Need help with pipe to redirect input/ output

[azi]

I have created two child processes. I am trying to tell one process to
write a command to the pipe, and I want the other child to read from the
pipe and print it out to the stdout. I want first child to write the
result of ps command with argument -aef to pipe, and the second child to
look at the pipe's output and run grep command to print all processes
running csh. I have used execl("usr/bin/ps -aef", "ps",0); to write the
result of ps to the pipe, but I don't know how to make the second child
read it and pick processors with csh??????????

I would appreciate your help.

int pipefd;
pipe(pipefd);
switch(pid2=fork())// This one writes to pipe
case -1: //error
case 0: close(1);
dup(pipefd[1]);
close(pipefd[0]);
execl("usr/bin/ps -aef", "ps",0);

switch(pid3=fork())// This one reads from pipe and prints to stdout
case -1: //error
case 0: close(0);
dup(pipefd[0]);
close(pipefd[1]);
//Need to read output from pipe and print to stdout???????

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[Jens.Toerring@physik.fu-berlin.de]

azi <member34294@dbforums.com> wrote:

> I have created two child processes. I am trying to tell one process to
> write a command to the pipe, and I want the other child to read from the
> pipe and print it out to the stdout. I want first child to write the
> result of ps command with argument -aef to pipe, and the second child to
> look at the pipe's output and run grep command to print all processes
> running csh. I have used execl("usr/bin/ps -aef", "ps",0); to write the
> result of ps to the pipe, but I don't know how to make the second child
> read it and pick processors with csh??????????

> I would appreciate your help.

> int pipefd;

This must be an array of 2 ints.

> pipe(pipefd);
> switch(pid2=fork())// This one writes to pipe
> case -1: //error
> case 0: close(1);
> dup(pipefd[1]);
> close(pipefd[0]);
> execl("usr/bin/ps -aef", "ps",0);

This should be

execl( "/usr/bin/ps", "ps" "-aef", NULL );

And check if 'ps' is really living in /usr/bin, on some systems it's
in /bin or even /sbin and if you get it wrong execl() will return
and not run ps.

> switch(pid3=fork())// This one reads from pipe and prints to stdout
> case -1: //error
> case 0: close(0);
> dup(pipefd[0]);
> close(pipefd[1]);
> //Need to read output from pipe and print to stdout???????

What's the problem? Just use read(2) to read on the pipe and whatever
you fancy to write it to stdout. Here's a working version of your
program:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main( void )
{
int pipefd[ 2 ];
char buffer[ 80 ];
ssize_t size;

if ( pipe( pipefd ) < 0 )
exit( EXIT_FAILURE );

switch( fork( ) )
{
case -1 :
exit( EXIT_FAILURE );

case 0 :
close( 1 );
dup( pipefd[ 1 ] );
close( pipefd[ 0 ] );
execl( "/bin/ps", "ps" "-aef", NULL );
exit( EXIT_FAILURE );
}

switch( fork( ) )
{
case -1 :
exit( 1 );

case 0 :
close( 0 );
dup( pipefd[ 0 ] );
close( pipefd[ 1 ] );
while ( ( size = read( 0, buffer, 80 ) ) > 0 )
write( 1, buffer, size );
}

return EXIT_SUCCESS;
}
Reagrds, Jens
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