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?
