CIS241

System-Level Programming and Utilities

C Functions

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

RECAP (from bash + 162/163)

What is a function?

• Blocks of reusable code
• Typically should accomplish one specific task

Why use them?

• Save time writing code
• Improves readability
• Modularity
• PREVENTS ERRORS

Compare to Python

In Python:

def get_smaller_value(a, b):
    if a < b: return a
    return b

In C:

int get_smaller_value(int a, int b) 
{
  if (a < b) return a;
  return b;
}

Calling it!
int result = get_smaller_value(7, x);

c/o Ferguson

And now - the fun of C

Function prototype

int get_smaller(int a, int b);

OR

int get_smaller(int, int);

Function definition

int get_smaller(int a, int b)
{
    if (a < b) return a;
    return b;
}

e.g.

int get_smaller(int a, int b); // prototype

int main(int argc, char* argv[])
{
    if (get_smaller(1, 2) == 1) return 1;
    return 0;
}

int get_smaller(int a, int b) // definition
{
    if (a < b) return a;
    return b;
}

Return types

ALL functions must declare a return type

• If no return, then must return void

int ex1(char ch)
{
    return (int)ch;
}

void ex2(int a, int b)
{
    // do things

    return;  // but not necessary
}

Wat does this do?

int increment(int x)
{
  x++;
  return x;
}
int main(void) 
{
  int i = 0;
  printf("i = %d\n", i);
  int res = increment(i);
  printf("res = %d\n", res);
  printf("i = %d\n", i);
  return;
}

Wat does this do?

int increment(int x)
{
  x++;
  return x;
}
int main(void) 
{
  int i = 0;
  printf("i = %d\n", i);     // 0
  int res = increment(i);
  printf("res = %d\n", res); // 1
  printf("i = %d\n", i);     // 0
  return;
}

Arguments

C is always pass-by-value

• Not pass-by-reference

How can we change arguments?

• Pass a pointer!

And wot does this do?

int increment(int* p)
{
  (*p)++;
  return *p;
}
int main(void)
{
  int i = 0;
  printf(“i = %d\n”, i);
  int res = Increment(&i);
  printf(“res = %d\n”, res);
  printf(“i = %d\n”, i);
  return 0
}

And wot does this do?

int increment(int* p)
{
  (*p)++;
  return *p;
}
int main(void)
{
  int i = 0;
  printf("i = %d\n", i);     // 0
  int res = Increment(&i);
  printf("res = %d\n", res); // 1
  printf("i = %d\n", i);     // 1
  return 0
}

And how about passing arrays (͡ ͡° ͜ つ ͡͡°)

And can we write a function to print an array nicely?

And how about passing arrays (͡ ͡° ͜ つ ͡͡°)

And can we write a function to print an array nicely?

void PrintArray(int* arr, int len)
{
  printf("[");
  int i = 0;
  for(i = 0; i < len; i++)
    printf(" %d", arr[i]);
  printf(" ]\n");
}

Must also pass length!

Scope

void DoStuff()
{
  int x = 10;            
  printf(“In func: x = %d\n”, x);
  ...
}
int main()
{
  int x = 0;
  printf("x = %d\n", x); 
  DoStuff();
  printf("x = %d\n", x);
}

Scope

void DoStuff()
{
  int x = 10;            // x = 10
  printf(“In func: x = %d\n”, x);
  ...
}
int main()
{
  int x = 0;
  printf("x = %d\n", x); // x = 0
  DoStuff();
  printf("x = %d\n", x); // x = 0
}

Scope

Functions have their own scope

• Variables in function don’t exist outside it
• Functions can’t access variables in main

Is this code valid?

int* MakeInt()
{
  int x = 10;
  return &x;
}

• Nope! x falls out of scope when we return
• Memory address is then pointing at nothing :(