Pass String to ASM

In this example, we will pass a string to ASM and print it out to STDOUT using sys_write.

C-Driver

passstringdriver.c

#include "cdecl.h"
#include "stdio.h"

int PRE_CDECL print( size_t, char * ) POST_CDECL;

int main() {
    int ret;
    size_t size;
    char string[] = "Hello, dear world!\n";

    size_t size = sizeof string;
    int ret = print( size, string );

    return ret;
}

NOTE: size_t will be a 32-bit integer in x86 compilation (64-bit in x64 mode) and that is exactly what we need as asm_main will return the RAM address of the first character of the string.

ASM Code

printfunction.asm

segment .text
    global print

print: ; int print(size_t size, char * string)
    STC ; set carry flag
    push ebp
    mov ebp, esp    ; put the head of the stack pointer on ebp
    pusha
    mov edx, [ebp + 8]  ; get the first element on stack which is the 32-bit int size
    mov ecx, [ebp + 12] ; get the char* and move on to print_asm
print_asm:      ; REQ: ECX <- char* string, EDX <- sizeof string
    mov eax, 4  ; sys_write
    mov ebx, 1  
    int 0x80    ; syscall kernel with sys_write
    JNC exit    ; jump to exit if no carry flag set, used to avoid popping unnecessarily

    popa        ; pop everything back
    pop ebp 
exit:
    mov eax, 0
    leave
    ret

This ASM code implements the "function" print which works like the function int print(size_t size, char* string).

NOTE: The label print_asm can be executed from within another part of the code by using call print_asm, but before executing print_asm, the requirements must be fulfilled (REQ: ECX <- char* string, EDX <- sizeof string).

Compilation

$ gcc -m32 -c passstringdriver.c
$ nasm -f elf printfunction.asm
$ gcc -m32 -o asm-printfunction passstringdriver.o printfunction.o

Execution

$ ./asm-printfunction
Hello, dear world!

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