Re: defn(`a b'): is it standard or extension? Is it reliable?

On Mon, Feb 02, 2026 at 01:52:07PM -0600, Eric Blake wrote:
> In practice, I've tested with BSD m4, which behaves almost like GNU m4
> in allowing arbitrary strings to be defined as a macro, and then later
> retrieved using defn, ifdef, or dumpdef, but with one caveat - BSD m4
> will not support the empty string as a macro name, even though GNU m4
> has no problem with that.  But beyond that, I can't definitively state
> whether there are other implementations of m4 that will choke on
> attempting to define an atypical macro name.

As a postscript not quite related your original question: Today I
noticed that in GNU m4, defn(`a', `b') produces the contents of `a'
first, then `b'; but BSD m4 produces the contents of `b' first, then
`a'.  So portable scripts should consider using defn(`a')defn(`b')
rather than getting defn() itself to do concatenation, even though my
read of POSIX says the GNU interpretation is desired.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org