Is it possible to see what is going on behind gcc and g++ compilation process?
I have the following program:
#include <stdio.h>
#include <unistd.h>
size_t sym1 = 100;
size_t *addr = &sym1;
size_t *arr = (size_t*)((size_t)&arr + (size_t)&addr);
int main (int argc, char **argv)
{
(void) argc;
(void) argv;
printf("libtest: addr of main(): %p\n", &main);
printf("libtest: addr of arr: %p\n", &arr);
while(1);
return 0;
}
Why is it possible to produce the binary without error with g++ while there is an error using gcc?
I'm looking for a method to trace what makes them behave differently.
# gcc test.c -o test_app
test.c:7:1: error: initializer element is not constant
# g++ test.c -o test_app
I think the reason can be in fact that gcc uses cc1 as a compiler and g++ uses cc1plus.
Is there a way to make more precise output of what actually has been done?
I've tried to use -v flag but the output is quite similar. Are there different flags passed to linker?
What is the easiest way to compare two compilation procedures and find the difference in them?
In this case, gcc produces nothing because your program is not valid C. As the compiler explains, the initializer element (expression used to initialize the global variable arr) is not constant.
C requires initialization expressions to be compile-time constants, so that the contents of local variables can be placed in the data segment of the executable. This cannot be done for arr because the addresses of variables involved are not known until link time and their sum cannot be trivially filled in by the dynamic linker, as is the case for addr1. C++ allows this, so g++ generates initialization code that evaluates the non-constant expressions and stores them in global variables. This code is executed before invocation of main().
Executables cc1 and cc1plus are internal details of the implementation of the compiler, and as such irrelevant to the observed behavior. The relevant fact is that gcc expects valid C code as its input, and g++ expects valid C++ code. The code you provided is valid C++, but not valid C, which is why g++ compiles it and gcc doesn't.