GCC Inline Assembly, str_set

gcc_inline.c

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


int str_set(char * str, const size_t n, const size_t pos, const uint8_t val) {
    uint8_t return_value = 0;

    asm 
    (
        "cmp %[pos], %[n]\n\t"
        "movb $1, %[retv]\n\t"
        "jle exit\n\t"
        "movb %[val], 0(%[str], %[pos], 1)\n\t"
        "movb $0, %[retv]\n\t"
        "exit:\n\t"
        : [str] "+r" (str), [retv] "=m" (return_value) // Out
        : [pos] "r" ((uintptr_t)pos), [val] "r" (val), [n] "m" (n) // In
    );

    return return_value;
}


int main()
{
    char text[14] = "ASM Xs great!";
    uint8_t ret_val = 0;
    
    ret_val = str_set(text, 13, 4, 'i');
    printf("Output: %s %d\n", text, ret_val);

    ret_val = str_set(text, 13, 13, 'x');
    printf("Output: %s %d\n", text, ret_val);
    return 0;
}