Tweaking the program name for <err.h> functions

[Guillem Jover]

Hi!

On Thu, 2024-03-07 at 23:24:32 +0100, Alejandro Colomar wrote:
> I'm interested in knowing if the <err.h> functions have a supported way
> of tweaking the prefix they print.
> 
> GNU's similar <error.h> functions support changing
> 
> 	extern char *program_invocation_name;
> 
> for that, as per the error(3) manual page:
> 
>      The  program  name  printed by error() is the value of the global
>      variable   program_invocation_name(3).    program_invocation_name
>      initially  has  the same value as main()’s argv[0].  The value of
>      this variable can be modified to change the output of error().
> 
> 
> I have experimented with err(3), and it seems you can tweak it by
> modifying 'program_invocation_short_name', but I'd like to know if
> that's supported and/or portable.  I guess that since the pointer is a
> GNU extension, while the functions come from the BSDs, that's not
> portable, and thus not supported.

That is not portable because the BSDs do not support that variable.
But all BSDs for which I've got a checkout (FreeBSD, NetBSD, OpenBSD,
DragonflyBSD) support changing it via setprogname(), but that's not
available in glibc (libbsd provides it though).

libbsd and cosmopolitan libc support setting it via setprogname() or
program_invocation_short_name.

musl libc supports setting it via program_invocation_short_name.

uclibc might support __progname and/or program_invocation_short_name
depending on how it has been configured, but it does not support
setting the err(3) program name via those.

> Maybe it would be interesting to get the BSDs to support these pointers?

They already have a way to control the program name though. (And
it seems to me that using functions instead of global variables is
superior. :)

Thanks,
Guillem