I'm studying computer systems and I've made this very simple function which uses fork() to create a child process. fork() returns a pid_t that is 0 if it's a child process. But calling the getpid() function within this child process returns a different, nonzero pid. In the code I have below, is newPid only meaningful in the context of the program, and not to the operating system? Is it possibly only a relative value, measured against the pid of the parent?

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>

void unixError(char* msg)
{
    printf("%s: %s\n", msg, strerror(errno));
    exit(0);
}

pid_t Fork()
{
    pid_t pid;
    if ((pid = fork()) < 0)
        unixError("Fork error");
    return pid;
}


int main(int argc, const char * argv[])
{
    pid_t thisPid, parentPid, newPid;
    int count = 0;
    thisPid = getpid();
    parentPid = getppid();

    printf("thisPid = %d, parent pid = %d\n", thisPid, parentPid);

    if ((newPid = Fork()) == 0) {
        count++;
        printf("I am teh child. My pid is %d, my other pid is %d\n", getpid(), newPid);
        exit(0);
    }
    printf("I am the parent. My pid is %d\n", thisPid);
    return 0;
}

Output:

thisPid = 30050, parent pid = 30049
I am the parent. My pid is 30050
I am teh child. My pid is 30052, my other pid is 0

Lastly, why is the child's pid 2 higher than the parent's, and not 1? The difference between the main function's pid and its parent is 1, but when we create a child it increments the pid by 2. Why is that?