[Bug c/112543] New: warning pointer used after free() for printf("%p")

[Bug c/112543] New: warning pointer used after free() for printf("%p")

[mario at klebsch dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112543

            Bug ID: 112543
           Summary: warning pointer used after free() for printf("%p")
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mario at klebsch dot de
  Target Milestone: ---

Created attachment 56592
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56592&action=edit
main.c preprocessed by gcc

Hello,

Compiling the following program with -Wall gives a use-after-free warning:


----8<--------8<--------8<--------8<--------8<--------8<----
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
        char* p = (char*)malloc(5);
        printf("allocated %p\n", p);
        free(p);
        printf("freed %p\n", p);
}
----8<--------8<--------8<--------8<--------8<--------8<----

Compiling with gcc gives this output:
mkl@C707 ~/tmp/g++-bug $ gcc -Wall main.c  -o main
main.c: In function ‘main’:
main.c:9:9: warning: pointer ‘p’ used after ‘free’ [-Wuse-after-free]
    9 |         printf("freed %p\n", p);
      |         ^~~~~~~~~~~~~~~~~~~~~~~
main.c:8:9: note: call to ‘free’ here
    8 |         free(p);
      |         ^~~~~~~
mkl@C707 ~/tmp/g++-bug $ gcc --version
gcc (Gentoo 13.2.1_p20230826 p7) 13.2.1 20230826
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

mkl@C707 ~/tmp/g++-bug $ 

Compiling with g++:
mkl@C707 ~/tmp/g++-bug $ g++ -Wall main.cpp  -o main
main.cpp: In function ‘int main(int, char**)’:
main.cpp:9:15: warning: pointer ‘p’ used after ‘void free(void*)’
[-Wuse-after-free]
    9 |         printf("freed %p\n", p);
      |         ~~~~~~^~~~~~~~~~~~~~~~~
main.cpp:8:13: note: call to ‘void free(void*)’ here
    8 |         free(p);
      |         ~~~~^~~
mkl@C707 ~/tmp/g++-bug $ g++ --version
g++ (Gentoo 13.2.1_p20230826 p7) 13.2.1 20230826
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

mkl@C707 ~/tmp/g++-bug $

[Bug c/112543] warning pointer used after free() for printf("%p")

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112543

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
J.2 Undefined behavior

(180) The value of a pointer that refers to space deallocated by a call to the
free or realloc function is used (7.24.3).

[Bug c/112543] warning pointer used after free() for printf("%p")

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112543

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |104075
           Keywords|                            |diagnostic

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yeah, this is diagnosed on purpose ...


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104075
[Bug 104075] bogus/missing -Wuse-after-free

[Bug c/112543] warning pointer used after free() for printf("%p")

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112543

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #3 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
I think the warning message is correct.  It says "pointer ‘p’ used after...",
not "pointer 'p' dereferenced after ...".

And as Andreas explained, *using* the value of p (without dereferencing) is
enough to invoke an undefined behavior.

Closing as invalid.