Re: Having Trouble with m4 Input (m4 version 1.4.13)

[Eric Blake]

On 09/03/2010 09:56 AM, Tom Browder wrote:
> I am trying to use m4 (version 1.4.13) to generate a C header file and
> am approaching it cautiously because my experience with m4 has not
> been comforting.

You may want to consider upgrading to 1.4.15, although I don't think 
that will impact your usage issue.

> My initial m4 input file has these lines:
>
> =====>
> divert(-1)
> define(`hashdef', `#define')dnl
> define(`Min', include(`conf/MINOR'))
> divert(0)dnl
> hashdef __PROGVER_MINOR__ Min
> <====
>
> When I execute "m4<m4 input file>" I get:
>
> ====>
> #define __PROGVER_MINOR__ Min
> <====
>
> Notice 'Min is not expanded.

> Is the '#' stopping it?

Correct.  And that's because, by default, # is the m4 comment character, 
and once hashdef is expanded, you have turned the rest of the line into 
an m4 comment.

>  If so, how can I get the line I want.

By either disabling m4 comments:

changecom()

or by using proper quoting (so that hashdef no longer expands to a 
comment character, but a quoted string that happens to contain #):

define(`hashdef', ``#'define')dnl

Also, to be robust to non-GNU m4, you should probably also double-quote 
define, or use other tricks to ensure that you don't inadvertently end 
up expanding the define built-in with zero arguments (GNU m4 explicitly 
documents that the define built-in when invoked with zero arguments 
expands to it's own name, as if it hadn't been a macro, but POSIX leaves 
that bit unspecified):

define(`hashdef', ``#define'')dnl

--
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org