C Char Array Define Length Using Variable Not Working

It seems that your compiler does not support variable length arrays.

Take into account that to write function reverse there is no need to define an auxiliary array. The function can be written the following way

char * reverse( char s[] )
{
    size_t i = 0, n = 0;

    while ( s[n] ) ++n;

    for ( ; i < n / 2; i++ )
    {
        char c = s[i];
        s[i] = s[n-i-1];
        s[n-i-1] = c;
    }

    return s;
}

Variable length arrays is one of the feature added to C99. You need to compile your code with flag -std=c99 in GCC.

You should also note, if you are using MSVS, that MSVS does not support VLAs.

The proper way to implement variable length arrays in C is with malloc.

#include <stdlib.h>

void makeArrayOfSize(int size) {
   // Create memory for array
   char *chars = (char *)malloc(size * sizeof(char));

   // Do business logic

   free(chars); // ALWAYS free pointers to allocated memory to avoid leaks
}

Though, you can make a string reverser without copying the string (IF it is not hard-coded to the string table)...

void reverse(char* string) {
   int length = 0;
   int index = 0;
   char temp;

   // Ensure string is not null
   if (!string) {
      return;
   }

   // Get length assuming string has proper '\0' char at the end
   while (string[length]) {
      ++length;
   }

   // Play catch!
   while (index < length / 2) {
      temp = string[index++];
      string[index - 1] = string[length - index];
      string[length - index] = temp;
   }
}

Disclaimer: I didn't compile or run this code... but it reminded me of an old homework assignment so I found it nostalgic to give it a shot :)

Edit: Compiled this code. Works perfectly unless the string is hard-coded in the string table (will produce a bus error). Also will not crash if string is empty.

Took longer to write a good test than the code but here's the test and the output showing working dynamic data, but an error on the hard string:

int main(int argc, char** arcv) {
   char *string;
   char string0[0];
   char string1[1];
   char string2[2];
   char string3[3];
   char string4[4];
   char string5[5];
   char string6[6];
   char *string7 = "abcdef";

   string2[0] = 'a';

   string3[0] = 'a';
   string3[1] = 'b';

   string4[0] = 'a';
   string4[1] = 'b';
   string4[2] = 'c';

   string5[0] = 'a';
   string5[1] = 'b';
   string5[2] = 'c';
   string5[3] = 'd';

   string6[0] = 'a';
   string6[1] = 'b';
   string6[2] = 'c';
   string6[3] = 'd';
   string6[4] = 'e';

   printf("String: %s\n", string);
   reverse(string);
   printf("Reverse String: %s\n", string);

   printf("String0: %s\n", string0);
   reverse(string0);
   printf("Reverse String0: %s\n", string0);

   printf("String1: %s\n", string1);
   reverse(string1);
   printf("Reverse String1: %s\n", string1);

   printf("String2: %s\n", string2);
   reverse(string2);
   printf("Reverse String2: %s\n", string2);

   printf("String3: %s\n", string3);
   reverse(string3);
   printf("Reverse String3: %s\n", string3);

   printf("String4: %s\n", string4);
   reverse(string4);
   printf("Reverse String4: %s\n", string4);

   printf("String5: %s\n", string5);
   reverse(string5);
   printf("Reverse String5: %s\n", string5);

   printf("String6: %s\n", string6);
   reverse(string6);
   printf("Reverse String6: %s\n", string6);

   printf("String7: %s\n", string7);
   printf("(error after this)\n");
   reverse(string7);
   printf("Reverse String7: %s\n", string7);

   return 0;
}

Output:

String: (null)
Reverse String: (null)
String0: 
Reverse String0: 
String1: 
Reverse String1: 
String2: a
Reverse String2: a
String3: ab
Reverse String3: ba
String4: abc
Reverse String4: cba
String5: abcd
Reverse String5: dcba
String6: abcde
Reverse String6: edcba
String7: abcdef
(error after this)
Bus error: 10

in earlier versions of C, a variable could only be defined immediately after an open brace '{'

so enclose the variable definition and the associated/following code in braces.

I.E.

{
    char copy[count+1];
    ..... // other associated code that uses the 'copy' variable
}