memfiles

nimony/lib/std/memfiles.nim

This module provides support for memory mapped files:idx: (Posix's mmap:idx:) on the different operating systems.

type MemFile = object
  mem: pointer
  size: int64
  handle: int32
  flags: int32

proc open(filename: string; mode: FileMode; mappedSize: int64; offset: int64; newFileSize: int64; allowRemap: bool; mapFlags: int32): MemFile
opens a memory mapped file. If this fails, OSError is raised.
newFileSize can only be set if the file does not exist and is opened with write access (e.g., with fmReadWrite).
mappedSize and offset can be used to map only a slice of the file.
offset must be multiples of the PAGE SIZE of your OS (usually 4K or 8K but is unique to your OS)
allowRemap only needs to be true if you want to call mapMem on the resulting MemFile; else file handles are not kept open.
mapFlags allows callers to override default choices for memory mapping flags with a bitwise mask of a variety of likely platform-specific flags which may be ignored or even cause open to fail if misspecified.
Example:
`nim var mm, mmfull, mmhalf: MemFile
mm = memfiles.open("/tmp/test.mmap", mode = fmWrite, newFileSize = 1024) # Create a new file mm.close()
# Read the whole file, would fail if newFileSize was set mm_full = memfiles.open("/tmp/test.mmap", mode = fmReadWrite, mappedSize = -1)
# Read the first 512 bytes mm_half = memfiles.open("/tmp/test.mmap", mode = fmReadWrite, mappedSize = 512) `
proc close(f: var MemFile)
closes the memory mapped file f. All changes are written back to the file system, if f was opened with write access.