通过管道进行进程间通信

Question

已知在Linux进程间通信时，进程之间通过一个名为"Pipe".[=10的特殊文件进行通信=]

还知道对该文件执行的操作是一个进程 write 和一个进程 read 以便相互交流。

现在，问题是：

Does these write and read operations are parallel performed during the communication (operations are executed parallely) ?如果不是，

当其中一个进程在通信过程中进入SLEEP状态时会发生什么？它是先为第二个进程执行 write 操作以 read 还是直接进入睡眠状态而不执行任何 write 和读取操作?

Answer 1

发送进程可以写入直到管道缓冲区已满（自 2.6.11 以来 Linux 上为 64k）。之后，write(2)会阻塞。

接收进程将阻塞，直到数据可用于 read(2)。

要更详细地了解管道缓冲，请查看 https://unix.stackexchange.com/a/11954。

比如这个程序

#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);
    }
}

当运行和 gcc pipes.c && strace -e trace=open,close,read,write,pipe,clone -f ./a.out 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, "7ELF[=11=][=11=][=11=][=11=][=11=][=11=][=11=][=11=][=11=]>[=11=][=11=][=11=][=11=]0[=11=][=11=][=11=][=11=][=11=]"..., 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，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供读取。

通过管道进行进程间通信

Interprocess communication via Pipes

linux

process

operating-system

ipc

interprocess