[PATCH v4] elf: fixes compile error when both enable -Werror and -DNDEBUG

[PATCH v4] elf: fixes compile error when both enable -Werror and -DNDEBUG

[Yang Yanchao]

Use -Werror and -DNDEBUG at the same time will
causes the following compilation errors:

cache.c: In function 'save_cache':
cache.c:758:15: error: unused variable 'old_offset' [-Werror=unused-variable]
  758 |       off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
      |               ^~~~~~~~~~

-DNDEBUG will disables the assertion.
Therefore, only the variables used by assertions do not take effect.
use DIAG_IGNORE_NEEDS_COMMENT to disable this warning.
---
 elf/cache.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/elf/cache.c b/elf/cache.c
index dbf4c83a7a..1e5427555b 100644
--- a/elf/cache.c
+++ b/elf/cache.c
@@ -36,6 +36,7 @@
 #include <dl-cache.h>
 #include <version.h>
 #include <stringtable.h>
+#include <libc-diag.h>
 
 /* Used to store library names, paths, and other strings.  */
 static struct stringtable strings;
@@ -751,6 +752,10 @@ save_cache (const char *cache_name)
       != (ssize_t) strings_finalized.size)
     error (EXIT_FAILURE, errno, _("Writing of cache data failed"));
 
+  /* Building with -DNDEBUG, assert will be ignored,
+     so that old_offset is not used */
+  DIAG_PUSH_NEEDS_COMMENT
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wunused-variable");
   if (opt_format != opt_format_old)
     {
       /* Align file position to 4.  */
@@ -758,6 +763,7 @@ save_cache (const char *cache_name)
       assert ((unsigned long long int) (extension_offset - old_offset) < 4);
       write_extensions (fd, str_offset, extension_offset);
     }
+  DIAG_POP_NEEDS_COMMENT
 
   /* Make sure user can always read cache file */
   if (chmod (temp_name, S_IROTH|S_IRGRP|S_IRUSR|S_IWUSR))
-- 
2.33.0

Re: [PATCH v4] elf: fixes compile error when both enable -Werror and -DNDEBUG

[Florian Weimer]

* Yang Yanchao:

> Use -Werror and -DNDEBUG at the same time will
> causes the following compilation errors:
>
> cache.c: In function 'save_cache':
> cache.c:758:15: error: unused variable 'old_offset' [-Werror=unused-variable]
>   758 |       off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
>       |               ^~~~~~~~~~
>
> -DNDEBUG will disables the assertion.
> Therefore, only the variables used by assertions do not take effect.
> use DIAG_IGNORE_NEEDS_COMMENT to disable this warning.
> ---
>  elf/cache.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/elf/cache.c b/elf/cache.c
> index dbf4c83a7a..1e5427555b 100644
> --- a/elf/cache.c
> +++ b/elf/cache.c
> @@ -36,6 +36,7 @@
>  #include <dl-cache.h>
>  #include <version.h>
>  #include <stringtable.h>
> +#include <libc-diag.h>
>  
>  /* Used to store library names, paths, and other strings.  */
>  static struct stringtable strings;
> @@ -751,6 +752,10 @@ save_cache (const char *cache_name)
>        != (ssize_t) strings_finalized.size)
>      error (EXIT_FAILURE, errno, _("Writing of cache data failed"));
>  
> +  /* Building with -DNDEBUG, assert will be ignored,
> +     so that old_offset is not used */
> +  DIAG_PUSH_NEEDS_COMMENT
> +  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wunused-variable");
>    if (opt_format != opt_format_old)
>      {
>        /* Align file position to 4.  */
> @@ -758,6 +763,7 @@ save_cache (const char *cache_name)
>        assert ((unsigned long long int) (extension_offset - old_offset) < 4);
>        write_extensions (fd, str_offset, extension_offset);
>      }
> +  DIAG_POP_NEEDS_COMMENT
>  
>    /* Make sure user can always read cache file */
>    if (chmod (temp_name, S_IROTH|S_IRGRP|S_IRUSR|S_IWUSR))

Personally I would prefer __attribute__ ((unused)) directly on the
old_offset declaration.  I think it's clearer.

Not sure what others think.

Thanks,
Florian

Re: [PATCH v4] elf: fixes compile error when both enable -Werror and -DNDEBUG

[Andreas Schwab]

On Apr 13 2022, Florian Weimer via Libc-alpha wrote:

> Personally I would prefer __attribute__ ((unused)) directly on the
> old_offset declaration.  I think it's clearer.

Perhaps even wrapping it inside #ifndef NDEBUG, like this:

#ifndef NDEBUG
      off64_t old_offset =
#endif
        lseek64 (fd, extension_offset, SEEK_SET);

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."