in the output
    unsigned int length = 0;
    char buffer[BUFSIZ];
       while (fgets_with_retry(buffer, BUFSIZ, stream) != NULL)
        length += strlen(buffer);

    if (ferror(stream) || pclose(stream))
        exit(1);

    // double-check the output
    if (length != strlen("hello")) {
        printf("unexpected length: %i\n", length);
        exit(2);
    }
}

// version of fgets() that retries on EINTR
char* fgets_with_retry(char *buffer, int size, FILE *stream) {
    for (;;) {
        if (fgets(buffer, size, stream))
            return buffer;
        if (feof(stream))
            return NULL;
        if (errno != EINTR)
            exit(1);
        clearerr(stream);
    }
}

如果错误发生在FILE阅读时流式传输fgets，之前读取的某些字节是否传输到缓冲区是未定义的fgets是否返回 NULL（C99 规范的 7.19.7.2）。因此，如果 SIGUSR1 信号发生在fgets调用并导致EINTR，某些字符可能会从流中丢失。

结果是你不能使用 stdio 函数来读/写FILE如果底层系统调用可能有可恢复的错误返回（例如EINTR or EAGAIN），因为无法保证标准库在发生这种情况时不会丢失缓冲区中的一些数据。您可以声称这是标准库实现中的一个“错误”，但这是 C 标准允许的错误。