Thread: Having a problem with const

Hi,

I have put together a small program that is supposed to sort strings into alphabetical order. However, I'm having a problem using "const".

The program uses a variable in main() which is passed as an argument to various functions. As these functions are not allowed to change the value of the (function) parameter that matches the argument, I have "marked" this parameter as const. However the gcc compiler doesn't like this and generates the following output.

Code:

prog8_4.c: In function ‘main’:
prog8_4.c:49:13: warning: passing argument 1 of ‘str_sort’ from incompatible pointer type [-Wincompatible-pointer-types]
   49 |    str_sort(pS, str_count);                        // Sort strings
        |                 ^~
        |                 |
        |                 char **
prog8_4.c:13:15: note: expected ‘const char **’ but argument is of type ‘char **’
   13 | void str_sort(const char **, size_t);              // Sorts an array of strings
      |                        ^~~~~~~~~~~~~

Here is the function prototype.

Code:

void str_sort(const char **, size_t);              // Sorts an array of strings

Here is main().

Code:

int main(void)
{
   size_t pS_size = INIT_NSTR;                     // count of pS elements
   char ** pS = calloc(pS_size, sizeof(char *));   // Array of string pointers
   if(!pS)
   {
      printf("Failed to allocate memory for string pointers.\n");
      exit(1);
   }
   
   char **pTemp = NULL;                            // Temporary pointers
   
   size_t str_count = 0;                           // Number of strings read
   char *pStr = NULL;                              // String pointer
   printf("Enter one string per line. Press Enter to end:\n");
   while((pStr = str_in()) != NULL)
   {
      if(str_count == pS_size)
      {
         pS_size += NSTR_INCR;
         if(!(pTemp = realloc(pS, pS_size * sizeof(char *))))
         {
            printf("Memory allocatio for array of strings failed.\n");
            return 2;
         }  // end inner if()
         pS = pTemp;
      }  // end outer if()
      pS[str_count++] = pStr;
   }  // end while()
   
   str_sort(pS, str_count);                        // Sort strings
   str_out(pS, str_count);                         // Output strings
   free_memory(pS, str_count);               // Free all heap memory
   
   return 0;
}  // end main()

Although the compiler generates the same error for functions str_sort() and str_out(), I will only show the code for str_sort() for brevity.

Code:

void str_sort(const char ** p, size_t n)
{
    bool sorted = false;                         // Strings sorted indicator
    while(!sorted)                                  // Loop until there are no swaps
    {
       sorted = true;                               // initialize to indicate no swaps
       for(int i = 0; i < n - 1; ++i)
       {
          if(strcmp(p[i], p[i + 1]) > 0)
         {
            sorted = false;                        // indicate we are out of order
            swap(&p[i], &p[i + 1]);            // Swap the string addresses
          }  // end if()
       }  // end for()
    }  // end while()
}  // end str_sort()

I thought this was the right way to use const but with the compiler objecting I am now not so sure. I have read through

Code:

https://stackoverflow.com/questions/14401856/how-do-i-best-use-the-const-keyword-in-c

but this seemed merely to agree with what I was already doing and so, sadly, did not help.

To compile this small program I use the following in Konsole.

gcc prog8_4.c -std=c11 -Wall -ggdb -o prog8_4

What am I not understanding about this use of const ?

Stuart