Is there a way to declare the variables a and b in this code?

Question

i am trying to call a function in main so that my code will execute through all parts of my code. Now when i call for compare_quads in main. i am getting an error code of a and b not being declared. but my problem is that i do not know how to get the variable declared because i have declared the function and variable at the top of the code i thought that would work. and if i try declaring the variable in main like

const void *a;
const void *b;

then when compiling i receive, warning a is used uninitialized in this function, and similarly with b.

here is my code,

//declare libraries
  #include <string.h>
  #include <stdio.h>
  #include <stdlib.h>
 
  //declare other functions/files to be used in the program
  void print_fun(void);
  void read_fun(void);
  static int compare(int arg, unsigned char networks[arg][4]);
  int compare_quads(const void *a, const void *b);
 
 
  //read command line input and store the information
  int main(int argc, char** argv){
      //declar variable
      int arg = 0;
 
      //make argv into an int
      arg = atoi(argv[1]);
      //assign size to networks
       unsigned char networks[arg][4];
 
      //assign input to networks
      for (int j =0; j<1; ++j){
          if(argc == 1)
               {
                  printf("ERROR ERROR, you messed up\n");
              }
  
         else
         {
          // hold network addresses in a 2-d array, with 4 unsigned char
 
              for(int k = 0; k<arg; k++){
                  for (int i =0; i<4; i++){
 
                  scanf("%hhu.", &networks[k][i]);
                  //checks to see if scanf was working properly
                 // printf(" %hhu",networks[k][i]);
 
              }
                  //printf("\n");
              }}}
 
 
      compare_quads(a, b);
      compare(arg, networks);
 
  return(0);
  }
 
 
  int compare_quads( const void *a, const void *b) {
      return memcmp (a, b, 4);
  }
 
 
  static int compare(int arg, unsigned char networks[arg][4])
  {
 
  qsort(networks, arg, sizeof(networks[0]), compare_quads);
 
      for (int k = 0; k< arg; k++){
          printf("%d.%d.%d.%d\n", networks[k][0],networks[k][1],networks[k][2],networks[k][3]);
          }
          return 0;
 }

I am pretty new to c, so please let me know if you need any clarification. thank you.

the exact warnings are

unitilazed

main.c: In function ‘main’:
main.c:47:19: error: ‘a’ undeclared (first use in this function)
   47 |     compare_quads(a, b);
      |                   ^
main.c:47:19: note: each undeclared identifier is reported only once for each function it appears in
main.c:47:22: error: ‘b’ undeclared (first use in this function)
   47 |     compare_quads(a, b);
      |                      ^

when const void *a; is used to initalize.

main.c: In function ‘main’:
main.c:48:5: warning: ‘a’ is used uninitialized in this function [-Wuninitialized]
   48 |     compare_quads(a, b);
      |     ^~~~~~~~~~~~~~~~~~~
main.c:48:5: warning: ‘b’ is used uninitialized in this function [-Wuninitialized]

EDIT I am taking in one input file that has, a various amount of lines with network address like

139.72.16.202

i am storing the values in an array of size [variable that is set by arg][4]

then after the main function the rest i am using to sort the code by column. the sorting function worked fine.

Answer 1

A pointer to the compare_quads function is getting passed to the qsort function as a comparison function, so it will get called internally by qsort.

Because of this, you don't need to call compare_quads in your main function. Passing it to qsort is enough.