How to access C variable for inline assembly manipulation?

Question

Given this code:

#include <stdio.h>

int main(int argc, char **argv)
{
    int x = 1;
    printf("Hello x = %d\n", x);
}

I'd like to access and manipulate the variable x in inline assembly. Ideally, I want to change its value using inline assembly. GNU assembler, and using the AT&T syntax.

Answer 1

In GNU C inline asm, with x86 AT&T syntax:
(But https://gcc.gnu.org/wiki/DontUseInlineAsm if you can avoid it).

// this example doesn't really need volatile: the result is the same every time
asm volatile("movl $0, %[some]"
    : [some] "=r" (x)
);

after this, x contains 0.

Note that you should generally avoid mov as the first or last instruction of an asm statement. Don't copy from %[some] to a hard-coded register like %%eax, just use %[some] as a register, letting the compiler do register allocation.

See https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html and https://stackoverflow.com/tags/inline-assembly/info for more docs and guides.

---

Not all compilers support GNU syntax. For example, for MSVC you do this:

__asm mov x, 0 and x will have the value of 0 after this statement.

Please specify the compiler you would want to use.

Also note, doing this will restrict your program to compile with only a specific compiler-assembler combination, and will be targeted only towards a particular architecture.

In most cases, you'll get as good or better results from using pure C and intrinsics, not inline asm.