[Bug c/37755] New: Mistaken Segmentation fault

[Bug c/37755] New: Mistaken Segmentation fault

[charpour at gnet dot gr]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 999 bytes --]

GCC Version: 4.2.3
Host: Ubuntu 8.04

When I compile the following code:

#include <stdio.h>

typedef struct person {
  char name[40];
  int age;
} Person;

static Person make_person(void);

int main(void) {
  printf("%s\n", make_person().name);

  return 0;
}

static Person make_person(void) {
  static Person p = { "alexander", 18 };

  return p;
}

I get a false warning:

warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char[40]’

and when I execute the file I get a segmentation fault.

If I use: printf("%s\n", &make_person().name[0]);

everything works as expected and the output "alexander" is printed


-- 
           Summary: Mistaken Segmentation fault
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: charpour at gnet dot gr


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37755

[Bug c/37755] Mistaken Segmentation fault

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-10-06 21:25 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-10-06 21:25:33
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37755

[Bug c/37755] Mistaken Segmentation fault

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-10-06 21:25 -------
I don't get a seg fault on ppc but I do on x86.  It works correctly for the C++
front-end where the array decays into a pointer.

The warning is also bogus but it shows what the issue is really.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic, wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37755

[Bug c/37755] Mistaken Segmentation fault

[joseph at codesourcery dot com]

------- Comment #3 from joseph at codesourcery dot com  2008-10-06 21:39 -------
Subject: Re:   New: Mistaken Segmentation fault

On Mon, 6 Oct 2008, charpour at gnet dot gr wrote:

>   printf("%s\n", make_person().name);

make_person().name is a non-lvalue array, so it only decays to a pointer 
for C99, not for C90.  If you use -std=c99/-std=gnu99 then the program 
works.

The program does not, however, have defined behavior for C99, only for 
C1x.  In C99 the lifetime of the array ends at the next sequence point, 
before the call to printf.  In C1x it instead ends at the end of the 
evaluation of the containing full expression, which is the call to printf.

I do not believe any changes to GCC are needed to implement this 
particular C1x requirement, since GCC discards information about variables 
lifetimes smaller than a function for gimplification and tree 
optimizations that may change those lifetimes, so it will in practice 
treat the lifetime as being anywhere it cannot show the temporary not to 
be live.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37755

[Bug c/37755] Mistaken Segmentation fault

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2008-10-07 07:37 -------
As Joseph said.  Invalid for -std=c89, worksforme for -std=c99.  No changes
needed for -std=c1x.


-- 

rguenth at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37755