通过管道进行进程间通信

Question

众所周知，在Linux的进程间通信期间，进程通过一个名为"Pipe">的特殊文件相互通信。

众所周知，对该文件执行的操作由一个进程写入并由一个进程读取，以便相互通信。

现在，问题是：

这些写入和读取操作在通信期间是否并行执行(操作并行执行(？ 如果不是，

当其中一个进程在通信期间进入 SLEEP 状态时会发生什么情况？它是先执行写入操作以便读取第二个进程，还是直接进入睡眠状态而不执行任何写入和读取操作？

Answer 1

发送过程可以写入，直到管道缓冲区已满(自 2.6.11 起在 Linux 上为 64k(。之后， write(2( 将阻塞。

接收进程将阻塞，直到数据可供读取(2(。

有关管道缓冲的更详细信息，请查看 https://unix.stackexchange.com/a/11954。

例如，该程序

#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int
main(int argc, char *argv[])
{
    int pipefd[2];
    pid_t cpid;
    char wbuf[32768];
    char buf[16384];
    /* Initialize writer buffer with 012...89 sequence */
    for (int i = 0; i < sizeof(wbuf); i++)
      wbuf[i] = '0' + i % 10;
    if (pipe(pipefd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }
    cpid = fork();
    if (cpid == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }
    if (cpid == 0) {    /* Child reads from pipe */
        close(pipefd[1]);          /* Close unused write end */
        while (read(pipefd[0], &buf, sizeof(buf)) > 0);
        close(pipefd[0]);
        _exit(EXIT_SUCCESS);
    } else {            /* Parent writes sequence to pipe */
        close(pipefd[0]);          /* Close unused read end */
        for (int i = 0; i < 5; i++)
          write(pipefd[1], wbuf, sizeof(wbuf));
        close(pipefd[1]);          /* Reader will see EOF */
        wait(NULL);                /* Wait for child */
        exit(EXIT_SUCCESS);
    }
}

使用gcc pipes.c && strace -e trace=open,close,read,write,pipe,clone -f ./a.out运行时将产生以下序列：

open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
close(3)                                = 0
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "177ELF21133>132032"..., 832) = 832
close(3)                                = 0
pipe([3, 4])                            = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f32117489d0) = 21114
close(3)                                = 0
write(4, "01234567890123456789012345678901"..., 32768) = 32768
write(4, "01234567890123456789012345678901"..., 32768) = 32768
write(4, "01234567890123456789012345678901"..., 32768strace: Process 21114 attached
 <unfinished ...>
[pid 21114] close(4)                    = 0
[pid 21114] read(3, "01234567890123456789012345678901"..., 16384) = 16384
[pid 21114] read(3,  <unfinished ...>
[pid 21113] <... write resumed> )       = 32768
[pid 21114] <... read resumed> "45678901234567890123456789012345"..., 16384) = 16384
[pid 21113] write(4, "01234567890123456789012345678901"..., 32768 <unfinished ...>
[pid 21114] read(3, "01234567890123456789012345678901"..., 16384) = 16384
[pid 21114] read(3,  <unfinished ...>
[pid 21113] <... write resumed> )       = 32768
[pid 21114] <... read resumed> "45678901234567890123456789012345"..., 16384) = 16384
[pid 21113] write(4, "01234567890123456789012345678901"..., 32768 <unfinished ...>
[pid 21114] read(3, "01234567890123456789012345678901"..., 16384) = 16384
[pid 21114] read(3,  <unfinished ...>
[pid 21113] <... write resumed> )       = 32768
[pid 21114] <... read resumed> "45678901234567890123456789012345"..., 16384) = 16384
[pid 21113] close(4)                    = 0
[pid 21114] read(3, "01234567890123456789012345678901"..., 16384) = 16384
[pid 21114] read(3, "45678901234567890123456789012345"..., 16384) = 16384
[pid 21114] read(3, "01234567890123456789012345678901"..., 16384) = 16384
[pid 21114] read(3, "45678901234567890123456789012345"..., 16384) = 16384
[pid 21114] read(3, "", 16384)          = 0
[pid 21114] close(3)                    = 0
[pid 21114] +++ exited with 0 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=21114, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
+++ exited with 0 +++

您会注意到读取和写入

是交错的，并且写入和读取过程将阻塞几次，因为管道已满或没有足够的数据可用于读取。