How do I compile assembly routines for use with a C program (GNU assembler)?

Question

I have a set of assembly function which I want to use in C programs by creating a header file. For instance, if I have asm_functions.s which defines the actual assembly routines and asm_functions.h which has prototypes for the functions as well as some standard #define's I needed. My goal is to use a C program, say test_asm.c to call the assembly functions.

asm__functions.h:

 #define ASM_CONST_1    0x80
 #define ASM_CONST_2    0xaf

uint8_t asm_foo( int, int, int );

asm__functions.s:

 /* dont need this: #include "asm_functions.h" */

.section .text
 .type asm_foo, @function
 asm__foo:
   /* asm code with proper stack manipulation for C calling conventions */
   ret

test__asm.c:

 #include "asm_foo.h"

int main() {
      uint8_t res = asm_foo( 1, 2, 3);
      return 0;
 }

In a situation like this what would be the proper way to compile a link the program? I was trying something like this:

gas -o asm_foo.o asm_foo.s
gcc -o test_asm test_asm.c

But I still get a linker error from GCC saying that my assembly routine is undefined. I hope this contrived example is good enough to explain the situation.

Thanks!

EDIT:

Here is a snippet of output when I compile with a single command:

tja@tja-desktop:~/RIT/SP2/latest$ gcc -o test_pci pci_config.s test_pci.c
/tmp/ccY0SmMN.o: In function _pci_bios_read_byte':
(.text+0x8): undefined reference toPCI_FUNCTION_ID'
/tmp/ccY0SmMN.o: In function _pci_bios_read_byte':
(.text+0xa): undefined reference toREAD_CONFIG_BYTE'
/tmp/ccY0SmMN.o: In function _pci_bios_read_byte':
(.text+0x18): undefined reference toPCI_BIOS_FUNCTION_INT'
/tmp/ccY0SmMN.o: In function _pci_bios_read_byte':
(.text+0x1b): undefined reference toBAD_REGISTER_NUMBER'
/tmp/ccY0SmMN.o: In function _pci_bios_read_word':
(.text+0x30): undefined reference toPCI_FUNCTION_ID'
...

All of those, such as PCI_FUNCTION_ID, are defined in my header file, which is included by the C program. When I compile the assembly code by itself there are no errors.

Answer 1

Based on the files in your question, I managed to compile it. I've changed both the file names and the file contents.

asm_const.h :

#define ASM_CONST_1    0x80
#define ASM_CONST_2    0xaf

asm_functions.h :

#include "asm_const.h"
unsigned char asm_foo( int, int, int );

asm_functions.S (the trailing S must be capital! #include needs it) :

#include "asm_const.h"
.section .text
.globl asm_foo
.type asm_foo, @function
asm_foo:
  mov $ASM_CONST_1, %eax
  /* asm code with proper stack manipulation for C calling conventions */
  ret

test_asm.c :

#include "asm_functions.h"
int main() {
  return asm_foo( 1, 2, 3);
}

Please note that you need the the assembly file extension .S with capital S. With .s, the .s file wouldn't be run through the preprocessor, thus #include wouldn't work, and you wouldn't be able to use ASM_CONST_1 in the .s file.

Compile with a single command:

gcc -o test_asm asm_functions.S test_asm.c

Or, as an alternative, compile with multiple commands, creating .o files:

gcc -c asm_functions.S
gcc -c test_asm.c
gcc -o test_asm asm_functions.o test_asm.o

The single-command gcc takes care of compiling the .S file using gas, the .c file with GCC's C compiler, and linking the resulting temporary .o files together using ld. gcc runs all those commands with the appropriate flags by default.

On some systems (but not on Linux with the default GCC installation) you have to prepend an underscore to the names of exported functions in the .S file (but not in the .c or .h files). So all instances of asm_foo would become _asm_foo only in the .S file.