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 in The GNU Emacs Manual.
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-
nil if built-in zlib decompression is available.
function zlib-decompress-region start end \&optional allow-partial
This function decompresses the region between
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. If
nil or omitted, then on failure, the function leaves the region unchanged and returns
nil. Otherwise, it returns the number of bytes that were not decompressed and replaces the region text by whatever data was successfully decompressed. This function can be called only in unibyte buffers.