Can't change heap size AMD64

Question

I'm trying to increase the heap size by 100 by changing the brk and I don't know why my code doesn't work.
Here is the part of the code that tries do it:

movq $0, %rdi
movq $12, %rax
syscall
movq %rax, InicialHeap

movq InicialHeap, %rsi
mov $str, %rdi
call printf

movq $100, %rdi
movq $12, %rax
syscall

movq %rax, %rsi
mov $str, %rdi
call printf

movq InicialHeap, %rdi
movq $12, %rax
syscall

movq InicialHeap, %rsi
mov $str, %rdi
call printf

movq $60, %rax
syscall

The program should print something like:
x (print InicialHeap)
x + 100 (print InicialHeap + 100)
x (print InicialHeap)
But it only prints 3 times the same result "x".
What do I have to do to increase my heap size?

Answer 1

As the NOTES section of the man page for int brk(void *addr); describes, the system call (__NR_brk = 12) actually implements brk(), not sbrk, but returns the current break rather than an integer.

As @osgx comments, try running your program under strace to see what return values you're getting. e.g. from strace /bin/true, you can see that it's normal for the dynamic linker to start off by using brk(0) to find out the current break:

brk(0)                                  = 0x24c6000

From there, you should save the return value, and make your next call to brk() with an offset from that.

---

Your current code clearly can't work:

movq $100, %rdi
movq $12, %rax
syscall             ;;  brk((void*)100)