What happens before main() is executed in C and why is it important?

Re:What happens before main() is executed in C and why is it... by saagarjha on Friday May 04, 2018 @07:10PM (#16999172)

Here's what works on my Mac, compiled with clang -m32.

  #include <stdio.h>
  
  int main() {
  	int i;                                // Another stack variable
  	printf("%s\n", **(char ***)(&i + 8)); // argv[0]
  	return 0;
  }

Of course, this won't work at all on x86_64 because arguments are passed on registers instead of the stack.

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Re:What happens before main() is executed in C and why is it... by doomjunky on Saturday May 05, 2018 @12:38AM (#17000356)

On Windows this dependes on the calling convention. The calling convention determines whether function arguments are passed: In order or in reverse order. Who takes care of cleaning the stack: caller or callee? And even if the arguments are passed by stack or CPU registers.

In Visual Studio you can control this behavoir with the __cdecl, __clrcall, __stdcall, __fastcall, __thiscall, __vectorcall [1] calling convention function prefixes, but this also depends on the optimization options [2] of the compiler.

Passing arguments via CPU registers has a huge preformance benefit, but also some drawbacks.

[1]: https://msdn.microsoft.com/en-us/library/984x0h58.aspx

[2]: https://msdn.microsoft.com/en-us/library/46t77ak2.aspx

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Re:What happens before main() is executed in C and why is it... by saagarjha on Saturday May 05, 2018 @07:53PM (#17004528)

> Passing arguments via CPU registers has a huge preformance benefit, but also some drawbacks.

Like what?

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Re:What happens before main() is executed in C and why is it... by pjmlp on Saturday May 05, 2018 @08:36PM (#17004681)

On architectures like x86, the set of registers is limited and some are special, so there is some contention going on.

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Re:What happens before main() is executed in C and why is it... by kbumsik on Friday May 04, 2018 @07:57PM (#16999379)

I don't know much about AMD64 but if the arguments are passed to registers then how it is possible to retrieve argc/argv later, as other functions likely to touch registers?

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Re:What happens before main() is executed in C and why is it... by saagarjha on Friday May 04, 2018 @08:04PM (#16999415)

They're pushed on to the stack if another function will touch them.

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