How Do I Declare a Function Pointer in C?

As a variable:

returnType (*variableName)(parameterTypes) = function_name;

As a static const variable:

static returnType (* const variableName)(parameterTypes) = function_name;

As an array:

returnType (*arrayName[])(parameterTypes) = {function_name0,
        ...};

(example code)

As a parameter to a function:

int my_function(returnType (*parameterName)(parameterTypes));

(example code)

As a return value from a function:

returnType (*my_function(int, ...))(parameterTypes);

(example code)

As a cast (but try not to cast functions):

... (returnType (*)(parameterTypes))my_expression ...

(example code)

As a function pointer typedef:

typedef returnType (*typeName)(parameterTypes);

(example code)

As a function typedef:

typedef returnType typeName(parameterTypes);

(example code)

How Do I Do It in C++? (C++11 and later)

C++ offers several compelling alternatives to C-style function pointers such as templates and std::function with std::bind. If you really want to use function pointers in C++, you can still use the same C-style syntax shown above or the type aliases below.

As a function pointer type alias:

using
        typeName = returnType (*)(parameterTypes);

(example code)

As a function type alias:

using
        typeName = returnType (parameterTypes);

(example code)

This site is not intended to be an exhaustive list of all possible uses of function pointers.
If you find yourself needing syntax not listed here, it is likely that a typedef would make your code more readable.