Can't call library function in user defined function

Question

I am making a user defined function of strcmp() in c.

#include <stdio.h>
#include <string.h>
int la,lb;    // la and lb for length of 2 strings
 
int scompare(char [],char []);

int main() {

 char a[50],b[50];     // a and b are 2 strings

printf("Enter 2 strings \n");
gets(a);
gets(b);
la =strlen(a);
lb =strlen(b);
printf("%d",scompare(a,b));

 }
 int scompare(char a[la],char b[lb])
   {
     for(int i=0 ;i<max(la,lb);i++)
     { // loop for comparing characters of 2 strings

       // here in vs code it is showing error --->
       // warning: implicit declaration of function 'max' [-Wimplicit-function-declaration]

for(int i=0 ;i<max(la,lb);i++)

         int k = a[i]-b[i];// k is the sum of ascii of characters of a and b
   
         if(k!=0 &&toupper(a[i])==toupper(b[i]))
         return (k>0)?1:-1;

         // other error is showing here in function toupper --->  warning: implicit declaration of function 'toupper' [-Wimplicit-function-declaration]

if(k!=0 &&toupper(a[i])==toupper(b[i])) 

         else if(k!=0)
        return k;
    
    }
  return 0;
 } 

Answer 1

Before calling a function you need to provide its declaration that can be present for example in a header.

There is no standard function max in C. You need to write such a function yourself.

To use the standard function toupper you need to include the header <ctype.h>

#include <ctype.h>

This loop (if to assume that the function max is defined somewhere)

for(int i=0 ;i<max(la,lb);i++)

can invoke undefined behavior then lengths of strings are not equal to each other.

Pay attention to that the function gets is not a standard C function. It is unsafe. You should use fgets.

And this function declaration

int scompare(char a[la],char b[lb]);

does not make a sense.

The function should be declared like

int scompare( const char a[], const char b[] );