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linux管道:捕获分叉子级的输出

fork pipe concurrency linux c

Rahn · 技术社区 · 3 年前

我有一个简单的程序,可以将数字0到2发送到管道中,然后函数 fork_child 从该管道接收编号,并打印出它接收的第一个编号,然后将以下编号发送到另一个管道中,该管道连接到由创建的下一个子管道 fork_child :

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

void fork_child(int port_in, int generation) {
    int number;
    int receive_first_number = 0;
    int pid = -1;
    int fd[2];

    while (read(port_in, &number, sizeof(number)) != 0) {
        if (receive_first_number == 0) {
            receive_first_number = 1;
            printf("child-%d (pid %d) receives: %d\n", generation, getpid(), number);
            continue;
        }

        if (pid == -1) {
            // execute only once to create child
            pipe(fd);
            pid = fork();
            if (pid == 0) {
                // child process
                close(fd[1]);
                fork_child(fd[0], generation + 1);
                close(fd[0]);
                exit(0);
            } else {
                // parent process
                close(fd[0]);
            }
        }
        // send number to next child
        write(fd[1], &number, sizeof(number));
    }

    close(fd[1]);
    wait(&pid);
}

int main() {
    int fd[2];
    pipe(fd);

    int pid = fork();
    if (pid == 0) {
        // child process
        close(fd[1]);
        fork_child(fd[0], 0);
        close(fd[0]);
        exit(0);
    } else {
        // parent process
        close(fd[0]);
        for (int number = 0; number < 3; number++) {
            write(fd[1], &number, sizeof(number));
        }
        close(fd[1]);
        wait(&pid);
    }

    return 0;
}

它在运行时按预期工作:

$ ./pipe
child-0 (pid 149437) receives: 0
child-1 (pid 149438) receives: 1
child-2 (pid 149439) receives: 2

execept捕获输出并重定向到文件中:

$ ./pipe > out
$ cat out
child-0 (pid 151246) receives: 0
child-1 (pid 151247) receives: 1
child-2 (pid 151248) receives: 2
child-0 (pid 151246) receives: 0
child-1 (pid 151247) receives: 1
child-0 (pid 151246) receives: 0

为什么捕获的输出与没有管道和重定向的情况不同?

0 回复 | 直到 3 年前

root 3 年前

这是一个依赖于缓冲的并发错误:

$ stdbuf -o 0 ./pipe 
child-0 (pid 864370) receives: 0
child-1 (pid 864371) receives: 1
child-2 (pid 864372) receives: 2
$ stdbuf -o 0 ./pipe > out && cat out
child-0 (pid 864355) receives: 0
child-1 (pid 864356) receives: 1
child-2 (pid 864357) receives: 2
$ stdbuf -o L ./pipe 
child-0 (pid 866493) receives: 0
child-1 (pid 866494) receives: 1
child-2 (pid 866495) receives: 2
$ stdbuf -o L ./pipe > out && cat out
child-0 (pid 866501) receives: 0
child-1 (pid 866502) receives: 1
child-2 (pid 866503) receives: 2
$ stdbuf -o 4096 ./pipe 
child-0 (pid 864399) receives: 0
child-1 (pid 864400) receives: 1
child-2 (pid 864401) receives: 2
child-0 (pid 864399) receives: 0
child-1 (pid 864400) receives: 1
child-0 (pid 864399) receives: 0
$ stdbuf -o 4096 ./pipe > out && cat out
child-0 (pid 864385) receives: 0
child-1 (pid 864386) receives: 1
child-2 (pid 864387) receives: 2
child-0 (pid 864385) receives: 0
child-1 (pid 864386) receives: 1
child-0 (pid 864385) receives: 0

请注意,这种类型的缓冲只影响 printf() 不 read() 或 write() 。

当没有缓冲时( stdbuf -o 0 ),或者当每行都进行缓冲时( stdbuf -o L ), 打印() 将在打印后立即刷新其写入缓冲区。

重定向到文件时,缓冲区要大得多(例如4k字节),并且 打印() 不会刷新该缓冲区。
然后你 fork() 但是 打印() 的缓冲区是用户空间的东西,内核并不知道,所以在 分叉() 父级和子级都有缓冲区的副本。
在2个这样的叉之后,缓冲区包含所有3个打印,第三个子将打印这些打印。
但第二个孩子没有意识到第三个孩子已经打印了所有内容,它将打印缓冲区,其中包含前两个打印。类似地,第一个子将打印其缓冲区,该缓冲区合法地仅包含其自己的打印。
这就是你得到0、1、2(来自第三个孩子)、0、1(来自第二个孩子)和0(来自第一个孩子)的方式。

你可以通过 strace (1) 通过检查写入的字节数:

$ strace -o /tmp/trace -f ./x && grep 'write.*child-' /tmp/trace
child-0 (pid 867548) receives: 0
child-1 (pid 867549) receives: 1
child-2 (pid 867550) receives: 2
867548 write(1, "child-0 (pid 867548) receives: 0"..., 33) = 33
867549 write(1, "child-1 (pid 867549) receives: 1"..., 33) = 33
867550 write(1, "child-2 (pid 867550) receives: 2"..., 33) = 33
$ strace -o /tmp/trace -f ./x > out && cat out && grep 'write.*child-' /tmp/trace
child-0 (pid 867515) receives: 0
child-1 (pid 867516) receives: 1
child-2 (pid 867517) receives: 2
child-0 (pid 867515) receives: 0
child-1 (pid 867516) receives: 1
child-0 (pid 867515) receives: 0
867517 write(1, "child-0 (pid 867515) receives: 0"..., 99) = 99
867516 write(1, "child-0 (pid 867515) receives: 0"..., 66) = 66
867515 write(1, "child-0 (pid 867515) receives: 0"..., 33) = 33

可能的解决方案:

明确缓冲区大小
fflush() 之后 打印()

另请参阅 this question 。