[Bug c/15048] New: Dereferincing char pointer to char array yields segfault.

[Bug c/15048] New: Dereferincing char pointer to char array yields segfault.

[sh4d0wstr1f3 at yahoo dot com]

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Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/3.3.2/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--disable-checking --with-system-zlib --enable-__cxa_atexit --host=i386-redhat-linux
Thread model: posix
gcc version 3.3.2 20031022 (Red Hat Linux 3.3.2-1)
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Maybe related to 6857? I've got two source files, and a header. Header (H) has a
declaration for 'extern char *string'. Source (A) defines 'string' as 'char
string[20]' and has a routine which takes a char * and strncpy's it into
'string'. Source (B) has the main routine and includes the header. The main
routine invokes the function in sA to set 'string' to "foo"; then proceeds to
printf 'string'. This causes a segmentation fault.

I notice the following things:

1. The address of 'string' gets annihilated on the call to sA.
2. The assembler code generated for calling printf in sB has a 'pushl address',
not a 'push $address' for the address of 'string'. This causes it to derefence
the first characters of 'string'.
3. WORKAROUND: if I change the extern of 'string' to 'extern char string[]',
everything works as expected.
4. If I have sA include header H, I get "error: conflicting types for `string'".

(4) makes me think I've answered my own question, but if I did, I as yet have no
idea what the answer means to me. :) I mean, if I can say 'char *c =
&string[3]', shouldn't what I'm describing here work?

Thanks for any help you can provide! 

--clip--

The invocation (from 'make'):

cc  -I. -ggdb  -c -o wg.o wg.c
cc  -I. -ggdb  -c -o swg.o swg.c

The preprocessed sources:

Source A:

# 1 "swg.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "swg.c"
char weak_char[20];

void set_wg_char(char *give)
{
    printf("copy work???");
    strncpy(weak_char, give, sizeof(weak_char));
    printf("yes---%s--%p\n", weak_char, weak_char);
}
---clip---
Source B:

# 1 "wg.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "wg.c"
# 1 "wg.h" 1



extern char *weak_char;
void set_weak_char(char *);
# 2 "wg.c" 2

int main()
{
    set_wg_char("it is a test");
    printf("pointer:-%p-\n", weak_char);
    printf("value:---%s---\n", weak_char);
}
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For case (4) where I include header H in sA:

swg.c:3: error: conflicting types for `weak_char'
wg.h:4: error: previous declaration of `weak_char'

--clip--

The interesting difference in assembly code generated when I change 'char *' to
'char []':

     printf("value:---%s---\n", weak_char);
- 80483b1:      83 ec 08                sub    $0x8,%esp
- 80483b4:      68 38 96 04 08          push   $0x8049638
- 80483b9:      68 ff 84 04 08          push   $0x80484ff
- 80483be:      e8 e9 fe ff ff          call   80482ac <_init+0x38>
- 80483c3:      83 c4 10                add    $0x10,%esp
+ 80483b2:      83 ec 08                sub    $0x8,%esp
+ 80483b5:      ff 35 3c 96 04 08       pushl  0x804963c
+ 80483bb:      68 03 85 04 08          push   $0x8048503
+ 80483c0:      e8 e7 fe ff ff          call   80482ac <_init+0x38>
+ 80483c5:      83 c4 10                add    $0x10,%esp

-- 
           Summary: Dereferincing char pointer to char array yields
                    segfault.
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sh4d0wstr1f3 at yahoo dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15048

[Bug c/15048] Dereferincing char pointer to char array yields segfault.

[rearnsha at gcc dot gnu dot org]

------- Additional Comments From rearnsha at gcc dot gnu dot org  2004-04-21 16:48 -------
Not a bug.  Your comment shows the problem.  extern char* is not the same as
extern char[].  One is a reference to a char array, the other a pointer to char.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15048