Tutorial introduction
This tutorial will be a basic demonstration of calling routines written in assembly from your C code. We will use NASM to compile our assembly code and GCC to compile our C code.
The assembly code
For the purpose of this tutorial, our assembly routine will be very simple, it will simply add two integers passed as parameters and return the result.
add.asm
; make the add function visible to the linker global _add ; prototype: int __cdecl add(int a, int B)/> ; desc: adds two integers and returns the result _add: mov eax, [esp+4] ; get the 2nd parameter off the stack mov edx, [esp+8] ; get the 1st parameter off the stack add eax, edx ; add the parameters, return value in eax ret ; return from sub-routine
We are using the __cdecl (default for C/C++) calling convention, so we must take some things into account...
We preserve the stack pointer, this is because the stack is cleaned up by the caller so we don't want to mess around with the stack pointer inside the routine.
The return value is returned in the EAX register.
Leading underscores are added because by default GCC will add these to function calls. We could use GCC compiler flags to change this behaviour but for simplicities sake, we will just add them in here.
The C code
The C code will simply be a basic main function that calls our add function
main.c
#include <stdio.h>
/*
* declaring add as extern tells the compiler that the definition
* can be found in a seperate module
*/
extern int add(int a, int B)/>;
int main() {
int ret = add(10, 20);
printf("add returned %d\n", ret);
return 0;
}
Compilation
Compiling the above code requires the use of object files, the format that we will use is ELF since NASM and GCC both understand this format.
The first thing we must do is compile our assembly code using NASM. We will compile add.asm into an object file called add.o using the following command.
nasm -f elf -o add.o add.asm
Now you should have an ELF file called add.o in your working directory. Next we need to compile our C code, main.c. We use GCC to do this using the following command.
gcc -c main.c -o main.o
Now you should have two object files, add.o and main.o. Now for the final step, we will link these object files together to create our final executable file. To do this, we will use GCC again which in turn will invoke LD for us. The following command will create our executable.
gcc -o test_asm add.o main.o
Now you should have an executable file asm_test.exe which will call our assembly routine and display the result, that is.
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add returned 30
Conclusion
Thanks for reading my tutorial, I hope you got something out of it.
Ryan
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