How to change the entry point of gcc generated asm code? - linux

Question

This experiment is on the 32 bit Linux.
I want to do a transformation on the asm level, and I am trying to implement
my transformation before the function main is called.
Currently I am trying to program a new entry point, implement my transformation code,
and hope this new entry point can successfully call main
Basically the default entry point of gcc generated assembly code is main, which I give an example as follow:
c code:
int main()
{
return 0;
}
I use this command to generate asm code:
gcc -masm=intel -fno-asynchronous-unwind-tables -S main.c
and this is what I got:
.file "main.c"
.intel_syntax noprefix
.text
.globl main
.type main, #function
main:
push ebp
mov ebp, esp
mov eax, 0
pop ebp
ret
.size main, .-main
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",#progbits
Could anyone tell me how to implement a new entry point(probably a function similiar like _start) and call main at the end of this new entry point?
Thank you!

Answer 1

I doubt you should replace _start() because it's very platform- and libc-specific. Either you write all code in assembler and so you don't need libc-specific initialization, or you should copy all _start() activity including things you aren't aware. The latter looks simply bogus.
If you agree not to replace start() but use a mechanism to run some code before main(), declare a function with __attribute__((constructor)). This is documented GCC extension and it's actively used e.g. for static object initializing in C++. Such function can't get arguments or return a real value, nor shall it override control flow in another way. I can't catch what you mean for "transformation" so it can contradict to your intention; if so, you would have explained this more detailedly.