[Bug 221533] New: strstr/c23/constness

[Bug 221533] New: strstr/c23/constness

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221533

            Bug ID: 221533
           Summary: strstr/c23/constness
           Product: Documentation
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: man-pages
          Assignee: documentation_man-pages@kernel-bugs.osdl.org
          Reporter: linux@treblig.org
        Regression: No

Hi,
  In C23 strstr can return a const char* if the first parameter is a const
char*;  this is also true of a bunch of the other routines.

  I'm not really sure how this should be documented, I guess two entries for
each function with some note saying it's only C23 and later?

See:
https://sourceware.org/cgit/glibc/commit/string/string.h?id=cd748a63ab1a7ae846175c532a3daab341c62690

for the glibc change that implemented it.

It can break some existing code, e.g. something like:


int foo(const char *a)
{
   char *bar = strstr(a, "frog");
}

gives:
t.c:9:16: warning: initialization discards ‘const’ qualifier from pointer
target type [-Wdiscarded-qualifiers]
    9 |    char *bar = strstr(a, "frog");
      |                ^~~~~~

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[Bug 221533] strstr/c23/constness

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221533

--- Comment #1 from Alejandro Colomar (alx@kernel.org) ---
Hi,

On 2026-05-16T16:47:42+0000, bugzilla-daemon@kernel.org wrote:
> Hi,
>   In C23 strstr can return a const char* if the first parameter is a const
> char*;  this is also true of a bunch of the other routines.
> 
>   I'm not really sure how this should be documented, I guess two entries for
> each function with some note saying it's only C23 and later?

I've been thinking about this.  At the moment, I've documented one
function that has this property: strnul(3) (which is a gnulib extension,
not part of C23).  There, I've used C++-style, documenting it with two
prototypes, as if it were a function overload.  I think we should do the
same for the C23 const-preserving macros.

        SYNOPSIS
             #include <string.h>

             char *strnul(char *s);
             const char *strnul(const char *s);

I'll work on some patches for the C23 APIs.

> See:
>
> https://sourceware.org/cgit/glibc/commit/string/string.h?id=cd748a63ab1a7ae846175c532a3daab341c62690
> 
> for the glibc change that implemented it.
> 
> It can break some existing code, e.g. something like:

Indeed, it broke mutt(1), for example.  That allowed finding a few bugs
in mutt(1), which were modifying read-only strings.  Most of the cases
were false positives, though, which could be fixed easily by adding
const.

> int foo(const char *a)
> {
>    char *bar = strstr(a, "frog");
> }
> 
> gives:
> t.c:9:16: warning: initialization discards ‘const’ qualifier from pointer
> target type [-Wdiscarded-qualifiers]
>     9 |    char *bar = strstr(a, "frog");
>       |                ^~~~~~

Yup.


Have a lovely day!
Alex

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[Bug 221533] strstr/c23/constness

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221533

--- Comment #2 from Dr. David Alan Gilbert (linux@treblig.org) ---
Yep, makes sense; I tripped over it yesterday in one of qemu's tests (someone
has already sent a test).

Thanks!

Dave

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[Bug 221533] strstr/c23/constness

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=221533

Artem S. Tashkinov (aros@gmx.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |ANSWERED

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