NULL problem in HP server

[KarthiKhivi]

Hi All
I am facing a problem in a C code which is working perfectly in AIX platform, but in HP it fails. In HP server, when i call a function in c using sample(NULL) it fails but when i call the same using sample(""), it works. This behaviour is looking strange in HP boxes. Can it be fixed say by caling that function using sample(NULL)? Can anybody suggest a solution?
The version of HP is B.11.11 U 9000/800.
Thx for your help inadvance.

[KarthiKhivi]

Adding to that, here is a sample code which works on AIX platform but not on HP.

#include
int main()
{
char **variable=NULL;
printf("Starting\n");
fflush(stdout);
while(*variable)
{
printf("Inside while\n");
fflush(stdout);
break;
}
printf("Ending\n");
fflush(stdout);
}

The output is:

Starting
Memory fault(coredump)


Can anybody make it work on a HP machine?
Thanks.

[Venkatesh BL]

It works on my 11.31. What is the compiler version and options you are using?

[Matti_Kurkela]

NULL is a pointer pointing to a "nowhere" memory location. The actual content is implementation-dependent: it may be e.g. 0x0000000000000, or it might be some other value that is known to be invalid. The invalidness of the value might even be enforced at the CPU hardware level: trying to access anything pointed to by a NULL pointer may (and *should*) cause an immediate segmentation fault.

An empty string ("") is a pointer to a valid memory location which only contains the string terminator character, '\0'.

These are two different things.

As we don't have access to the source code of your function, I cannot see what is wrong. But I guess that the function does not properly check its input for NULLness: it might make *some* checks that are adequate for AIX, but not good for portable C code.

From a document "Recommended C Style and Coding Standards":
http://www.psgd.org/paul/docs/cstyle/cstyle.htm

Chapter 16, "Portability":

[...]
# On some machines, the null character pointer ((char *)0) is treated the same way as a pointer to a null string. Do not depend on this.
[...]

Obviously, the AIX programmer that wrote the code has not read this document.

MK

[Don Morris_1]

So you're doing --

char **variable = NULL;

while (*variable) {

So really, you've written while (*NULL). NULL should not be dereferenced. If you set variable to "", that still isn't exactly right (since "" is a constant array of char, not a char **... this is just the empty string. You could set *variable to "", but first you'd need to allocate memory for a char * to do that).

I would suspect the problem in the real code may be similar -- if the sample() function expects a valid string -- calling with NULL means the function will attempt to check the string length of an invalid pointer. Boom. Calling with "" on the other hand _is_ a valid [empty] string, hence why it works.

The obvious fix if you truly want to call sample() at times with no string is to have a check at the start of sample() for if (input == NULL ) { return; // Or do whatever is reasonable here, but don't deref! } .

[KarthiKhivi]

Hi All

Thanks for your quick and valuable response. i ve already given a sample code which works on a AIX machine but not in HP. That is a very simple version of our huge application. The actual application has 100k such codes. So we are in a state to find out a solution other than code change.

[KarthiKhivi]

Hi Venkatesh

Thanks for your reply.

Wats the compiler version using? we haven't used any options while compiling. we are using GCC as compiler and its version is "gcc version 4.1.2". Since GCC is a freeware we are using this compiler.
Wats the difference in ur version of server and ours? Can we make it work in our machine with any configuration settings change?

[KarthiKhivi]

Hi Don Morris

Thanks for your inputs

We cant afford to make such code changes in our application since it has around 100K LOC. :(
Need to fix without code changes. Thats the sad thing here.

[KarthiKhivi]

is it possibile to instruct compiler while compiling not to use "0x0000000000000, or it might be some other value that is known to be invalid" and instead use a valid segment area? Any options can be added for the same?

[Don Morris_1]

With gcc? I'm not entirely sure -- Dennis probably will know. On newer releases of HP-UX, there's a chatr option (-Z) that disables the trap for NULL dereferences. I believe on v1, you can pass a flag to the compiler or linker (http://docs.hp.com/en/1694/comp_run.html, http://docs.hp.com/en/B2355-90689/ld.1.html), but that's the HP compilers -- not gcc. If gcc ends up invoking ld, then you just need it to pass "-Z" to ld in the linking stage. I expect it does (since a portable linker seems like an odd choice) -- but I'm not even vaguely up to date on gcc choices/internals.

That said, this is not what I'd call a good idea and you've got some risk here. If all this thing ever does is treat NULL like a string, attempt to look for the '\0' terminator... then you're probably okay [since I believe you get to read zeros from the Zero page when NULL deref is disabled, and 0 looks like '\0' if my memory serves]. Hence a string operation on NULL in this model will see an "empty" string right off and you'll survive. If your code tries to do anything later like store something to that string buffer, you're still hosed. At least since it runs on AIX (and has for a while), the odds are it doesn't do that.

So "-Z" will probably get you going -- but I'd still fix that source in the future to have more clean portability.