Functions

Just as with variables functions need to be defined before they can be used. To declare a function we use function prototypes, which include a name, the type of value it return and a list of parameters it takes. Parameters being values that the function it takes as input, also with a name and type just like variables.

Parameters are pass by value in C, meaning that a copy of the input is made on the stack which is local to the function body. Later on you can also pass by reference using pointers.

#include <stdio.h>
int addNumbers(int a, int b); // function prototype
 
int main()
{
    int n1 = 1, n2 = 2, sum;
 
    sum = addNumbers(n1, n2);
    printf("sum = %d",sum);
 
    return 0;
}
 
int addNumbers(int a, int b) {
    return a + b;
}

Pass by reference

You might find yourself often swapping values between two variables which would lead you to implementing a swap function and your first attempt might look something like this

#include <stdio.h>
void swap(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
    printf("swap: a=%d, b=%d\n", a, b);
}
int main()
{
    int a = 10;
    int b = 5;
 
    swap(a, b);
    printf("main: a=%d, b=%d\n", a, b);
 
    return 0;
}

When executing the above code you will notice that the desired result was not reached due to functions in java being pass by value. To fix this we can use pointers and create functions which are pass by reference.

#include <stdio.h>
void swap(int* a, int* b) {
    int temp = *a;
    *a = *b;
    *b = temp;
    printf("swap: a=%d, b=%d\n", a, b);
}
int main()
{
    int a = 10;
    int b = 5;
 
    swap(&a, &b);
    printf("main: a=%d, b=%d\n", a, b);
 
    return 0;
}

Multiple return values

By using pointers as so called output parameters you can have functions return more then one value.

Pointers to functions

Map

We can use pointer for functions for a multitude of things for example passing a function to a map function which applies the function to every element in the array.

#include <stdio.h>
#include <stdlib.h>
 
#define LENGTH 5
 
void map(int a[], int len, int (*f)(int))
{
    for (int i = 0; i < len; i++)
    {
        a[i] = f(a[i]);
    }
}
 
int inc(int i)
{
    return i + 1;
}
 
int main()
{
    int i;
    int values[LENGTH] = {88, 56, 100, 2, 25};
 
    printf("Before: ");
    for (i = 0; i < LENGTH; i++)
    {
        printf("%d ", values[i]);
    }
 
    map(values, LENGTH, inc);
 
    printf("\nAfter: ");
    for (i = 0; i < LENGTH; i++)
    {
        printf("%d ", values[i]);
    }
 
    return (0);
}

output

Before: 88 56 100 2 25
After: 89 57 101 3 26

QSort

Another common use case is when you want to use the qsort function from the standard library to sort an array.

c void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void*))

#include <stdio.h>
#include <stdlib.h>
 
#define LENGTH 5
 
int compareInts(const void *a, const void *b)
{
    return (*(int *)a - *(int *)b);
}
 
int main(void)
{
    int i;
    int values[LENGTH] = {88, 56, 100, 2, 25};
 
    printf("Before: ");
    for (i = 0; i < LENGTH; i++)
    {
        printf("%d ", values[i]);
    }
 
    qsort(values, LENGTH, sizeof(int), compareInts);
 
    printf("\nAfter: ");
    for (i = 0; i < LENGTH; i++)
    {
        printf("%d ", values[i]);
    }
 
    return (0);
}

output

Before: 88 56 100 2 25
After: 2 25 56 88 100

Macros

Macros are based on the define preprocessor directive, and work very similarly to functions. Just as when defining a key value pair you are limited to one line unless you use a backslash, "", at the end. Macros can be faster then normal functions because in the end they are just text substitutions and you therefore don't have the overhead when using functions like creating a new memory space. You must however be careful when using macros as they can not be debugged and because they really are just text substitute they can cause unexpected side effects.

#include <stdio.h>
#define PRINT(a) \
    printf("value=%d\n", a);
 
#define MAX(a, b) ((a) > (b)) ? (a) : (b)
 
int main(void)
{
    int a = 5;
    int b = 4;
 
    PRINT(a);            // 5
    int c = MAX(++a, b); // becomes ((++a) > (b)) ? (++a) : (b)
    PRINT(c);            // 7
}

Macro operators

We have already seen the first one in action \. Another one is defined which can be used to check if a symbol is already defined #if defined(DEBUG) which is very similar to #ifdef.

The # operator

If you place a # in front of a parameter in a macro definition is inserts double quotes around the actual marco argument and therefore makes it to a constant string. Strings that are separated by a white space are concatenated during preprocessing so you can do something like this

#include <stdio.h>
#define PRINTINT(var) printf(#var "=%d\n", var)
 
int main(void)
{
    int count = 100;
    PRINTINT(count); // printf("count" "=%d\n", count); -> printf("count=%d\n", count);
    return 0;
}

Control Flow Arrays