Creating a Pipe with parent and child processes

[jre247]

Hello,

I am trying to create a pipe that will direct stdout to in side of the pipe, and stdin to the out side of the pipe - I created two child processes to handle this. However, my pipe doesn't seem to be working correctly. Did I use execv() correctly? Here is my code below for the pipe function. I'd greatly appreciate help from someone. Thanks!

/* cisshPipe.c
*/

/* External functions --
*/
extern void error(char* message);
//extern wait(int*status);



/* cisshPipe(char* command1[], char* command2[])
* handles command lines with pipes.
*/

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

void
cisshPipe(char* command1[], char* command2[])
{
pid_t pid;
pid_t pid1;
int status;
int commpipe[2]; /* This holds the input and output of the pipe */

if ( (pid = fork()) == -1 ){
fprintf(stderr,"Fork error. Exiting.\n"); /* something went wrong with forking */
}

if (pid ==0) {
// first child process

if(pipe(commpipe)){ /* Setup communication pipeline */
fprintf(stderr,"Pipe error\n");
exit(1);
}

close(commpipe[1]);
dup2(commpipe[0],1); /* Replace in side of pipe with stdout */
close(commpipe[0]);
execv(command1[0], command1);



if ( (pid1 = fork()) == -1 ){
fprintf(stderr,"Fork error. Exiting.\n"); /* something went wrong with forking */
}

if(pid1 == 0){
/* second child process */

close(commpipe[0]);
dup2(commpipe[1],0); /* Replace out side of pipe with stdin */
close(commpipe[1]);
execv(command2[0], command2);


}

else {
close(commpipe[1]);
close(commpipe[0]);


if(wait(&status) < 0)
{
error("cissh: error waiting for child.");
perror("wait");
}

}
}


else {
close(commpipe[1]);
close(commpipe[0]);


if(wait(&status) < 0)
{
error("cissh: error waiting for child.");
perror("wait");
}
}



}

[jsmith]

You need to create the pipe in the parent before forking.

[jre247]

I created the pipe in the parent process, however, the pipe still doesn't work. Are you able to let me know what I'm still doing wrong? I've pasted the new code below. Thanks!

/* cisshPipe.c
*/

/* External functions --
*/
extern void error(char* message);
//extern wait(int*status);



/* cisshPipe(char* command1[], char* command2[])
* handles command lines with pipes.
*/

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

void
cisshPipe(char* command1[], char* command2[])
{
pid_t pid;
pid_t pid1;
int status;
int commpipe[2]; /* This holds the input and output of the pipe */

if ( (pid = fork()) == -1 ){
fprintf(stderr,"Fork error. Exiting.\n"); /* something went wrong with forking */
}




if (pid ==0) {
// first child process



close(commpipe[1]);
dup2(commpipe[0],1); /* Replace in side of pipe with stdout */
close(commpipe[0]);
execv(command1[0], command1);



if ( (pid1 = fork()) == -1 ){
fprintf(stderr,"Fork error. Exiting.\n"); /* something went wrong with forking */
}

if(pid1 == 0){
/* second child process */

close(commpipe[0]);
dup2(commpipe[1],0); /* Replace out side of pipe with stdin */
close(commpipe[1]);
execv(command2[0], command2);


}

else {
close(commpipe[1]);
close(commpipe[0]);


if(wait(&status) < 0)
{
error("cissh: error waiting for child.");
perror("wait");
}

}
}


else {

if(pipe(commpipe)){ /* Setup communication pipeline */
fprintf(stderr,"Pipe error\n");
exit(1);
}


close(commpipe[1]);
close(commpipe[0]);


if(wait(&status) < 0)
{
error("cissh: error waiting for child.");
perror("wait");
}
}



}

[jsmith]

You need to create the pipe in the parent before forking.