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Q&A When does it not work to dereference the pointer for sizeof during malloc?

In addition to all the examples I gave in my answer to the linked post, all scenarios where p is a pointer to incomplete type fails. Not just the void* scenario, but also when p is a pointer to an ...

posted 2y ago by Lundin‭

Answer
#1: Initial revision by user avatar Lundin‭ · 2022-02-14T14:58:33Z (about 2 years ago)
In addition to all the examples I gave in my answer to the linked post, all scenarios where `p` is a pointer to incomplete type fails. Not just the `void*` scenario, but also when `p` is a pointer to an array of incomplete type:

    int (*p)[] = malloc(n * sizeof *p); // will not compile cleanly

And also when using forward-declared structs/unions:

    struct fwd_declared* p;
    p = malloc(n * sizeof *p); // will not compile cleanly

And when using function pointers:

    void (*p)(void);
    p = malloc(n * sizeof *p); // will not compile cleanly

---

All of these examples (including de-referencing `void` pointers) are invalid C as per C17 6.5.3.4:

> **Constraints**
> 
> The `sizeof` operator shall not be applied to an expression that has function type or an incomplete type

Regarding "compile cleanly" please see [What must a C compiler do when it finds an error?](https://software.codidact.com/posts/277340) 

And please note that gcc/clang are by default in a non-standard mode until you pass along `-std=c17 -pedantic-errors` to tell them to become conforming implementations.

---

These are weak arguments against using a certain `malloc` style though.