GNU m4は、昔ながらのUnixマクロ・プロセッサの実装です。若干、拡張していますが（例えば、９以上の位置パラメータをマクロで処理）、ほとんどSVR4と互換性があります。また、GNU m4には、ファイルのインクルード、シェル・コマンドの実行、算術演算の実行などのための機能が組み込まれています。Autoconfは、configure'スクリプトの生成にGNU m4を必要としますが、実行には必要ありません。
A longstanding bug where the "format" builtin could crash on an invalid format string was fixed. Compilation against newer glibc and on AIX 7.1BETA was fixed. A number of portability improvements were inherited from gnulib.
A security issue in "make dist", reported as CVE-2009-4029, was resolved. The new "-g"/"--gnu" command-line option now overrides "-G"/"--traditional", in preparation for not supporting GNU extensions and behaving POSIXly correct without it. The "-L"/"--nesting-limit" command-line option now defaults to 0 for unlimited on platforms that can detect and deal with stack overflow. The "--debugfile" command line option now defaults to stderr. The input engine was optimized for faster processing. Several performance improvements were made. Several portability issues and regressions were resolved.
Security fixes were made for the -F option. A
regression introduced in 1.4.9b in the "divert"
builtin was fixed. The output of the "maketemp"
and "mkstemp" builtins is now quoted if a file was
created. The "defn" builtin was enhanced to
support concatenation of multiple text arguments,
as required by POSIX. The "format" builtin was
enhanced to parse all C99 floating point numbers.
The "index" builtin was enhanced to guarantee