我在Linux系统中遇到了一个奇怪的问题，涉及到
该程序：
popen和fgets库函数。下面是一个简短的程序示例：
- 安装
SIGUSR1信号处理程序。 - 创建一个辅助线程来反复向主线程发送
SIGUSR1信号。 - 在主线程中，通过
popen()执行一个非常简单的shell命令，通过fgets()获取输出，并检查输出是否符合预期长度。
$ gcc -Wall test.c -lpthread && ./a.out
iteration 0
iteration 1
iteration 2
iteration 3
iteration 4
iteration 5
unexpected length: 0
我的机器详情（该程序也可以使用这个在线 C 编译器编译和运行）：
$ 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);
}
}
read系统调用被信号中断了。除非你能在某个地方找到这种行为的文档，否则这似乎是fgets中的一个错误。 - Dark Falconlibc中的某种错误。经过一些更正，我在OS X上运行了它，在未经修改的情况下在RHEL 6上也没有问题。 - Sergey L.fgets调用之一的read系统调用返回了完整的“hello”文本。这似乎意味着我要么使用fgets不正确，要么fgets的实现中存在错误。 - Josh Johnson