【发布时间】：2019-12-28 10:06:41
【问题描述】：
我试图让一个程序创建 n 个子进程来读取文件，并且父进程不断循环它们并尝试发送信号以继续读取文件，如果该进程是空闲的。否则，如果该进程被占用，它会移动到下一个进程。这是我使用信号的方法，但输出是意外的。如果我使用 printf 在父级的 while 循环中调试代码，我只会得到 printf 语句本身。为什么会这样？
据我了解，预期的输出应该是几乎同时从文件中读取的所有 n 个子进程（文件很小），然后间隔 1 秒再发生这种情况。这是正确的，还是由于竞争条件而不确定？
我知道使用管道可能有更好的方法，但是使用信号可以做到这一点吗？提前致谢。
我知道 signal() 的使用不如 sigaction() 多，如手册页中所述，但这是否可以解决？
会不会是printf本身对IPC的干扰造成的printf输出异常？
编辑：用于测试的 Pastebin 链接：https://pastebin.com/ZWFnkJDA
#include<stdio.h>
#include<stdlib.h>
#include<sys/types.h>
#include<unistd.h>
#include<signal.h>
#include<fcntl.h>
#include<sys/wait.h>
int counter = 0;
int fd;
const char* filename;
int j = 0;
int no_of_children;
int* cpid;
//User define signal handler
static void sig_usr1(int);
static void sig_usr2(int signo, siginfo_t* info, void* context);
static void sig_usr1(int signo)
{
//Now the child process waits for reading the Filename
//Block SIGUSR1 until it's complete
signal(SIGUSR1, SIG_IGN);
printf("Blocked now.\n");
printf("Child no %d is reading now.\n\n",getpid());
fd = open(filename, O_RDONLY | O_CREAT);
char buf = 'a';
int k=0;
char* op = (char*) malloc (255*sizeof(char));
while(read (fd, &buf, 1))
{
if (buf == '\n')
{
op[k] = '\0';
break;
}
else
{
op[k++] = buf;
}
}
//Now wait for a second and then send a signal
sleep(1);
//Print the contents of the buffer via op
printf("Output: %s\n\n", op);
//Now unblock the signal
kill(getppid(), SIGUSR2);
signal(SIGUSR1, sig_usr1);
printf("Unblocked now\n");
}
static void sig_usr2(int signo, siginfo_t* info, void* context)
{
if (signo == SIGUSR2)
{
child_pid = info->si_pid;
printf("Parent Received SIGUSR2. Child Process with PID %d is now free\n\n", child_pid);
}
}
int main(int argc, char* argv[])
{
//Filename is the first argument
filename = argv[1];
//Number of Child Processes to be spawned
no_of_children = atoi(argv[2]);
cpid = (int*) malloc (no_of_children*sizeof(int));
//Create a sigaction() handler for SIGUSR2
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = sig_usr2;
sigaction(SIGUSR2, &sa, NULL);
//Create no_of_children children
for(int i=0; i<no_of_children; i++)
{
cpid[i] = fork();
if (cpid[i] == 0)
{
//Inside a child
printf("Created %dth child process", i);
printf(" with Process ID = %d\n", getpid());
signal(SIGUSR1, sig_usr1);
while(1)
{
pause();
}
//Every child process must exit so control goes back to the parent
exit(0);
}
}
//Returns to the parent process
while(1)
{
int fpid = cpid[j];
//Send the signal to the free child process
//printf("Sending to PID %d\n", fpid); //----> Uncommenting this line only prints this statement. Why does this happen?
kill(fpid, SIGUSR1);
j = (j < no_of_children - 1) ? j + 1 : 0; //----->Does not work as expected if i uncomment this line. I want to switch between PIDs and send signals to all of them
}
return 0;
}
- 样本输出：
$ ./a.out sample.txt 4
Created 0th child process with Process ID = 15734
Created 1th child process with Process ID = 15735
PID: 15737 -> PID: 15736 -> PID: 15735 -> PID: 15734
Created 2th child process with Process ID = 15736
Blocked now.
Child no 15734 is reading now.
Created 3th child process with Process ID = 15737
Output: This is a sample file
Unblocked now
Parent Received SIGUSR2. Child Process with PID 15734 is now free
Blocked now.
Child no 15734 is reading now.
Blocked now.
Child no 15735 is reading now.
Output: This is a sample file
Unblocked now
Output: This is a sample file
Parent Received SIGUSR2. Child Process with PID 15735 is now free
Blocked now.
Child no 15734 is reading now.
Unblocked now
Parent Received SIGUSR2. Child Process with PID 15736 is now free
Blocked now.
Child no 15735 is reading now.
Blocked now.
Blocked now.
Child no 15736 is reading now.
Child no 15737 is reading now.
^\%
【问题讨论】：
-
请提供minimal reproducible example 并将编译器的警告级别提高到最大值。尝试编译您的示例会产生太多错误。
-
我已经编辑了我的帖子以生成一个工作示例。你现在可以检查一下吗？谢谢
-
抱歉，现在不行，因为我在工作，必须使用 Windows。为了增加其他人加入的机会，您还可以显示您期望的输出以及您得到的输出。
-
是的。我也添加了输出。我期望的是“输出”行每 1 秒一次显示 4 次，因为此时所有 4 个进程都读取文件
-
pintf是信号不安全的。永远不要在信号处理程序中这样做。
标签： c signals fork parent-child ipc