C小程序 - pipe与fork结合

                                   pipe(p)
                                   fork()
                                      |
--------------------------------------------------------------------------------
| child process                                                         | parent process
close(p[0]);                                                              close(p[1])
dup2(p[1], 1)                                                           dup2(p[0],0)
close(p[1])                                                               close(p[0])
execlp "who"                                                           exec "sort"

这里要了解一个系统调用函数 execlp 的用法
#include
int execlp(const char * file,const char * arg,...,(char *)0);

execlp()会从PATH 环境变量所指的目录中查找符合参数file的文件名，找到后便执行该文件，然后将第二个以后的参数当做该文件的argv[0]、argv[1]……，最后一个参数必须用空指针(NULL)作结束。如果用常数0来表示一个空指针，则必须将它强制转换为一个字符指针，否则将它解释为整形参数，如果一个整形数的长度与char * 的长度不同，那么exec函数的实际参数就将出错。如果调用成功,进程自己的执行代码就会变成加载程序的代码,execlp()后边的代码也就不会执行了.

#include    <stdio.h>
#include    <unistd.h>

#define    oops(m,x)    { perror(m); exit(x); }

main(int ac, char **av)
{
    int    thepipe[2],            /* two file descriptors    */
        newfd,                /* useful for pipes    */
        pid;                /* and the pid        */

    if ( ac != 3 ){
        fprintf(stderr, "usage: pipe cmd1 cmd2\n");
        exit(1);
    }
    if ( pipe( thepipe ) == -1 )        /* get a pipe        */
        oops("Cannot get a pipe", 1);

    /* ------------------------------------------------------------ */
    /*    now we have a pipe, now let's get two processes        */

    if ( (pid = fork()) == -1 )            /* get a proc    */
        oops("Cannot fork", 2);

    /* ------------------------------------------------------------ */
    /*     Right Here, there are two processes            */
    /*             parent will read from pipe            */

    if ( pid > 0 ){            /* parent will exec av[2]    */
        close(thepipe[1]);    /* parent doesn't write to pipe    */

        if ( dup2(thepipe[0], 0) == -1 )
            oops("could not redirect stdin",3);

        close(thepipe[0]);    /* stdin is duped, close pipe    */
        execlp( av[2], av[2], NULL);
        oops(av[2], 4);
    }

    /*     child execs av[1] and writes into pipe            */

    close(thepipe[0]);        /* child doesn't read from pipe    */

    if ( dup2(thepipe[1], 1) == -1 )
        oops("could not redirect stdout", 4);

    close(thepipe[1]);        /* stdout is duped, close pipe    */
    execlp( av[1], av[1], NULL);
    oops(av[1], 5);
}

可以这样运行：
#./pipe who sort