How to pass an array by void pointer in C/C++

Asked Feb 10 '21 at 19:24

Active Feb 10 '21 at 19:48

Viewed 464 times

The function interface being used requires a single generic void pointer argument:

void* function(void* p_void) { ... }

The argument I need to pass happens to be an array of objects. Let's go back to something simpler; for a normal datatype, it's possible to write:

void* function(void* p_void) {
    type* p_object = static_cast<type*>(p_void);
}

Is replacing this type of syntax for arrays possible? If it is, I'm not sure how that works -- how would the function know how long the array is?

Is there a reference that shows how to do this? Something like Range[]* p_array = static_cast<[Range]*>(p_void) obviously doesn't work in C.

edited Feb 10 '21 at 19:40

asked Feb 10 '21 at 19:24

Denis G. Labrecque

1,023
13
29

You need to pass also the number of elements in the array. – Vlad from Moscow Feb 10 '21 at 19:25
And then do some pointer arithmetic or array indexing on each element. As an aside, C and C++ are two different languages. – Robert Harvey Feb 10 '21 at 19:26
2

Are you using C or C++? In C++, you'd probably use a template. – NathanOliver Feb 10 '21 at 19:27
@NathanOliver C++, the reason I mention C is that it's actually using C libraries and compiled in gcc then using the g++ linker. – Denis G. Labrecque Feb 10 '21 at 19:28
Why not use a vector? See https://stackoverflow.com/questions/2923272/how-to-convert-vector-to-array for how to convert it to an array when calling the C library. – Barmar Feb 10 '21 at 19:31
@Barmar the function I'm actually calling is `pthread_create()`. I'm not sure about the implications for thread safety. – Denis G. Labrecque Feb 10 '21 at 19:32
1

The implications should be no different than for a normal array. Just make sure you don't access the vector in multiple threads. – Barmar Feb 10 '21 at 19:34
C and C++ have two *completely* different approaches here. Please don't conflate the two. – tadman Feb 10 '21 at 19:34
1

Is there any reason you're not using `sdt::thread` so you can avoid all of this `void*` trouble? – NathanOliver Feb 10 '21 at 19:35
@NathanOliver yes, it's an assignment on `pthread`. – Denis G. Labrecque Feb 10 '21 at 19:36
@Barmar your idea works, sort of. Because the address to the vector is held in shared memory, concurrent threads get confused about which vector they are pointing to, causing faults. – Denis G. Labrecque Feb 10 '21 at 19:53

How to pass an array by void pointer in C/C++

0 Answers0