Tracing the execution of a process - c

Question

I have this code that gets its process ID and its parent process:
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main(){
int pid;
printf("I am the original process with PID %d and PPID %d. \n", getpid(), getppid());
pid = fork();
if (pid >0){
printf("I am the original process with PID %d and PPID %d \n", getpid(), getppid());
printf("My child’s pid is %d \n" , pid);
}
else if(pid == 0) {
printf ("I am the original process with PID %d and PPID %d \n", getpid(), getppid());
}
else if (pid == 1){
printf ("Error – no child process was created \n");
}else{
printf ("Error – system error \n");
}
printf (" PID %d terminates \n", getpid()); /* both processes execute this instruction */
return 0;
}
OUTPUT
I am the original process with PID 1009 and PPID 964.
I am the original process with PID 1009 and PPID 964
My child’s pid is 1010
PID 1009 terminates
I am the original process with PID 1010 and PPID 1009
PID 1010 terminates
Few questions that confuses me..
how does this code is executed?
In the output, you can see that it runs the code under under if(pid == 0) while the condition if(pid > 0) is already executed. how come pid is equal to 0? while it was set already to greater than 0.
Lastly , what does fork() really do?

Answer 1

fork() makes multiple processes, or a "child". So the parent and child execute the code. The parent's pid is > 0 and the child's is ==0.
So, the parent and child execute at similar times starting at the fork command. So let's start with the parent. The parent checks the first statement (pid > 0) and finds it is true, so it prints out the two statements. Then it goes all the way to the print statement after the last else.
Now to the child. The child checks the first if statement, and is false. Checks the next (pid == 0) and finds it is true. So now it's going to print out that statement. Now it will skip to the print statement after the else and prints again.
Note: The parent and child can execute at different times, so if you run the code multiple times the output may be in a different order.