Question

a program about pipe

Asked by: changxing

I am a Linux beginner. I wrote a small program about pipe to simulate the ls -s the current dircetory. But it can't work. So can you tell me what error happens?

codes as these:

#include <stdio.h>

int jp( cmd1, cmd2 )
char *cmd1[], *cmd2[];
{
     int status, pfd[2], pid;
     char *mesg;
     FILE *fp;
     if( pipe(pfd) == -1 )
     {
          perror( "pipe" );
          exit( 5 );
     }
     if( (pid=fork()) == -1 )
     {
          perror( "fork" );
          exit( 1 );
     }
     else
     {
          if( pid == 0 )
          {
               close( 1 );
               dup( pfd[1] );
               close( pfd[0] );
               close( pfd[1] );
               execvp( cmd1[0], cmd1 );
               perror( "execvp" );
               exit( 2 );
          }
          else
          {
               wait( NULL );
               close( 0 );
               dup( pfd[0] );
               close( pfd[0] );
               execvp( cmd2[0], cmd2 );
               perror( "execvp 2" );
               exit( 3 );
          }
     }
}
#define CL 30

main()
{
     static char *cmd1[CL]={ "ls", "-s", (char *)0 };
     static char *cmd2[CL]={"sort" };
     if( jp(cmd1,cmd2) != 0 )
     {
          fprintf( stderr, "\nPipe error\n" );
          exit( 4 );
     }
     exit( 0 );
}



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Asked On
2001-12-04 at 19:44:29ID20243722
Tags

pipe

,

linux

,

execvp

Topic

Linux Programming

Participating Experts
3
Points
50
Comments
4

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Answers

 

by: MFCRichPosted on 2001-12-05 at 11:28:42ID: 6667961

First, you dont have the NULL pointer at the end of cmd2.
Second, the function jp never returns (no return statement)

 

by: bearwarePosted on 2002-03-19 at 02:58:25ID: 6879529

Should check return of pipe notequ 0 is fail.

Use dup2 its better than dup, dont need the close.
dup2( Pipe[1],STDOUT_FILENO ) );
dup2( Pipe[0], STDIN_FILENO ) );
(one in each task)

Why does the parent wait?
Get rid of Wait.

jp does not exit.

Main should be:
int main ( int argc, char *argv[] )
at it returns (via exit)

P.S. you program does not exist once the to commands have run. You could produce to children to do the work.

P.S its K&R style not ANSI style for function arguments.

 

by: jmcgPosted on 2003-12-06 at 22:05:37ID: 9891169

No comment has been added lately, so it's time to clean up this TA.
I will leave the following recommendation for this question in the Cleanup topic area:

Split: MFCRich {http:#6667961} & bearware {http:#6879529}

Please leave any comments here within the next seven days.
PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!

jmcg
EE Cleanup Volunteer

20120131-EE-VQP-002

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