How to avoid "use of possibly uninitialized data"

help
1

I have a DLL written in C and a main program also written in C to call the DLL. Recently I am translating the main program from C to rust, but keep the DLL unchanged. Today I encounter the following issue.

In the main program, the following C function exported by the DLL is called:

int php_sys_stat_ex(const char *path, zend_stat_t *buf, int lstat);

Bindgen translates this declaration into rust:

extern "C" {
     pub fn php_sys_stat_ex(
         path: *const ::std::os::raw::c_char,
         buf: *mut zend_stat_t,
         lstat: ::std::os::raw::c_int,
     ) -> ::std::os::raw::c_int;
}

In this function the second argument is a pointer to a data structure, which is filled according to the information supplied by the first argument. So the initialization of the data structure *buf is done in this function. I tried to translate the statements calling this function into rust, but I saw the following error:

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Then my question is: How could I avoid this error? The initialization is supposed to be done in the function I want to call, but the rust compiler stops me calling the function because of "use of possibly uninitialized data"!

2

If you don't want to initialize the var with dummy data, use MaybeUninit in std::mem - Rust
Calling as_mut_ptr

3

Thank you! Can you give more detailed guide? I am a newcomer to rust.

4

MaybeUninit is not about dummy data. It's about uninitialized data, and touching it's content before initialization is UB means you're violating the language's rule.

5

So basically the usage could look like this

let mut sb = MaybeUninit<zend_stat_t>::uninit();
if php_sys_stat_ex(document_root.as_ptr() as *const i8, sb.as_mut_ptr(), 0) != 0 {
    …
    panic!(…);
}
// If php_sys_stat_ex returning success
// guarantees `sb` got initialized, then this is safe:
let sb = unsafe { sb.assume_init() }:

// use and/or return `sb`

To avoid UB, you must make sure that php_sys_stat_ex is okay with being passed a pointer to uninitialized memory and that it properly initializes it.

1 Like
6

Whopse, my phrasing was off.
I meant that as MaybeUninit or dummy value

7

Thank you so much!

9

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