基本shell实施中的管道

Question

In an assignment we were asked to implement piping. I was able to do so, but the problem which I am encountering is that when I enter the command

ls | grep 'a.out'

then the output is

a.out

a.out

but when I do

ls | wc

The output comes only once. Can anyone point out the mistake in the code ?. The code is as follows :

void execute_pipe(char **argv,char **args)
{

int pfds[2];
pid_t pid,pid2;
int status,status2;
pipe(pfds);
if ((pid = fork()) < 0) {    
      printf("*** ERROR: forking child process failed\n");
      exit(1);
 }
if ((pid2 = fork()) < 0) {    
      printf("*** ERROR: forking child process failed\n");
      exit(1);
 }
if (pid==0) {
    close(1);     
    dup(pfds[1]);  
    close(pfds[0]); 
close(pfds[1]);
    if(execvp(argv[0],argv)<0){
        printf("**error in exec");
    }
} 
else if(pid2==0){

    close(0);       
    dup(pfds[0]);  
    close(pfds[1]);
close(pfds[0]);
    if(execvp(args[0],args)<0){
    printf("**error in exec");

}
}
else{
    close(pfds[0]);
    close(pfds[1]);
    while (wait(&status) != pid)  ;
    while (wait(&status2) != pid2)  ;
        }
}

I ma quite sure that there are no stray prints and there is only one a.out in the directory.

Answer 1

You run the second fork in both the parent and first child processes. You should only do the second fork if pid is non-zero.