fitsio | A python package for FITS input/output wrapping cfitsio | Grid library

Well, first of all, you need to know that Vec and &mut [u8] are not quite the same as C or C++'s uint8_t *. The main difference is that Vec and &mut [u8] have the size of the array or slice saved within themselves, while uint8_t * doesn't. The Rust equivalent to C/C++ pointers are raw pointers, like *mut [u8]. Raw pointers are safe to build, but requires unsafe to be used. However, even tho they are different types, a smart pointer as &mut [u8] can be casted to a raw pointer without issue AFAIK.

Secondly, the capacity of a Vec is different of its size. Indeed, to have good performances, a Vec allocates more memory than you use, to avoid reallocating on each new element added into vector. The length however is the size of the used part. In your case, you ask the Vec to allocate a heap space of length length_buffer, but you don't tell them to consider any of the allocated space to be used, so the initial length is 0. Since C++ doesn't know about Vec and only use a raw pointer, it can't change the length written inside the Vec, that stays at 0. Thus the panicking.

To resolve it, I see multiple solutions:

• Changing the Vec::with_capacity(length_buffer) into vec![0; length_buffer], explicilty asking to have a length of length_buffer from the start

• Using unsafe code to explicitly set the length of the Vec without touching what is inside (using Vec::from_raw_parts). This might be faster than the first solution, but I'm not sure.

• Using a Box<[u8; length_buffer]>, which is like a Vec but without reallocation and with the length that is the capacity

• If your length_buffer is constant at compile time, using a [u8; length_buffer] would be much more efficient as no allocation is needed, but it comes with downsides, as you probably know

If you check the related documentation :

read_healpix_map

This routine reads a full sky HEALPix map from a FITS file

Location in HEALPix directory tree: src/C/subs/read_healpix_map.c

FORMAT float *read_healpix_map(char *infile, long *nside, char *coordsys, char *ordering)

ARGUMENTS

Per the documentation of read_healpix_map():

ordering and coordsys are output arguments - they are pointers to space into which the function will write results. You have passed pointers to string constants in memory that will normally be marked read-only and may not be sufficiently large to receive the result in any case.

nside is also an output argument, initialising it to 512 serves no purpose. You do not need p_nside; you can simply pass &nside

It is not the most clear of interfaces; specifically it is not clear whether coordsys is a string or a single char. The documentation uses single quotes so you might assume char but then it does that for ordering too and that is clearly a string. It is safest to assume coordsys is a string - it will do no harm.

From the error output:

configure: error: Unable to locate bz2 library needed when enabling bzip2 support; try specifying the path

It looks very likely you will need the bz2 library and its development headers. How you install these will depend on your OS. For example on Debian/Ubuntu it's:

No need for sudo. On Colab, you're already running as root.

Here's a complete example notebook:

https://colab.research.google.com/drive/1RqtDwzhL8vWEJ-3ruGUGo-QckWr6gHUJ