1 /* pipe.c 2  * Demostrates how to create a pipeline from one process to another 3  * * take two args, each a command ,and connects 4  *   argv[1]s output to intput of argv[2] 5  *   effect: command1 | command2 6  */ 7 #include <stdio.h> 8 #include <unistd.h> 9 10 #define oops(x,n) { perror(x); exit(n); }11 12 int main(int argc, char *argv[]){13   int pid, thepipe[2];14   if(argc!=3) {15     fprintf(stderr, "usage: pipe cmd1 cmd2.\n");16     exit(1);17   }18   if(pipe(thepipe) == -1)19     oops("pipe", 2);20   if((pid=fork()) == -1)21     oops("fork", 3);22   if(pid==0) {                  /*child execs argv[1] and writes into pipe*/23     close(thepipe[0]);          /*      child doesnt read from pipe       */24     if((dup2(thepipe[1], 1)) == -1)     /*child change the stdout, set the stdout into the pipe*/25       oops(argv[1], 6);26     close(thepipe[1]);          /*stdout is duped , close pipe*/27     execlp(argv[1], argv[1], NULL);28     oops("child", 4);29   }else if(pid>0) {             /*parent execs argv[0] and read from pipe*/30     close(thepipe[1]);          /*     parent doesnt write to pipe       */31     if(dup2(thepipe[0], 0) == -1)32       oops("could not redirect stdin", 6);33     close(thepipe[0]);          /*     stdin is duped, close pipe        */34     execlp(argv[2],argv[2], NULL);35     oops(argv[2], 5);36   }37   return 0;38 }