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<unistd.h> 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 |