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* [Bug c/109708] New: [c, doc] wdangling-pointer example broken
@ 2023-05-03  7:54 vries at gcc dot gnu.org
  2023-05-03  8:04 ` [Bug c/109708] " pinskia at gcc dot gnu.org
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: vries at gcc dot gnu.org @ 2023-05-03  7:54 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109708

            Bug ID: 109708
           Summary: [c, doc] wdangling-pointer example broken
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

I ran into a Wdangling-pointer warning and decided to read the docs and try out
an example.

The first one listed is:
...
int f (int c1, int c2, x)
{
  char *p = strchr ((char[]){ c1, c2 }, c3);
  // warning: dangling pointer to a compound literal
  return p ? *p : 'x';
}
...

It's not a complete example, x is missing a declared type and c3 is undeclared. 

After fixing that (and adding the implicit "#include <string.h>"), we have an
example that compiles:
...
#include <string.h>

int f (int c1, int c2, int c3)
{
  char *p = strchr ((char[]){ c1, c2 }, c3);
  return p ? *p : 'x';
}
...
but no warning, not at O0, O1, O2 or O3:
...
$ gcc test.c -Wdangling-pointer=1 -c
$
...

After reading the description of the warning, I managed to come up with:
...
char
f (char c1, char c2)
{
  char *p;

  {
    p = (char[]) { c1, c2 };
  } 

  return *p;
}
...
which does manage to trigger the warning for O0-O3:
...
$ gcc test.c -Wdangling-pointer=1 -c
test.c: In function ‘f’:
test.c:10:10: warning: using dangling pointer ‘p’ to an unnamed temporary
[-Wdangling-pointer=]
   10 |   return *p;
      |          ^~
test.c:7:18: note: unnamed temporary defined here
    7 |     p = (char[]) { c1, c2 };
      |                  ^
$
...

It might be worth mentioning that it's a C example, when using g++ we have:
...
$ g++ test.c -Wdangling-pointer=1 -c
test.c: In function ‘char f(char, char)’:
test.c:7:18: error: taking address of temporary array
    7 |     p = (char[]) { c1, c2 };
      |                  ^~~~~~~~~~
...

BTW, note that the warning can be fixed by doing:
...
 char
 f (char c1, char c2)
 {
   char *p;
+  char c;

  {
    p = (char[]) { c1, c2 };
+   c = *p;
  }

-  return *p;
+  return c;
}
...

^ permalink raw reply	[flat|nested] 5+ messages in thread
end of thread, other threads:[~2024-01-21  4:00 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-05-03  7:54 [Bug c/109708] New: [c, doc] wdangling-pointer example broken vries at gcc dot gnu.org
2023-05-03  8:04 ` [Bug c/109708] " pinskia at gcc dot gnu.org
2024-01-20 22:39 ` sandra at gcc dot gnu.org
2024-01-21  3:57 ` cvs-commit at gcc dot gnu.org
2024-01-21  4:00 ` sandra at gcc dot gnu.org

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