linux - In C programming language, how can variable store two values? -

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• how possible fork() return 2 values? 5 answers

i've decided learn c, , here snippet 1 of books use:

#include <sys/types.h> #include <unistd.h> #include <stdio.h>  int main() {     pid_t result = fork();      if (result == -1){         fprintf(stderr, "error\n");         return 1;     }      if (result == 0)         printf("i'm child pid = %d\n", getpid());     else         printf("i'm parent pid = %d\n", getpid());      return 0; } 

its output is:

i'm parent pid = 5228 i'm child pid = 5229 

everything's clear, how result == 0 , result != 0 @ same time? looks variable stores 2 values, because printf instruction executed twice. know, fork() returns 0 , parent's pid, how result check if returns true different conditions?

because it's not same variable. when fork process, end 2 totally different processes (see this answer more detail).

hence result variable in parent not same 1 in child. you're seeing two processes, both attached same output device, each writing own message.

in fact, fork documentation covers that:

on success, pid of child process returned in parent, , 0 returned in child.

so can use return value fork (as do) see if you're parent or child (and see if worked well, it'll return -1 if fails , you'll parent no child).

the idea parent gets process id of child can (like wait() finish) , child gets zero. child can process id of parent calling getppid().