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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: function pointer question
Date: Sat, 03 Jan 2026 16:39:14 -0800
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David Brown <david.brown@hesbynett.no> writes:
> On 02/01/2026 23:18, Chris M. Thomasson wrote:
[...]
>> typedef void (*generic_func_ptr)(void)
>
> There is no generic function pointer type equivalent to "void *" - you
> are always going to need casts when converting between different
> pointer types.  And you /really/ need to make sure you convert to
> exactly the correct type before calling the pointed-to function.  But
> the definition you gave here is commonly used when people want to have
> generic function pointers.

Yes, but ...

There is no generic function pointer type equivalent to void* in the
sense that values can be converted to and from the type implicitly.
For example:

    int n = 42;
    void *vp = &n;   // implicit conversion
    double *dp = vp; // implicit conversion, potentially dangerous

There are no such implicit conversions for function pointer types.

However, in the sense of a round-trip conversion yielding the
original result *all* function pointer types can be treated as
generic function pointer types.  You just have to use casts to do
any conversions.

It's very common for all pointer types in a given implementation
to have the same representation, and for round-trip conversions
to safely yield the original value, but it's not guaranteed by
the language, and there are implementations where, for example,
function pointer types are bigger than object pointer types, or
void* is bigger than int*.  Certain collections of pointer types
are guaranteed by the language to have the same representation:

- void*, char*, signed char*, unsigned char*
- All struct pointer types
- All union pointer types
- All function pointer types

Incidentally, if I want a generic function pointer type, I might
define it so that it can't be used accidentally without a cast.
For example:

    typedef struct dummy__ (generic_function)(void);

where struct dummy__ is an incomplete type.  (Note that I've
typedef'ed the function type, not the pointer type.)  But that
might be considered overkill; treating a void function with
no parameters as generic is probably good enough.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */