In C how is this parameter declared in the function?

Question

I am trying to learn the basics of C using The C Programming Language - Brian Kernighan and Dennis Ritchie

In the program below, I don't understand where the value for 'maxlineLength' comes from?

The for loop is set to run while 'i' is smaller than maxLineLength-1, but what is the value of maxLineLength and where does it come from?

From what I understand, when parameters are declared in a function like this a value is being passed to them, so they must surely be declared somewhere else to have the value passed in?

#include <stdio.h>
#define MAXIMUMLINELENGTH 1000
#define LONGLINE 20

main() {
 int stringLength;
 char line[MAXIMUMLINELENGTH];

  while((stringLength = getLineLength(line, MAXIMUMLINELENGTH)) > 0)
    if(stringLength < LONGLINE){
  printf("The line was under the minimum length\n");
    }
    else if (stringLength > LONGLINE){
        printf("%s", line);
    }
  return 0;
}


int getLineLength(char line[], int maxLineLength){
  int i, c;

  for(i = 0; i< maxLineLength-1 && ((c = getchar())!= EOF) && c != '\n'; i++)
    line[i] = c;

  if(c == '\n') {
      line[i] = c;
      i++;
  }

  line[i] = '\0';
  return i;
}

Answer 1

From what I understand, when parameters are declared in a function like this a value is being passed to them, so they must surely be declared somewhere else to have the value passed in?

In this case, the function is being declared and defined at the same time. Pre-ANSI C allowed that. This is considered a bad practice today. You should add a forward declaration

int getLineLength(char line[], int maxLineLength);

above main in order to avoid declaring the function implicitly.

When this forward declaration is missing, the first use of the function becomes its implicit declaration. The compiler takes types of the expression parameters that you pass, and assumes that the function takes corresponding types as its parameters. It also assumes that the function returns an int. In this case the compiler has all the information necessary to figure out the correct signature, so the function is defined correctly. However, in more complex cases (say, if maxLineLength was declared as long long, not int) it would lead to undefined behavior, so you should always include a forward declaration of your functions.

Answer 2

getLineLength(line, MAXIMUMLINELENGTH)

MAXLINELENGTH is defined constant

Inside function getLineLength is this value "mapped" to maxLineLength variable