Unused stack space

michael_quinlivan_at_hotmail.com
Date: 09/30/05

  • Next message: QNils_O=2E_Sel=E5sdal=22?=: "Re: Unused stack space" 
    Date: 29 Sep 2005 22:45:53 -0700

    In a C module that I have compiled, it seems to generate object files
    which waste stack space. The C file is as follows

    print1(char * str)
    {
       printf(str);
    }

    yes I know it is useless, it was simply created so that I could examine
    how GCC generates asm code, which is below

    >objdump -d print1.o
    00000013 <print1>:
      13: 55 push %ebp
      14: 89 e5 mov %esp,%ebp
      16: 83 ec 08 sub $0x8,%esp
      19: 8b 45 08 mov 0x8(%ebp),%eax
      1c: 89 04 24 mov %eax,(%esp)
      1f: e8 fc ff ff ff call 20 <print1+0xd>
      24: c9 leave
      25: c3 ret

    The code is relatively simple, but I don't understand the third
    instruction (sub 0x8, esp). obviously it allocates 8 bytes to the
    stack, but why does it allocate this much? It only uses 4 bytes of it
    (to pass the str parameter to printf), and leaves four bytes unused.
    Why is this? Or am I confusing something??

    My mental image of the stack just before the call to printf is as
    follows. Is this accurate?

    |Param to print1 |
    |--------------------|
    |Addr of caller |
    |--------------------|
    |Caller EBP |
    |--------------------|
    |UNUSED DWORD |
    |--------------------|
    |Param to printf |
    |--------------------|

    cheers
    MQ

  • Next message: QNils_O=2E_Sel=E5sdal=22?=: "Re: Unused stack space"

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