[SOLVED] Perverse mixing of C and assembly

hydraMax · 02-25-2012, 04:31 AM

I've been finding lately that it is really convenient to write a whole bunch of assembly and then glue it together with a little bit of C. However, there is one issue I've been wondering about: is there some way to use an undefined symbol in a file of C code?

Say, as my primary example, I create assembly file data.S that contains a .data section, which contains object "specialdat" consisting of 64 bytes of various data. I can use that symbol in various other assembly files, because the assembler (gas) allows me to have undefined symbols in individual files which are resolved later. But what if I want to use specialdat in the C file (say, to pass it to a function that takes a void pointer to refer to a block of memory). Is there a way I can do that? GCC will allow me to use the functions from the assembly files in the C file without declaring them, so one would think there would be a way to do it with a symbol that simply points to data.

Of course, I could create the data block in the C file and then reference it from the assembly files, but lets say that I preferred to create the data in an assembly file instead.

millgates · 02-25-2012, 04:42 AM

I am not sure I understand what you mean, but if you use a variable in a c file, it should be declared. If it is defined in a different compilation unit, you should declare it in a c file where you use it like extern void* specialdat or something like that. Also, the symbol in your assembly file should be exported so the linker can see it (.globl or something like that on x86)

NevemTeve · 02-25-2012, 04:47 AM

Yes, it is 'extern char specialdat [64]' in the C program. Or perhaps (for advanced coders):

Code:

typedef SpecialType {
...
} SpecialType;
extern SpecialType specialdat;

hydraMax · 02-25-2012, 01:05 PM

I tried that, and it seems to work fine:

Code:

$ cat main.c 
extern char mydata[8];

int main() {
  puts(mydata);
  return 0;
}
$ cat mydata.S 
.globl 	mydata
.data
.align 8
.type	mydata, @object
mydata:
	.byte 'h
	.byte 'e
	.byte 'l
	.byte 'l
	.byte 'o
	.byte '!
	.byte '!
	.byte 0
$ gcc -O2 mydata.S main.c -o test
$ ./test 
hello!!

Thanks!