Printing behavior using fork() in C

I started learning about forks, but I don't understand why this prints 4. More specifically I don't understand when the child process will finish and the parent process will exit.

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

int main() {
    int i, status;
    int x =1;
    for ( i = 0; i < 3; i++) {
        int pid = fork();
        x++;
        if (pid > 0) {
            wait(&status);
            x--;
            exit(-1);
        }
    }

    printf ("x = %d\n", x);
    return 0;
}

I tried a diagram to understand it better

Click here to see the diagram

You've made a chain of fork()s in which only the last child will be alive after the for loop. At each iteration, x is increased as such:

original process
x = 1
  |
fork() --> child1
  |          |
x++        x++ (x==2)
parent     fork() --> child2
waits        |          |
for        x++        x++ (x==3)
child1     child1     fork() --> child3
  |        waits        |          |
  |        for        x++        x++ (x==4)
  |        child2     child2       |
  |          |        waits        |
  |          |        for          |
  |          |        child3       |
  |          |          |          |
  |          |          |          |
  |          |          |        printf ("x = %d\n", x);
  |          |          |        return 0;
  |          |        x--
  |          |        exit
  |        x--
  |        exit
x--
exit 

The initial process (670743) forks a child (670744) which subsequently increments it's copy of x. It doesn't matter what the parent does to it's copy of x before it eventually exits (after a child exits).

In the next iteration the first child is now the parent with a copy of an increment value of x (now 2). It forks another child (670745) and the process repeat till last child (670746) exists the loop and prints x = 4.

I modified your code as follows to demonstrate this (also added the missing headers for Linux, reduced scope of i and eliminated the unused status variable):

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

int main() {
	int x = 1;
	for (int i = 0; i < 3; i++) {
		printf("pid=%jd, x=%d, i=%d\n", (intmax_t) getpid(), x, i);
		int pid = fork();
		x++;
		if (pid > 0) {
			wait(&(int) { 0 });
			x--;
			exit(-1);
		}
	}
	printf("pid=%jd, x=%d\n", (intmax_t) getpid(), x);
}

which shows it's the last child process that prints out x:

pid=670743, x=1, i=0
pid=670744, x=2, i=1
pid=670745, x=3, i=2
pid=670746, x=4