Trivial-MMAP is a Common Lisp library that aims to provide an easy-to-use API for working with memory-mapped files.

The library exports the following symbols:

  • mmap-file a function that maps a file into memory
  • munmap-file a function that removes a memory-mapped file from memory
  • mmap-read-char a helper function that facilitates reading a character from a memory-mapped file
  • mmap-read-byte a helper function for reading one byte from a memory-mapped file
  • with-mmap-file a helper macro that ensures that a memory-mapped file is safely unmapped (using munmap-file function) after we are done with it

Trivial-MMAP is based on code snippets in a blog post by Nicolas Martyanoff.

Getting Started

The following code snippet shows how to use Trivial-MMAP's with-mmap-file and mmap-read-char to read one character at a time from a file containing Twitter social graph (780 MB uncompressed).

CL-USER> (time
          (trivial-mmap:with-mmap-file (ptr size "/home/zodmaner/")
            (loop :for offset :below size :do
              (assert (characterp (trivial-mmap:mmap-read-char ptr offset))))))
Evaluation took:
  3.400 seconds of real time
  3.400000 seconds of total run time (3.360000 user, 0.040000 system)
  100.00% CPU
  8,479,536,815 processor cycles
  65,952 bytes consed

The little toy benchmark above was run on my laptop (an i5 ThinkPad X230 (Ivy Bridge) with 8 GB of RAM and an SSD). For comparison, reading the same file using with-open-file and read-char takes around 14 seconds of real time on the same machine.



  • Currently the library only supports reading data from memory-mapped files.
  • The library only supports Linux. It may work on MacOS, but definitely not on Windows.

Author & Maintainer

Smith Dhumbumroong ()


Public Domain

Smith Dhumbumroong <>
Public Domain