Reverse a string using pointers in a function, output in main is garbled

You're returning a pointer to a local variable, (automatic variable to speak in ISO standard terms,) which is allocated on the stack, as soon as you return from your function that memory is released leaving you with a dangling pointer pointing to memory that may or may not still contain the string you put there, it depends entirely on circumstances. You should provide the output buffer as a function argument, or allocate it with malloc, or in C++ with new.

edit; added some example code

void reverse(const char* s1, char* s2) {
  const int l = strlen(s1);
  const char* p = s1 + l - 1;
  do {
    *s2++ = *p;
  } while (p-- != s1);
  *s2 = 0;
}

int main() {
  // some code here
  char s1[5] = "abcd";
  char s2[5] = "";

  reverse(s1, s2);

  // some more code here

  return 0;
}

or

char* reverse(const char* s) {
  const int l = strlen(s);
  char* rs = malloc(l+1);
  const char* p = s + l - 1;
  do {
    *rs++ = *p;
  } while (p-- != s);
  *rs = 0;
  return rs - l;
}

int main() {
  // some code here
  char s1[5] = "abcd";

  char* s2 = reverse(s1);

  // some more code here

  free(s2);

  return 0;
}

The problem description sounds like you can just reverse the string in place. Keep it simple.

void reverse_range(char *first, char *last) // [first, last)
{
    for (; first != last && first != --last; ++first)
    {
        char temp = *first; *first = *last; *last = temp;
    }
}

void reverse(char *str)
{
    reverse_range(str, str + strlen(str));
}

int main()
{
    char text[] = "0123456789";

    printf("before: %s\n", text);
    reverse(text);
    printf("after : %s\n", text);
}

s2 points to your local ar3 array. Therefore when str_rev returns, it is no longer valid to look at the memory where ar3 was via s2. Now s2 is called a dangling pointer, and this is one of the huge pains in learning to use C pointers correctly.

For a simple solution that doesn't use malloc and meets the exercise requirement of "replaces the contents of a string", try copying the result into the function argument pointer (the original string; but be careful since your current code has since changed the pointer string). You know this pointer points at memory with enough characters and isn't local to your function.

Part of the problem is that you can't actually "replace the contents of a string with the string reversed" when you are dealing with string literals, ie. strings delcared in the form char * s1 = "abcd";

Without using literals, I made a relatively easy to understand recursive example:

/*   A function that replaces the contents of a string with the string reversed. */
#include <stdio.h>
#include <string.h>
void str_rev(char * string);

int main(void)
{

   char s1[] = "abc";
   char s2[] = "even";
   char s3[] = "quodd";


   printf("This is s1 before: %s\n", s1);
   str_rev(s1);
   printf("This is s1 after: %s\n", s1);

   printf("This is s2 before: %s\n", s2);
   str_rev(s2);
   printf("This is s2 after: %s\n", s2);

   printf("This is s3 before: %s\n", s3);
   str_rev(s3);
   printf("This is s3 after: %s\n", s3);

    return 0;

}

void str_rev(char * string) {

    //Store the first char of the string locally
    char firstChar = string[0];
    //Store the last char of the string locally
    int lastCharPos = strlen(string)-1;
    char lastChar = string[lastCharPos];

    //Shorten the string (temporarily)
    string[lastCharPos] = '\0';


    if (string[1] != '\0') {
        //Call on the now shortened string, eg.
        //"abc" becomes "b"
        //"even" becomes "ve"
        //"quodd" becomes "uod"
        str_rev(string+1);
    }

    //Swap the first and last characters
    string[0] = lastChar;
    string[lastCharPos] = firstChar;

}

On my system the output is as follows:

This is s1 before: abc
This is s1 after: cba
This is s2 before: even
This is s2 after: neve
This is s3 before: quodd
This is s3 after: ddouq

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

void my_strrev(char* begin){
    char temp;
    char* end;
    end = begin + strlen(begin)-1;

    while(end>begin){
        temp = *end;
        *end = *begin;
        *begin = temp;
        end--;
        begin++;
    } 
}

main(){
    char string[]= "foobar";
    my_strrev(string);
    printf("%s", string);
}