I am a university student and I am trying to get the fork command to work into the following program but for some reason when i compile it on windows it gives me this error warning: implicit declaration of function 'fork' [-Wimplicit-function-declaration] 32 | pid1=fork();.

#include <stdio.h>
#include <unistd.h>

int main(){
int pid1,pid2,pid3,pid4,pid5;
    pid1=fork();
    if (pid1 == 0) pid2 = fork();
    else 
        pid3=fork();
        if (pid3!=0){
            pid4=fork();
            if (pid4!=0)
                pid5=fork();
        }
    printf("this is my pid: %d",getpid());
    return 0;
}

(I included the unists header file because my linux vm doesn’t run the code unless i include it, again no clue why since from what i know the fork command should be included in the stdio file)

The weird thing is that when I try to compile it into linux it works perfectly fine for some reason. So im guessing it has something to do with my compiler but I dont know what exactly so I hope somebody can help me figure this out.