Question

I have done the following function to reverse a string:

char* reverseString(char *original_string)
{
    int length = strlen(original_string);
    char *end_of_string = original_string + (length - 1);
    char *reversed_string = malloc(sizeof(char) * length);

    int count = 0;
    int top_limit = length;

    while (count < length) {
        reversed_string[count] = end_of_string[top_limit];
        top_limit--;
        count++;
    }

    return reversed_string;
}

The strategy that I was planning is to put a pointer at the end of the original string and go backwards copying into a new string. I am not sure what am I doing wrong here, but when doing:

char* prova = "hello";
char* reversed_string;
reversed_string = reverseString(prova);
printf("%d", strlen(reversed_string));

It doesn’t show me the correct length, and in compile time I get the following warning:

test.c:46: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘size_t’

Answer 1

The type returned by strlen is size_t, which is an unsigned type. Using %d format to print it is in an undefined behavior. You can either use %zu in C99, or use %u and cast to unsigned int in C89.

#include <string.h>

/* C89 */ printf("%u\n", (unsigned int)strlen(reversed_string));
/* C99 */ printf("%zu\n", strlen(reversed_string));

C11 (n1570), § 7.19 Common definitions <stddef.h>
size_t, which is the unsigned integer type of the result of the sizeof operator […].

Your algorithm is also wrong, because you use both end_of_string and top_limit. You should choose one of the two solutions.

#include <string.h>

char *reverseString(char *original_string)
{
    size_t length = strlen(original_string);
    char *reversed_string = malloc(length + 1);
    size_t start;
    size_t end;

    for (start = 0, end = length - 1; start < length; start++, end--)
    {
      reversed_string[start] = original_string[end];
    }

    reversed_string[start] = '\0';

    return reversed_string;
}