I have this C code where I am trying to reverse a string using inline assembly. However, I am curious to know how the reversed string is displayed at the end and if there is any relation to the lack of outputs declared in the constraints.

#include <stdio.h>
#include <string.h>

int main() {
    char str[10000]; // Allow for 10000 characters

    // Prompt user for input
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    // Remove newline character if present
    char *nl = strchr(str, 'n');
    if (nl != NULL) {
        *nl = '';
    } else {
        printf("Line too long or no newline character found.n");
    }

    // Validate string length
    int length = strlen(str);
    if (length > 50) {
        printf("String exceeds 50 characters.n");
        return 1;
    }

    // Inline assembly to reverse the string
    __asm__ (
        "mov %[len], %%esint"    // esi = length
        "decl %%esint"            // esi = length - 1 (last character index)
        "xor %%ecx, %%ecxnt"      // ecx = 0 (starting index)
        "reverse_loop:nt"
        "cmp %%esi, %%ecxnt"      // Compare start and end indexes
        "jge loop_endnt"          // If start >= end, end loop
        "movb (%[str], %%ecx), %%alnt"    // al = str[ecx]
        "movb (%[str], %%esi), %%blnt"    // bl = str[esi]
        "movb %%bl, (%[str], %%ecx)nt"    // str[ecx] = bl
        "movb %%al, (%[str], %%esi)nt"    // str[esi] = al
        "incl %%ecxnt"            // Increment start index
        "decl %%esint"            // Decrement end index
        "jmp reverse_loopnt"      // Repeat loop
        "loop_end:nt"
        :
        : [str] "r" (str), [len] "r" (length)
        : "%esi", "%ecx", "%al", "%bl"
    );

    // Print the reversed string
    printf("Reversed string: %sn", str);

    return 0;
}

Constraints:

:
: [str] "r" (str), [len] "r" (length)
: "%esi", "%ecx", "%al", "%bl"

I’ve tried to understand the logic behind the inline assembly section, but I’m not entirely clear on why the reversal occurs as it does.

Could someone please shed some light on this behavior and provide insights into how the absence of declared outputs in the constraints might affect it?

Any help or guidance would be greatly appreciated. Thank you!