Re: what is $ac_t ?

[Eric Blake]

On 09/17/2010 10:25 AM, Sam Steingold wrote:
> google appears to suggest that $ac_t is widely used in older configure
> scripts as a tab character, e.g.:
> echo "$ac_t""found" 1>&6
> it appears that it it not mentioned in any docs and not defined to
> anything useful for some time.

Commit ab7322949, 1124 commits before the long-ancient 2.50 release:

2000-02-25  Akim Demaille  <address@hidden>

        Export the knowledge on disabling echo's trailing new line.

        * acspecific.m4 (AC_PROG_ECHO_N): Rename as...
        (_AC_PROG_ECHO): this.
        Set ECHO_N, ECHO_C, ECHO_T instead of ac_n, ac_c and ac_t.

At that point, $ac_t was no longer defined by autoconf, so anyone using 
it now gets "" instead of "<tab>".

Really, anyone with $ac_t in their script should modernize, and use 
ECHO_T instead.

> since recently, however, it is used in a loop setting ac_install_sh, and
> now it is set to "install-sh". thus now I see configure messages like
>
> checking for getrlimit declaration... (cached) install-shextern int
> getrlimit (enum __rlimit_resource, struct rlimit *);

Thanks for the report.  That's an unfortunate side effect of trying to 
use a temporary variable to work around a shell portability issue with 
"`""`", introduced in commit c0b647a8c, Oct 2007, 2.62 release.

> Thus my questions are:
>
> 1. optional: what is the history of the matter?

I think I've covered that :)

> 2. required: is there a replacement for ac_t or should it be dropped?

Yes, we can avoid using $ac_t for our temporary variable name, to 
respect the fact that there are some _really_ old scripts that are 
exploiting formerly undocumented internals that no longer exist. 
Although those configure.in should be modernized, we can at least make 
their output look prettier with a simple change to avoid the problematic 
name.

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