Casting unsafe pointers for test case?

MarcWeber · April 30, 2024, 8:40am

I have written bindings to CSV ZSV library which allows to read CSV files
without utf-8 checking using SIMD.

The C struct allows to assign a processing function, a FILE and fread like function.

for testing I'd like to just use some memory b"..." and custom read implementation.
But I horribly fail at unsafely passing pointer to &[u8] as STREAM to my
reading function.

Or is it best to use temporary file and be done ?

The fear I have is that I hit the problem again and again and again ?

It's about this myread function which should behave like read but just copy
some memory to zsv to make it happy.

github.com

MarcWeber/rust-zsv-bindings/blob/master/tests/zsv_test.rs#L25


      
            let cell_count: usize =  unsafe { zsv_cell_count(my_opts.parser) } ;
            let row: Vec<_> = (1..cell_count).map(|x| {
              let c = unsafe { zsv_get_cell(my_opts.parser, x) };
              let s = unsafe { slice::from_raw_parts(c.str_, c.len) };
              Box::from(s)
            }).collect();
            my_opts.cells.borrow_mut().push(row)
          }
          
          
          pub unsafe extern "C" fn myread(
              mut ptr: *mut libc::c_void,
              mut size: usize,
              mut nmemb: usize,
              mut stream: *mut ::std::os::raw::c_void,
          
          ) -> usize {
            let x: Box<[u8]> = Box::from_raw(transmute(stream));
            return 0;
          }

Not sure wheret he Box::into_raw etc is best or whether I can just unsafe {
&csv_contents[..] } or such ?

The basic question is does Rust allow to turn any pointer into untyped pointer
and back into a different pointer even if its unsafe and 'know what you're
doing' thing ?

yyogo · April 30, 2024, 10:35am

A boxed slice is a fat pointer, it also contains the length of the slice, which isn't included in the pointer. You need to use slice::from_raw_parts_mut for that. Additionally, since Box is an owned pointer, when the function returns it immediately gets dropped and the memory gets freed. You need to use a borrowed slice if you want the memory to stay allocated.

system · July 29, 2024, 10:35am

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Casting unsafe pointers for test case?

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