error : implicit declaration of function 'xxx'

This error occurs when you try to call a function that has not been declared or defined in your code.

To solve this error, you need to either declare the function before calling it or include the header file that contains the function declaration.

For example, if you are using a function called "foo" in your code, you need to either declare it before calling it:

void foo(); // function declaration

int main() {
  foo(); // function call
  return 0;
}

void foo() {
  // function definition
}

Or include the header file that contains the function declaration:

#include "myheader.h" // header file that contains function declaration

int main() {
  foo(); // function call
  return 0;
}

Make sure that the function name and parameters match the declaration or definition.