do parent and child process execute concurrently - c

Question

I'm trying to figure out how the following program i wrote for a raspberry pi with led matrix works.
The program is supposed to have a child process forked by the main process light up the LED from 0,0 -0,8 in first iteration for num=0. But instead, the LEDs light up randomly in the 0th column.
Example : in 0th column 0,6,4,5,2,3,1
in 1st column 0,6,5,4,2,3,1
and so on till 8th column.
I'm confused as to why the "main" for loop controlling the rows runs randomly ,but the second loop controlling the columns works in an orderly increasing fashion.
What am I missing in the working of fork together with a for loop?
From the output I can only guess that the parent is somehow creating random child processes all at once that are calling function2 with their randomized num argument.
<header files>
function1
{ creates a delay of a second through calculations }
function2(n)
{ for(int i = 0; i<=8 ;i++)
function1;
printf("LED(%d,%d)\n",n,i);
}
main()
{
for(int num = 0; num <=7; num++)
{
pid_t pid = fork();
if (pid ==0)
{ function2(num); }
}
for (int num =0; num <= 7; num++)
{ wait(NULL);}
}

Answer 1

Father and child processes do run concurrently, and it is not predictable which process is running at what time.
From a first inspection you have a loop that starts from the father process and creates 8 child processes, that on their hand each creates other child processes! You should terminate the child process after calling the function2, otherwise the child processes will create other processes.
I think the solution to your problem is something like this:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
void function1() {
//0.5 seconds sleep
usleep(500000);
}
void function2(int n) {
for(int i = 0; i <= 8; i++) {
function1();
printf("LED(%d,%d)\n", n, i);
}
}
int main()
{
pid_t pid;
for(int num = 0; num <= 7; num++) {
pid = fork();
if (pid == 0) {
function2(num);
exit(0);
}
}
for (int num = 0; num <= 7; num++){
wait(NULL);
}
return 0;
}
Please for future questions please post a Minimal, Complete, and Verifiable example.
The code could also have a concurrency problem, I don't think the printf is an atomic function, so a semaphore may be necessary to avoid having multiple processes calling the printf.
Another note is that a check should be fork() to check if the child process was successfully generated.