auto-compression-mode is enabled, Emacs automatically
uncompresses compressed files when you visit them, and automatically
recompresses them if you alter and save them. See Compressed Files.
The above feature works by calling an external executable (e.g., gzip). Emacs can also be compiled with support for built-in decompression using the zlib library, which is faster than calling an external program.
This function returns non-
nilif built-in zlib decompression is available.
This function decompresses the region between start and end, using built-in zlib decompression. The region should contain data that were compressed with gzip or zlib. On success, the function replaces the contents of the region with the decompressed data. On failure, the function leaves the region unchanged and returns
nil. This function can be called only in unibyte buffers.