Spawning C child process for different jobs? - c

Question

So I am trying do a application which fork()s 2 children.
first does a for(i=1; i<=50000; i++) loop
the second a for(i=50000; i<=100000; i++) loop
the parent does for(asciic=65; asciic<=90; asciic++)-> loop for printing A to Z letters
I need all three to do they're work simultaneous not one after another.
I've looked over the internet and I couldn't found a proper way, all I could find are loops which create the children processes but they do almost the same thing and most are created one after another.
Any help is appreciated.
To be more understood, this is what I've done before posting:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
int main(void) {
pid_t child_pid,child_pid1;
int i=0;
int stare;
int asciic;
child_pid=fork();
child_pid1=fork();
if (child_pid==0) {
//printf("Father PID: %d -> Child 1: %d\n",getppid(),getpid());
for(i=1; i<=50000; i++){
printf("%d-%d\n",getpid(),i);
}
exit(0);
} else if (child_pid1==0) {
//printf("Father PID: %d -> Child 2: %d\n",getppid(),getpid());
for(i=50000; i<=100000; i++) {
printf("%d-%d\n",getpid(),i);
}
exit(0);
} else {
//printf("PID-ul procesului parinte: %d\n", getpid());
pid_t rez=waitpid(child_pid,&stare,WNOHANG);
pid_t rez1=waitpid(child_pid1,&stare,WNOHANG);
while(rez==0 || rez1==0){
for(asciic=65; asciic<=90; asciic++){
printf("%d- %c\n",getpid(),asciic);
}
rez=waitpid(child_pid,&stare,WNOHANG);
rez1=waitpid(child_pid1,&stare,WNOHANG);
}
}
return 0;
}
If I comment out the loops I can see that the children have different PIDs, 1 child has the right parent PID and other it has other parent PID.

Answer 1

Your 2 lines with forks:
child_pid=fork();
child_pid1=fork();
do not create 2 children, but three: the parent creates a first child in the first fork(). From that moment there are 2 processes: parent and child. And each of them executes the second fork(). You will have in total 1 parent, 2 children and 1 grandchild.
In order to have just 1 parent and 2 children, you will have to:
pid1 = fork();
if (pid1 < 0) {
/* Error in fork() */
} else if (pid1 == 0) {
/* first child */
exit(0);
}
pid2 = fork();
if (pid2 < 0) {
/* Error in fork() */
} else if (pid2 == 0) {
/* second child */
exit(0);
}
/* parent */
Moreover, even when your code is correct, you cannot "see" if the processes are running concurrently or not just looking at their outputs. In fact, even if there are multiple processes executing "at the same time", you may see that one of the processes finishes before another one starts. That is because typically the kernel does time multiplexing to offer each child some CPU time. You can see concurrency if the processes take longer to complete, for example adding some sleep().

Answer 2

The problem is here:
child_pid=fork();
child_pid1=fork(); // This line will be executed by both parent and first child!!!
You need to move the second fork into the parent part of the first if, and then have a separate if for the second child.

Answer 3

fork() returns the pid_t of the child to the caller (parent), or -1 if it failed. In the child, 0 is returned. You can simply test
if (fork())
{
//do one thing
}
else
{
//do something else
}