linux系统编程之管道（二）：管道读写规则和Pipe Capacity、PIPE_BUF

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我是靠谱客的博主认真雨，最近开发中收集的这篇文章主要介绍linux系统编程之管道（二）：管道读写规则和Pipe Capacity、PIPE_BUF，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

http://tech.ddvip.com/2013-05/1369683339196204_2.html

一、当没有数据可读时
O_NONBLOCK disable：read调用阻塞，即进程暂停执行，一直等到有数据来到为止。

O_NONBLOCK enable：read调用返回-1，errno值为EAGAIN。

示例程序如下：

/*************************************************************************
    > File Name: process_.c
    > Author: Simba
    > Mail: dameng34@163.com
    > Created Time: Sat 23 Feb 2013 02:34:02 PM CST
 ************************************************************************/
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>
#include<signal.h>
#define ERR_EXIT(m) 
    do { 
        perror(m); 
        exit(EXIT_FAILURE); 
    } while(0)
     
int main(int argc, char *argv[])
{
    int pipefd[2];
    if (pipe(pipefd) == -1)
        ERR_EXIT("pipe error");
     
    pid_t pid;
    pid = fork();
    if (pid == -1)
        ERR_EXIT("fork error");
     
    if (pid == 0)
    {
        sleep(3);
        close(pipefd[0]);
        write(pipefd[1], "hello", 5);
        close(pipefd[1]);
        exit(EXIT_SUCCESS);
    }
     
    close(pipefd[1]);
    char buf[10] = {0};
    int flags = fcntl(pipefd[0], F_GETFL);
    fcntl(pipefd[0], F_SETFL, flags | O_NONBLOCK); //enable fd的O_NONBLOCK
    int ret = read(pipefd[0], buf, 10); //默认是disable fd的O_NONBLOCK
    if (ret == -1) // 父进程不会阻塞，出错返回
        ERR_EXIT("read error");
    printf("buf=%sn", buf);
     
    return 0;
}

特意在子进程中sleep了3s，让父进程先被调度运行，而且读端文件描述符标志设置为非阻塞，即立刻出错返回，如下。

simba@ubuntu:~/Documents/code/linux_programming/APUE/pipe$ ./pipe_block
read error: Resource temporarily unavailable

二、当管道满的时候
O_NONBLOCK disable： write调用阻塞，直到有进程读走数据
O_NONBLOCK enable：调用返回-1，errno值为EAGAIN

管道是一块内存缓冲区，可以写个小程序测试一下管道的容量Pipe Capacity：

/*************************************************************************
    > File Name: process_.c
    > Author: Simba
    > Mail: dameng34@163.com
    > Created Time: Sat 23 Feb 2013 02:34:02 PM CST
 ************************************************************************/
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>
#include<signal.h>
#define ERR_EXIT(m) 
    do { 
        perror(m); 
        exit(EXIT_FAILURE); 
    } while(0)
     
int main(int argc, char *argv[])
{
    int pipefd[2];
    if (pipe(pipefd) == -1)
        ERR_EXIT("pipe error");
     
    int ret;
    int count = 0;
    int flags = fcntl(pipefd[1], F_GETFL);
    fcntl(pipefd[1], F_SETFL, flags | O_NONBLOCK); // 设置为非阻塞
    while (1)
    {
        ret = write(pipefd[1], "A", 1);
        if (ret == -1)
        {
            printf("err=%sn", strerror(errno));
            break;
        }
     
        count++;
    }
    printf("count=%dn", count); //管道容量
     
    return 0;
}

程序中将写端文件描述符标志设置为非阻塞，当管道被写满时不会等待其他进程读取数据，而是直接返回-1并置errno，输出如下：
simba@ubuntu:~/Documents/code/linux_programming/APUE/pipe$ ./pipe_capacity
err=Resource temporarily unavailable
count=65536

打印了错误码，可以看到管道的容量是64kB，man 7 pipe中也有提到在2.6.11内核以前是4096，现在是65536。

三、如果所有管道读端对应的文件描述符被关闭（管道读端的引用计数等于0），则write操作会产生SIGPIPE信号，默认终止当前进程

示例代码如下：

/*************************************************************************
    > File Name: process_.c
    > Author: Simba
    > Mail: dameng34@163.com
    > Created Time: Sat 23 Feb 2013 02:34:02 PM CST
 ************************************************************************/
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>
#include<signal.h>
#define ERR_EXIT(m) 
    do { 
        perror(m); 
        exit(EXIT_FAILURE); 
    } while(0)
     
void handler(int sig)
{
    printf("recv sig=%dn", sig);
}
     
int main(int argc, char *argv[])
{
    signal(SIGPIPE, handler);
     
    int pipefd[2];
    if (pipe(pipefd) == -1)
        ERR_EXIT("pipe error");
     
    pid_t pid;
    pid = fork();
    if (pid == -1)
        ERR_EXIT("fork error");
     
    if (pid == 0)
    {
        close(pipefd[0]);
        exit(EXIT_SUCCESS);
    }
    close(pipefd[0]);
    sleep(1);
    int ret = write(pipefd[1], "hello", 5);
    if (ret == -1)
    {
        printf("err=%sn", strerror(errno));
    }
     
    return 0;
}

输出测试：

simba@ubuntu:~/Documents/code/linux_programming/APUE/pipe$ ./close_fd_read
recv sig=13
err=Broken pipe

父进程睡眠1s确保所有读端文件描述符都已经关闭，如果没有安装SIGPIPE信号的处理函数，则默认终止当前进程，即write函数不会返回，现在write错误返回-1，并置errno=EPIPE，对应的出错信息是Broken pipe。

四、如果所有管道写端对应的文件描述符被关闭（管道写端的引用计数等于0），那么管道中剩余的数据都被读取后，再次read会返回0

示例程序如下：

/*************************************************************************
    > File Name: process_.c
    > Author: Simba
    > Mail: dameng34@163.com
    > Created Time: Sat 23 Feb 2013 02:34:02 PM CST
 ************************************************************************/
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>
#include<signal.h>
#define ERR_EXIT(m) 
    do { 
        perror(m); 
        exit(EXIT_FAILURE); 
    } while(0)
     
void handler(int sig)
{
    printf("recv sig=%dn", sig);
}
     
int main(int argc, char *argv[])
{
    signal(SIGPIPE, handler);
     
    int pipefd[2];
    if (pipe(pipefd) == -1)
        ERR_EXIT("pipe error");
     
    pid_t pid;
    pid = fork();
    if (pid == -1)
        ERR_EXIT("fork error");
     
    if (pid == 0)
    {
        close(pipefd[1]);
        exit(EXIT_SUCCESS);
    }
     
    close(pipefd[1]);
    sleep(1);
    char buf[10] = {0};
    int ret = read(pipefd[0], buf, 10);
    printf("ret = %dn", ret);
     
    return 0;
}

输出测试如下：

simba@ubuntu:~/Documents/code/linux_programming/APUE/pipe$ ./close_fd_write
ret = 0

同样地父进程睡眠1s确保所有的写端文件描述符都已经关闭，read返回0。

五、当要写入的数据量不大于PIPE_BUF时，linux将保证写入的原子性；当要写入的数据量大于PIPE_BUF时，linux将不再保证写入的原子性。

On Linux, PIPE_BUF is 4096 bytes。

The precise semantics depend on whether the file descriptor is nonblocking (O_NONBLOCK), whether there are multiple writers to the pipe, and on n, the number of bytes to be written。即由文件描述符的标志，是否有多个进程向管道写入以及写入的字节数所决定准确的语义，总共分4种情况，具体可man一下。

下面的程序演示O_NONBLOCK disabled ，size > PIPE_BUF(4K)的情况：

/*************************************************************************
    > File Name: process_.c
    > Author: Simba
    > Mail: dameng34@163.com
    > Created Time: Sat 23 Feb 2013 02:34:02 PM CST
 ************************************************************************/
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>
#include<signal.h>
#define ERR_EXIT(m) 
    do { 
        perror(m); 
        exit(EXIT_FAILURE); 
    } while(0)
     
#define TEST_SIZE 68*1024 // 68KB
/* 默认O_NONBLOCK disabled ，这里验证 size > PIPE_BUF(4K)的情况 */
int main(int argc, char *argv[])
{
    char a[TEST_SIZE];
    char b[TEST_SIZE];
     
    memset(a, 'A', sizeof(a));
    memset(b, 'B', sizeof(b));
     
    int pipefd[2];
    int ret = pipe(pipefd);
    if (ret == -1)
        ERR_EXIT("pipe error");
     
    int pid = fork();
    if (pid == 0)
    {
     
        close(pipefd[0]);
        ret = write(pipefd[1], a, sizeof(a)); // 全部写完才返回
        printf("apid=%d write %d bytes to pipen", getpid(), ret);
        exit(0);
    }
     
    pid = fork();
     
    if (pid == 0)
    {
     
        close(pipefd[0]);
        ret = write(pipefd[1], b, sizeof(b));
        printf("bpid=%d write %d bytes to pipen", getpid(), ret);
        exit(0);
    }
     
    close(pipefd[1]);
     
    sleep(1);
     
    int fd = open("test.txt", O_WRONLY | O_CREAT | O_TRUNC, 0664);
    char buf[1024 * 4] = {0};
    int n = 1;
    while (1)
    {
        ret = read(pipefd[0], buf, sizeof(buf)); //当管道被写入数据，就已经可以开始读了,每次读取4k
        if (ret == 0) // 管道写端全部关闭,即读到了结尾
            break;
        printf("n=%02d pid=%d read %d bytes from pipe buf[4095]=%cn",
               n++, getpid(), ret, buf[4095]);
        write(fd, buf, ret);
    }
     
    return 0;
}

输出测试如下：

simba@ubuntu:~/Documents/code/linux_programming/APUE/pipe$ ./pipe_buf
n=01 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=02 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=03 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=04 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=05 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=06 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=07 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=08 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=09 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=10 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=11 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=12 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=13 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=14 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=15 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=16 pid=7137 read 4096 bytes from pipe buf[4095]=B
n=17 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=18 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=19 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=20 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=21 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=22 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=23 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=24 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=25 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=26 pid=7137 read 4096 bytes from pipe buf[4095]=A
apid=7138 write 69632 bytes to pipe
n=27 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=28 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=29 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=30 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=31 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=32 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=33 pid=7137 read 4096 bytes from pipe buf[4095]=A
n=34 pid=7137 read 4096 bytes from pipe buf[4095]=B
bpid=7139 write 69632 bytes to pipe

分析一下：现在的情况是有两个子进程在对管道进行阻塞写入各68k，即每个子进程完全写入68k才返回，而父进程对管道进行阻塞读取，每次读取4k，打印每4k中的最后一个字符。需要注意的是是边写边读，因为前面说过管道的容量只有64k，当管道被写满时子进程就阻塞等待父进程读取后再写入。由上面输出可以看出B进程先写入64k的B，然后A进程写入68k的A之后B进程接着写完最后4K的B，然后write返回。由A进程write完毕输出的提示可知此时A进程已经写完成了，但父进程还没读取A完毕，当两个子进程全部写完退出时关闭写端文件描述符，则父进程read就会返回0，退出while循环。可以得出结论：当多个进程对管道进行写入，且一次性写入数据量大于PIPE_BUF时，则不能保证写入的原子性，即可能数据是穿插着的。man 手册的解释如下：

O_NONBLOCK disabled, n > PIPE_BUF
The write is nonatomic: the data given to write(2) may be interleaved with write(2)s by other process; the write(2) blocks until n bytes have been written.

注意我们这里设定了size=68k，则写端不能设置成非阻塞，因为PIPE_BUF只有64k，不能一次性写入68k，故只能返回-1，且一个字符也不写入，读端也不能设置为非阻塞，因为有可能此时管道还没被写入数据或者不够一次性read的数据量的时候去读，则直接返回-1，不会阻塞等待足够数据量后进行读取。总之测试4种不同情形下的情况也应设置不同的条件。

管道的前4种读写规则具有普遍意义，Tcp socket 也具有管道的这些特性。