how to synchronize two process - c

Question

Hello I'm struggling right now with C language and process so basically I've just learnt about pipe and I want to use them just to exercise myself on it, so I want to try a code that basically use two child and 1 father, by one child the user enter some number then this child send those numbers to the other child and then this second child send them to the father who show them.
here my code so far
`
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <limits.h>
#include <string.h>
int main(int argc, char const *argv[])
{
pid_t son1;
pid_t son2;
int Pipe1[2];
int pipe2[2];
int sent=0;
int sent2=0;
int recive=0;
int recive2=0;
int j=0;
int mem[3];
if (pipe(Pipe1)==-1)
{
printf("error pipe1");
exit(0);
}
if (pipe(pipe2)==-1)
{
printf("error pipe2");
exit(0);
}
son1=fork();
if (son1==0)
{
close(Pipe1[0]);
//close(pipe2[0]);
//close(pipe2[1]);
printf("i'am the child 1\n");
for (int i = 0; i < 3; i++)
{
printf("type your number \n");
scanf("%d",&sent);
write(Pipe1[1],&sent,sizeof(int));
}
close(Pipe1[1]);
}
son2=fork();
if (son2==0)
{
close(Pipe1[1]);
close(pipe2[0]);
printf("i'am the son number 2 \n");
recive=read(Pipe1[0],&sent,sizeof(int));
while(recive == sizeof(int))
{
printf("nb reçu %d \n",sent);
mem[j]=sent;
recive= read(Pipe1[0],&sent,sizeof(int));
j++;
}
close(Pipe1[0]);
for (int p = 0; p < 3; p++)
{
sent2=mem[p];
printf("sent2 %d",sent2);
write(pipe2[1],&sent2,sizeof(int));
}
close(pipe2[1]);
when i run this code it does work but not how i expect, the commmunication between children work but not between child 2 and the father actualy if you look at the second part of the code of son2 "mem[]" value is not the same before close(pipe[1]) and after and this is why the comunication is mestup but i realy dont know how the value can change.. if someone can explain me it will be really kind of him
`

Answer 1

You create two pipes in the parent which are shared with the children, and the children do not share a pipe with each other. The easiest thing is probably to move the 2nd pipe to the child and have it fork the 2nd child:
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
int main(void) {
int parent_child[2];
if(pipe(parent_child) == -1) {
printf("pipe failed\n");
exit(1);
}
if(!fork()) {
close(parent_child[0]);
int child_grand_child[2];
if(pipe(child_grand_child) == -1) {
printf("pipe failed\n");
exit(1);
}
printf("child write\n");
write(child_grand_child[1], "1", 1);
if(!fork()) {
char buf[1];
ssize_t n = read(child_grand_child[0], buf, sizeof buf);
printf("grand child read\n");
write(parent_child[1], buf, n);
exit(0);
}
close(parent_child[1]);
wait(&(int) {0});
exit(0);
}
close(parent_child[1]);
char buf[1];
ssize_t n = read(parent_child[0], buf, sizeof buf);
printf("parent read %*s\n", n, buf);
wait(&(int) {0});
}
which outputs:
child write
grand child read
parent read 1