Mirror of forum.nim-lang.org

5958 :: How to use pointer to mmaped file?

• readysloth (orginal) [2020-02-20T19:13:20+01:00] view original

  Hello guys! I have a quick question to resolve. I mmap some file from disk with memfiles module as streams and it works great, but some cases involve indexing mmaped file. Here is what i tried to do:

  var mmaped_file = memfiles.open("file", mode = fm_read_write)
  var mem_pointer = mmaped_file.mem
  var first_byte = mem_pointer[]
  Run

  results into:

  Error: type mismatch: got <pointer>
  Run

  Why? How can i access mmaped file as buffer?

---

• enthus1ast (orginal) [2020-02-20T19:43:43+01:00] view original

  You must cast the pointer to a datatype.

---

• readysloth (orginal) [2020-02-20T20:02:15+01:00] view original

  Thank you, that solves my issue :)

---

• cblake (orginal) [2020-02-20T20:28:54+01:00] view original

  You can probably use cstring if on a C backend, but if you wanted to index some other element type than char (like int or Foo) you can also do

  let foos = cast[ptr UncheckedArray[Foo]](mmaped_file.mem)
  if i * Foo.sizeof < mmapped_file.size:
    doSomethingWith foos[i]  # e.g. i == 0 for 1st Foo
  Run

  You may well be able to skip the range check every time if you are, e.g. looping over the file & know you won't overflow from that, etc. To recover automatic bounds checks, you can write new/use existing procs accepting openArray[Foo] (e.g. openArray[T]) and pass via toOpenArray explicitly in the parameter list:

  fun(toOpenArray(mf.mem, 0, mf.size div Foo.size - 1), a, b, c)
  Run

  That can be abbreviated with a template oaFoo(mf) if you need to pass it or various files in several places or something. If such procs want to modify the data the file needs to have been opened writable or you will get segmentation violations.

---