using pointers in C in a function code example

Example 1: c function pointer in argument

void f(void (*a)()) {
    a();
}

void test() {
    printf("hello world\n");
}

int main() {
     f(&test);
     return 0;
}

Example 2: c function pointer

//Declaration of a pointer to a function that takes an integer 
//and returns an integer.
int (*f_ptr)(int);

//Assignment of a function foo to the function pointer f_ptr declared above.
f_ptr = foo;

//Calling foo indirectly via f_ptr, passing the return value of foo to r.
int r = f_ptr(v);

//Assigning an address of a function to the function pointer f_ptr,
//then calling foo by dereferencing the function pointer.  
f_ptr = &foo;
int r = (*f_ptr)(v);