Calling function pointers

Learn to call function pointers.

Two ways to call

You can call a function pointer in two ways: directly fp(arg) or indirectly (*fp)(arg). Both are valid and give the same result.

Example: after defining int (*fp)(int) = negate;, both fp(7) and (*fp)(7) return -7. The first form is more common and shorter. The second makes the indirection explicit; some old code prefers it. Which to use is a style choice; modern C commonly uses fp(arg) directly.

When to use a function pointer?

A function pointer is used when you want to choose the behavior at runtime. For a fixed function call it's unnecessary.

For example in a calculator the user may enter 'add' or 'sub'; you keep a function pointer and call it. Or a sort algorithm takes the comparison via a function pointer (like qsort). Calling a fixed function directly is simpler; a function pointer earns its place when flexibility is needed. That's why real-world code shows it only in specific patterns.

Exercise Call both ways

Write int negate(int x) returning -x. Declare the pointer with int (*fp)(int) = negate;. Call both ways: printf("%d\n", fp(7)); and printf("%d\n", (*fp)(7));.

Show example solution

#include <stdio.h>

int negate(int x) {
    return -x;
}

int main(void) {
    int (*fp)(int) = negate;
    printf("%d\n", fp(7));
    printf("%d\n", (*fp)(7));
    return 0;
}

Test yourself

Match the call forms

fp(7)

Direct call
(*fp)(7)

Indirect call
fp = negate

Assignment
int (*fp)(int)

Type
What's the relation between fp(7) and (*fp)(7)?
- fp(7) is invalid
- (*fp)(7) is invalid
- They give the same result
- They give different results
Show answer

They give the same result — Both forms call the function pointer; same result.

Complete the indirect call

int (*fp)(int) = negate;
int r = (____)(7);

Show answer

int (*fp)(int) = negate;
int r = (*fp)(7);

Next lesson: qsort

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