Can I use pipe as read at parent and write in child? - c

Question

I'm now learning how to use pipes correctly. I found examples only for write in parent and read in child, but I want to know how can I do it reverse. I tried like this:
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <sys/types.h>
int main(){
int p1[2], p2[2];
char original[]="This is a string\n ";
pid_t child2;
pid_t child=fork();
if (child>0)
{
child2 = fork();
if(child2>0){
wait();
printf("I'm the parrent.\n");
close(p1[1]);
close(p2[1]);
printf("Parrent read p1:\n");
if (read(p1[0], original, sizeof(original)) == -1)
perror("read() error in parent p1");
else printf("parent read '%s' from pipe 1\n", original);
printf("Parrent read p2:\n");
if (read(p2[0], original, sizeof(original)) == -1)
perror("read() error in parent p2");
else printf("parent read '%s' from pipe 2\n", original);
}
else{
printf("Child2 \n");
pipe(p2);
close(p2[0]);
if (write(p2[1], original, sizeof(original)+1) == -1)
perror("write() error in child2");
//close(p2[1]);
}
}
else
{
printf("Child1 \n");
pipe(p1);
close(p1[0]);
if (write(p1[1], original, sizeof(original)+1) == -1)
perror("write() error in child1");
//close(p1[1]);
}
return 0;
}
But this way give to me error at reading in parent. read() error in parent p1: Bad file descriptor both times, at p1 and also at p2. So, can I do this this way or not? Or this is just something trivial error?

Answer 1

As already said you just need to invert the indexes for closing the pipes and the read/write.
Your code was almost correct. Two things were wrong: on the parent you were first closing the pipes and then calling pipe(): it should be the other way round: first you create both of the pipes and then close the corresponding element, all in the parent.
Secondly you should call wait after setting the pipes otherwise it does not work. Guessing you want to wait for all the children you should call wait(NULL). I do not know what you meant with wait().
The complete code:
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
int main(){
int p1[2], p2[2];
char original[]="This is a string\n ";
pid_t child2;
pid_t child=fork();
if (child>0)
{
child2 = fork();
if(child2>0){
printf("I'm the parrent.\n");
pipe(p1); //ADDED
pipe(p2); //ADDED
close(p1[1]);
close(p2[1]);
wait(NULL); //DON'T GET IT WHY? if u want to wait all children,
//wait after setting the pipes.
printf("Parrent read p1:\n");
if (read(p1[0], original, sizeof(original)) == -1)
perror("read() error in parent p1");
else printf("parent read '%s' from pipe 1\n", original);
printf("Parrent read p2:\n");
if (read(p2[0], original, sizeof(original)) == -1)
perror("read() error in parent p2");
else printf("parent read '%s' from pipe 2\n", original);
}
else{
printf("Child2 \n");
//pipe(p2); ERROR HERE
close(p2[0]);
if (write(p2[1], original, sizeof(original)+1) == -1)
perror("write() error in child2");
//close(p2[1]);
}
}
}