C example of using `fork()`.

fork-example.c

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int var_global = 0;

int main()
{
    int var_local          = 0;
    pid_t process_id_child = fork();

    if ( process_id_child >= 0 )
    {
        if ( process_id_child == 0 )
        {
            var_global++;
            var_local++;

            puts( "Process [child]:" );
            printf( "\tvar_global => %2d\n", var_global );
            printf( "\tvar_local  => %2d\n", var_local );
            puts( "" );
        }
        else
        {
            var_global = 20;
            var_local  = 10;

            puts( "Process [parent]:" );
            printf( "\tvar_global => %2d\n", var_global );
            printf( "\tvar_local  => %2d\n", var_local );
            puts( "" );
        }
    }
    else
    {
        puts( "`fork(2)` failed. Exiting." );

        return 1;
    }

    return 0;
}