why out put is blank....

[alokdotnet]

hi friends,

in this program, i m creating a pipe and creating two child process of main which will write and read in this pipe.
no error no warning, but when i m running the a.out, screen is blank, and waiting for somthing, for infinite time. ( its a gcc code)

      1 // create pipe between two processes
      2 #include<stdio.h>
      3 #include<unistd.h>
      4
      5 main()
      6     {
      7         int fd[2];
      8         int report[2];
      9         int freturn ;
     10         freturn = pipe(fd);
     11         if(freturn < 0)
     12             {
     13                 fprintf(stderr,"Cant create pipe..\n");
     14                 exit(1) ;
     15             }
     16         switch(fork())
     17             {
     18                 case -1 : fprintf(stderr,"Cant fork..\n") ;
     19                           exit(2) ;
     20                 case 0 : // fork is successfull
     21                         close(1) ; //1 is writing fd
     22                         if(dup(fd[1]) == -1)
     23                             {     
     24                                 fprintf(stdout,"Can't dup..\n") ;
     25                                 perror("dup");
     26                                 exit(2);
     27                             }
     28                         if(execlp("ls","ls", "-l","-a", NULL) == -1)
     29                             {
     30                                 fprintf(stderr,"Cant exec..\n") ;
     31                                 exit(3) ;
     32                             }
     33                         break ;
     34             }// end of switch1
     35
     36         switch(fork())
     37             {
     38                 case -1 :fprintf(stderr,"Cant fork..\n") ;
     39                          exit(4) ;
     40                 case 0 : //fork is successfull
     41                          close(0);
     42                          if(dup(fd[0]) == -1)
     43                             {
     44                                 fprintf(stderr,"Cant dup..\n") ;
     45                                 perror("2nd dup") ;
     46                                 exit(5) ;
     47                             }
     48                         if(execlp("wc","wc", NULL) == -1 )
     49                             {
     50                                 fprintf(stderr,"Cant exec..\n") ;
     51                                 exit(6) ;
     52                             }
     53                         break ;
     54             } // end of switch 2
     55
     56         wait(&report[0]) ;
     57         wait(&report[1]) ;
     58     } // end of main
     59

[Yasir_Malik]

You are creating a parent with two child, or sibling, processes. I don't think siblings can communicateover a pipe. The childs can only communicate to the parent over a pipe. I assume you are trying to implement "pipe" functionality. What you should do is have the parent fork a child, have the parent exec ls, the child exec wc, and redirect the parent's output to the child's input. Therefore, you should have something like:

rc = fork();

if(rc)
{
  //parent
  exec("ls");
}
else if(!rc)
{
  exec(wc);
}

[fnoyan]

Well, are you sure (or does the system guarantee) which process is the one that will be run next?! If I were you, I would use some mutex mechanisms to synchonize the processes.

[Yasir_Malik]

I thought about that too. However, if you run wc, you'll notice that it waits for input, so synchronization is done. For example:

#include <unistd.h>

int main()
{
 int fds[2];
 int pid;

 pipe(fds);

 if((pid = fork()) < 0)
 {
  perror("fork");
  exit(1);
 }
 else if(pid)
 {
  /* parent */
  close(fds[0]);

  /* made standard output the same as pipe's write end */
  dup2(fds[1], 1);

  close(fds[1]);
  execlp("ls","ls", "-l","-a", NULL);
 }
 else
 {
  /* child */
  close(fds[1]);

  /* made standard input the same as pipe's read end */
  dup2(fds[0], 0);

  close(fds[0]);
  execlp("wc","wc", NULL);
 }
}