"%s" format directive with a NULL pointer

[Collin Funk]

Moving this to bug-gnulib@gnu.org and CC'ing Bruno because this
discussion made me notice something.

Paul Eggert <egg...@cs.ucla.edu> writes:

> On 2025-09-05 20:14, Collin Funk wrote:
>> This patch works fine, with some formatting adjustments.
>> But maybe it is just better to use fprintf, which can handle NULL
>> pointers.
>
> fprintf can handle null pointers?

Well, it depends. It is a GNU extension.

Using the test program:

    $ cat main.c
    #include <stdio.h>
    int
    main (void)
    {
      fprintf (stdout, "%s\n", NULL);
      return 0;
    }

On GNU/Linux:

    $ gcc main.c 
    $ ./a.out 
    (null)

On Solaris 10:

    $ gcc main.c
    $ ./a.out 
    Segmentation Fault (core dumped)

My original message assumed that Gnulib would support it. In
lib/printf-args.c we have the following:

      case TYPE_STRING:
        ap->a.a_string = va_arg (args, const char *);
        /* A null pointer is an invalid argument for "%s", but in practice
           it occurs quite frequently in printf statements that produce
           debug output.  Use a fallback in this case.  */
        if (ap->a.a_string == NULL)
          ap->a.a_string = "(NULL)";
        break;

This is also the case for wide character strings.

However, it appears the m4/*printf.m4 files do not check for the
behavior so we cannot depend on it.

Bruno, do you recall if this was intentional? Or do you think it should
be added?

Collin