To better understand how processes work, fork, I wrote this test program:
#include <unistd.h>
#include <stdio.h>
int main(){
int value = 9 ;
int pid = fork();
if(pid==0) value+=2 ;
else value-=5 ;
printf("%d",value);
}
The output was
411
This was not quite what I expected.
After fork() (if there was no error) there are two distinct variables with name value . One is in the original process (with 9 value), another is in the child process (also with 9 value).
if (pid == 0) does different things in 2 active processes. One of them increases 2 to its valor and in the other it decreases 5 to other valor .
Then the two processes write their variable, without request of the order by which this writing is done.
One of the processes has valor with 4 (9 - 5), another has 11 (9 + 2).
Both 411 and 114 are perfectly acceptable results.