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+--- Thread: How to use a global variable in gcc inline assembly (/Thread-How-to-use-a-global-variable-in-gcc-inline-assembly)
How to use a global variable in gcc inline assembly - horeb350 - 07-24-2023
I am trying to use inline assembly like this, for a global variable, but the compiler gives an error by saying undefined reference to **saved_sp**.
__asm__ __volatile__ (
"movq saved_sp, %rsp\n\t" );
**saved_sp** is declared as `static long saved_sp` globally (for a file that is). What mistake am I doing here?
RE: How to use a global variable in gcc inline assembly - inhalators281896 - 07-24-2023
As I indicated in the comments, the following compiles (and generates the correct machine code) using `gcc 4.4.4` on 64-bit Ubuntu:
long saved_sp;
int main() {
__asm__ __volatile__ (
"movq saved_sp, %rsp\n\t" );
}
Perhaps the issue could be something else entirely (a missing `#include` so that `saved_sp` is actually not defined? **edit**: now that you say it's `static`, I guess that's not very likely.)
RE: How to use a global variable in gcc inline assembly - karolienrxrioelwpc - 07-24-2023
If it fails with "undefined reference to \`saved_sp' " (which is really a *linker* error, not a compiler error) when `saved_sp` is `static`, but works when it is not, then it seems likely that the compiler has decided that `saved_sp` is not used in your source file, and has therefore decided to omit it completely from the compiled code that is passed to the assembler.
The compiler does not understand the assembly code inside the `asm` block; it just pastes it into the assembly code that it generates. So it does not know that the `asm` block references `saved_sp`, and if nothing else in the C code ever reads from it, it is at liberty to decide that it is completely unused - particularly if you have any optimisation options enabled.
You can tell `gcc` that `saved_sp` is used by something that it can't see, and therefore prevent it from choosing to throw it away, by adding the `used` attribute (see the [documentation of variable attributes](
[To see links please register here]
), about half-way down the page), e.g.:static long __attribute__((used)) saved_sp;
Here's a fully worked example:
$ cat test.c
#ifdef FIXED
static long __attribute__((used)) saved_sp;
#else
static long saved_sp;
#endif
int main(void)
{
__asm__ __volatile__ (
"movq saved_sp, %rsp\n\t" );
}
$ gcc -m64 -o test test.c
$ gcc -m64 -O1 -o test test.c
/tmp/ccATLdiQ.o: In function `main':
test.c:(.text+0x4): undefined reference to `saved_sp'
collect2: ld returned 1 exit status
$ gcc -m64 -DFIXED -O1 -o test test.c
$
(This is from a 32-bit Debian squeeze system with gcc 4.4.5, which is the closest thing I have to hand; `-m64` may well be unnecessary on your system.)
RE: How to use a global variable in gcc inline assembly - Mrwaffled126 - 07-24-2023
Preferably you would use input and output parameters:
__asm__ __volatile__ (
"movq %0, %%rsp\n\t"
: : "r"(saved_sp) : "memory"
);
Often there can be some variables that are not symbols at all at the assembling stage (for example stack variables, or registers. Also, you'd want to clobber the whole memory, to ensure that no stack variables are kept in register after the `saved_sp` is stored in RSP.