g++ dumped assembly output doesn't work - c++

Question

I have following C++ code in main.cpp file.
int add(int a,int b)
{
int c = a + b;
return c;
}
int main()
{
int a = 2;
int b = 4;
int d = add(2,4);
}
when I ran g++ -S main.cpp I got the following assembly code.(after removing all the debug symbols). Also I have changed the code to print the sum of the 2 numbers using sys_write system call.
.text
.globl _Z3addii
_Z3addii:
pushq %rbp
movq %rsp, %rbp
movl %edi, -20(%rbp)
movl %esi, -24(%rbp)
movl -24(%rbp), %eax
movl -20(%rbp), %edx
addl %edx, %eax
movl %eax, -4(%rbp)
movl -4(%rbp), %eax
popq %rbp
ret
.globl main
main:
pushq %rbp
movq %rsp, %rbp
subq $16, %rsp
movl $2, -12(%rbp)
movl $4, -8(%rbp)
movl $4, %esi
movl $2, %edi
call _Z3addii
movl %eax, -4(%rbp)
movl $4, %edx #message length
movl -4(%rbp), %esi #message to write
movl $1, %edi #file descriptor (stdout)
movl $1, %eax #system call number (sys_write)
syscall #call kernel
movl $60, %eax # Invoke the Linux 'exit' syscall
movl $0, %edi # With a return value of 0
syscall # call kernel
ret
My problem is when I run the above assembly it gives nothing as output. I can't understand what I am missing here? Can someone please tell me what I am missing? Thanks.
commands used:
g++ -o main main.s and ./main -->no output
OS: Ubuntu 12.04 64bit and g++ version: 4.8.2

Answer 1

There are two things you're doing wrong:
Firstly, you're the 64-bit syscall instruction, but initialize only the %e part of the registers. Secondly, this:
movl -4(%rbp), %esi
loads the value that is at -4(%rbp) (the 6 you just calculated) into %esi, when sys_write expects the memory address of that value there (by which I mean in %rsi). It works with this:
movq $1, %rax #system call number (sys_write)
movq $1, %rdi #file descriptor (stdout)
leaq -4(%rbp), %rsi #message to write
movq $4, %rdx #message length
syscall #call kernel
Of course, you're not going to get formatted output this way. To see that the 6 is printed, you will have to pipe the output through hexdump or something similar.
Addendum: That you only initialize the %e part of the registers is actually only really critical here in the case of %rsi. %rbp holds, at the time of reading, a value with set high bits, and these are lost if only -4(%ebp) is written to %esi. Technically this also works:
movl $1, %eax #system call number (sys_write)
movl $1, %edi #file descriptor (stdout)
leaq -4(%rbp), %rsi #message to write
movl $4, %edx #message length
syscall #call kernel
...but I feel that it is rather poor style.