What c code would compile to something like `call *%eax`? - c

Question

I'm working with c and assembly and I've seen call *%eax in a few spots. I wanted to write a small c program that would compile to something like this, but I'm stuck.
I was thinking about just writing up some assembly code like in this question: x86 assembly instruction: call *Reg only using AT&T syntax in my case to get a small example with the call in it. However, that wouldn't solve my burning question of what kind of c code compiles to that?
I understand that it is a call to the address that eax is pointing to.

Answer 1

Documentation: http://gcc.gnu.org/onlinedocs/gcc/Local-Reg-Vars.html#Local-Reg-Vars
Try this
#include <stdio.h>
typedef void (*FuncPtr)(void);
void _Func(void){
printf("Hello");
}
int main(int argc, char *argv[]){
register FuncPtr func asm ("eax") = _Func;
func();
return 0;
}
And its relative assembly:
.file "functorTest.c"
.section .rdata,"dr"
LC0:
.ascii "Hello\0"
.text
.globl __Func
.def __Func; .scl 2; .type 32; .endef
__Func:
pushl %ebp
movl %esp, %ebp
subl $24, %esp
movl $LC0, (%esp)
call _printf
leave
ret
.def ___main; .scl 2; .type 32; .endef
.globl _main
.def _main; .scl 2; .type 32; .endef
_main:
pushl %ebp
movl %esp, %ebp
andl $-16, %esp
call ___main
movl $__Func, %eax
call *%eax ; see?
movl $0, %eax
movl %ebp, %esp
popl %ebp
ret
.def _printf; .scl 2; .type 32; .endef

Answer 2

Just to add another example with local variables instead.
#include <stdio.h>
// A normal function with an int parameter
// and void return type
void fun(int a)
{
printf("Value of a is %d\n", a);
}
int main()
{
// fun_ptr is a pointer to function fun()
void (*fun_ptr)(int) = &fun;
// Invoking fun() using fun_ptr
(*fun_ptr)(10);
return 0;
}
The Assembly (There are some extra lines, for this was compiled in x86 on an x64 computer. I can provide the x64 if requested by new viewers)
000011c7 <main>:
11c7: 8d 4c 24 04 lea 0x4(%esp),%ecx
11cb: 83 e4 f0 and $0xfffffff0,%esp
11ce: ff 71 fc pushl -0x4(%ecx)
11d1: 55 push %ebp
11d2: 89 e5 mov %esp,%ebp
11d4: 51 push %ecx
11d5: 83 ec 14 sub $0x14,%esp
11d8: e8 28 00 00 00 call 1205 <__x86.get_pc_thunk.ax>
11dd: 05 23 2e 00 00 add $0x2e23,%eax
11e2: 8d 80 99 d1 ff ff lea -0x2e67(%eax),%eax
11e8: 89 45 f4 mov %eax,-0xc(%ebp)
11eb: 83 ec 0c sub $0xc,%esp
11ee: 6a 0a push $0xa
11f0: 8b 45 f4 mov -0xc(%ebp),%eax
11f3: ff d0 call *%eax
11f5: 83 c4 10 add $0x10,%esp
11f8: b8 00 00 00 00 mov $0x0,%eax
11fd: 8b 4d fc mov -0x4(%ebp),%ecx
1200: c9 leave
1201: 8d 61 fc lea -0x4(%ecx),%esp
1204: c3 ret