Geek Answers Handbook

Why is there no "sub rsp" instruction in this function, and why are function parameters stored at negative rbp offsets?

This is what I understood by reading some memory segmentation docs: when a function is called, there are several instructions (called the prolog function) that save the frame pointer on the stack, copy the value of the stack pointer to the base pointer, and save some memory for local variables.

Here is the trivial code I'm trying to debug using GDB:

void test_function(int a, int b, int c, int d) {
    int flag;
    char buffer[10];

    flag = 31337;
    buffer[0] = 'A';
}

int main() {
    test_function(1, 2, 3, 4);
}

The purpose of debugging this code was to understand what happens on the stack when the function is called: therefore, I had to examine the memory at different stages of the program execution (before the function was called and during its execution). Although I managed to see things like the return address and the saved frame pointer by examining the base pointer, I really don't understand what I will write after the parsed code.

dismantling:

(gdb) disassemble main
Dump of assembler code for function main:
   0x0000000000400509 <+0>: push   rbp
   0x000000000040050a <+1>: mov    rbp,rsp
   0x000000000040050d <+4>: mov    ecx,0x4
   0x0000000000400512 <+9>: mov    edx,0x3
   0x0000000000400517 <+14>:    mov    esi,0x2
   0x000000000040051c <+19>:    mov    edi,0x1
   0x0000000000400521 <+24>:    call   0x4004ec <test_function>
   0x0000000000400526 <+29>:    pop    rbp
   0x0000000000400527 <+30>:    ret    
End of assembler dump.
(gdb) disassemble test_function 
Dump of assembler code for function test_function:
   0x00000000004004ec <+0>: push   rbp
   0x00000000004004ed <+1>: mov    rbp,rsp
   0x00000000004004f0 <+4>: mov    DWORD PTR [rbp-0x14],edi
   0x00000000004004f3 <+7>: mov    DWORD PTR [rbp-0x18],esi
   0x00000000004004f6 <+10>:    mov    DWORD PTR [rbp-0x1c],edx
   0x00000000004004f9 <+13>:    mov    DWORD PTR [rbp-0x20],ecx
   0x00000000004004fc <+16>:    mov    DWORD PTR [rbp-0x4],0x7a69
   0x0000000000400503 <+23>:    mov    BYTE PTR [rbp-0x10],0x41
   0x0000000000400507 <+27>:    pop    rbp
   0x0000000000400508 <+28>:    ret    
End of assembler dump.

I understand that "saving the frame pointer on the stack" is done using "push rbp", "copying the value of the stack pointer to the base pointer" is done "mov rbp, rsp", but what gets me confused is the lack of "sub rsp $ n_bytes" to "save some memory for local variables". I have seen this in many exhibits (even in some sections here in stackoverflow).

, ( ), , , , , , , . , , , . , ( ). rsp "offsetted"?

, , 64- , , , .

+4
2

V ABI x86-64 red zone 128 %rsp. 128 , ( leaf function). , .
test_function, , , . ( , ).

+6

, , , ,

x86_64 , . , rbp rsp, .

:

, :

  • x86_64 . , , .
  • IA-32, .

"sub rsp $n_bytes" " ".

sub rsp : - GCC x86-64 , ?

+2

More articles:


All Articles