Understanding the code in C involving processes - c

Question

Someone can explain me why the first code the variable "v" dont change the value in the last prinft() and se second code the variable "v" change.
First code:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
int v = 5;
int main(){
pid_t piid;
piid = fork();
if(piid==0){
v += 15;
return 0;
}
else if(piid >0){
wait(NULL);
printf("Final value = %d\n",v);
return 0;
}
}
I know this code be involving fork() for create another process maybe
Second code:
#define _GNU_SOURCE
#include <stdlib.h>
#include <malloc.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <sched.h>
#include <stdio.h>
// 64kB stack
#define FIBER_STACK 1024*64
int v = 5;
int threadFunction( void* argument )
{
v += 10;
return 0;
}
int main(){
void* stack;
pid_t pid;
stack = malloc( FIBER_STACK );
if ( stack == 0 )
{
perror("malloc: could not allocate stack\n");
exit(1);
}
pid = clone( &threadFunction, (char*) stack + FIBER_STACK,
SIGCHLD | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_VM, 0 );
if ( pid == -1 )
{
perror( "clone" );
exit(2);
}
pid = waitpid( pid, 0, 0 );
if ( pid == -1 )
{
perror( "waitpid" );
exit(3);
}
free( stack );
printf("Final value = %d\n", v);
return 0;
}
This code is a thread, but I can not understand the so-called clone () and what's inside there.

Answer 1

Forking a new process creates a standalone and distinct copy of the original process. Changing something in the process copy will not change anything in the original process.
Threads are different, they share everything. All threads are still parts of the same process.
While the clone system call can create a new process (that's what happens when you call fork in Linux actually) it can also be used to create a thread, which is what happens in the second program.

Answer 2

threads run in a shared memory space, while processes run in separate memory spaces.
so in the first case using fork each process has different reference of the variable v while in the thread both has the same reference.