How to pass memory address of a location in Stack from assembly?

Question

I am trying to store a stack memory address into a register. The point is to call a c function from assembly code that takes in three arguments.

1) A pointer to a function: which I get using leaq label(%rip), %rdi

2) An int: which is simple, movq $2, %rsi

3) A pointer to a memory address: This is the tricky part. This address is lets say stack location 2 = -16(%rsp).

How to pass this stack location to the register %rdx and have the c function write its results there?

Note: This c function is pthread threaded if you need to know.

So why wasn't that critical information stated in the question? — user207421
– user207421, Commented Nov 1, 2017 at 3:36
Look at compiler output for foo() { int k; bar(&k); } godbolt.org/g/kfPqic. — Peter Cordes
– Peter Cordes, Commented Nov 1, 2017 at 4:11

Bibrak · Accepted Answer · 2017-11-01 05:31:38Z

Ok so I did it, thanks. Here is the full solution:

assembly code:

 1 .globl main 2 main: 3 4 pushq %rbp 5 movq %rsp, %rbp 6 subq $8, %rsp 7 8 movq $50, -8(%rbp) 9 10 leaq PrintHello(%rip), %rdi 11 movq $17, %rsi 12 leaq -8(%rbp), %rdx 13 14 callq dispatcher 15 callq wait 16 17 movq -8(%rbp), %rax 18 movq %rax, %rdi 19 callq print_int 20 21 addq $8, %rsp 22 movq $0, %rax 23 popq %rbp 24 retq

output:

In dispatcher arg = 17 In wait wrap_func: args_data.ret = 50 Hello World! It's me, thread # 17! wrap_func: after assignment-> args_data.ret = 20 Exit from wait

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