Calling C from Assembly

Why would you want to combine C and Assembly code? It's a powerful technique in low-level and system programming!

• Assembly: Great for performance-critical tasks, direct hardware access, and understanding system internals.
• C: Offers high-level structure, portability, and access to vast libraries.

By calling C functions from Assembly, you can leverage C's complex logic and libraries while retaining Assembly's low-level control for specific tasks.

When one function calls another, they need to agree on a set of rules. This 'contract' is known as a calling convention.

It defines crucial aspects of how functions interact:

• How arguments are passed (e.g., on the stack, in registers).
• The order in which arguments are passed.
• Which function is responsible for cleaning up the stack after the call.
• How return values are transmitted back to the caller.

Without these conventions, your assembly code wouldn't know how to prepare data for a C function, or how to interpret its results.

The cdecl (C declaration) calling convention is very common, especially for 32-bit x86 systems (like Linux).

Let's look at its key characteristics:

• Argument Order: Arguments are pushed onto the stack from right to left.
• Stack Cleanup: The caller (your assembly code) is responsible for cleaning up the stack after the function returns.
• Return Values: Integer return values are typically placed in the EAX register.

Understanding cdecl is fundamental for successful interaction between your assembly and C code.

With 32-bit cdecl, arguments are pushed onto the stack in reverse order. This means the last argument is pushed first, and the first argument is pushed last.

For a C function like my_func(arg1, arg2, arg3);, the assembly call would involve:

1. push dword arg3_value
2. push dword arg2_value
3. push dword arg1_value
4. call my_func

This ensures that arg1 is at the 'top' of the arguments on the stack, just below the return address pushed by call.

One of the defining features of cdecl is that the caller (your assembly code) is responsible for removing the arguments from the stack after the C function returns.

This is typically done by adjusting the stack pointer (ESP) using the ADD ESP, N instruction, where N is the total size of the arguments pushed (e.g., 4 bytes per argument on 32-bit systems).

This cleanup mechanism allows C functions to accept a variable number of arguments (like printf) because the caller knows exactly how many arguments it pushed.

When a C function returns a value using cdecl, it places that value in a specific register for the caller to retrieve.

• For integer types (like int, char, short), the return value is typically stored in the EAX register (on 32-bit x86).
• For larger or floating-point types, other registers or memory locations might be used, but EAX is the most common for simple integer returns.

After the CALL instruction returns, you can simply access the EAX register to get the result from your C function.

Before your assembly code can call a C function, you need to tell the assembler that the function exists but is defined elsewhere. This is done using the extern directive.

Example: extern printf

This directive informs the assembler that printf is an external symbol. During the linking phase, the linker (e.g., gcc) will resolve this symbol to the actual C function's address, allowing your assembly program to execute it.

Let's call a simple C function that takes no arguments and returns nothing. We'll use 32-bit x86 assembly.

First, compile the C code (c_funcs.c):

#include <stdio.h>
#include <stdlib.h>

void greet_c() {
    printf("Hello from C's greet_c()!\n");
}

// Placeholder for next example
int add_c(int a, int b) {
    return a + b;
}

Now, try running this assembly code:

Now, let's call a C function that takes arguments and returns a value. Remember the cdecl rules for 32-bit x86:

• Arguments pushed right-to-left.
• Caller cleans the stack.
• Return value in EAX.

Add the add_c function to your c_funcs.c file (from the previous scene).

Then, run this assembly code:

Consider a C function int calculate(int x, int y, int z); that you want to call from 32-bit x86 assembly using the cdecl calling convention.

Which sequence of assembly instructions correctly prepares the stack and calls calculate with arguments x=5, y=10, z=15, and correctly cleans up the stack?

You've successfully learned how to integrate C functions into your assembly programs!

• Calling Conventions: These are crucial rules for function interaction.
• cdecl (32-bit): Arguments are pushed onto the stack from right-to-left.
• Caller Cleanup: The assembly code (caller) is responsible for removing arguments from the stack using ADD ESP, N.
• Return Values: Integer results from C functions are typically found in the EAX register.
• extern Directive: Use this to declare C functions to your assembler.

This skill allows you to combine the performance and low-level control of assembly with the rich features and libraries of C!