Behavior of logical or

[michaelp]

Hello!

I am running gcc 2.95 on linux debian, and have come across some strange
behavior.

I have an expression like this:

if (
*charVariable == "\0"
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{
//do something
}

When charVariable is null, I expect the construct not to attempt at
evaluating the next sub-expression. It seems to me that it does,
resulting in a segmentation fault.

Is it possible to control this behavior?

Michael

[Conor Hughes]

"michaelp" <mi******@hio.nowrote in message
news:eh**********@dolly.uninett.no...

Hello!

I am running gcc 2.95 on linux debian, and have come across some strange
behavior.

I have an expression like this:

if (
*charVariable == "\0"
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{
//do something
}

When charVariable is null, I expect the construct not to attempt at
evaluating the next sub-expression. It seems to me that it does, resulting
in a segmentation fault.

Is it possible to control this behavior?

Are you trying to test if charVariable points to the null character '\0' or
if it is a null reference? Your code insinuates that you're trying the
former, your explanation insinuates you're trying the latter.

[Nils O. Selåsdal]

michaelp wrote:

Hello!

I am running gcc 2.95 on linux debian, and have come across some strange
behavior.

I have an expression like this:

if (
*charVariable == "\0"

This is likely wrong. "\0" evaluates to a pointer. Compare strings
with strcmp.

If you want to chech wether charVariable is a NULL pointer you'd
do
charVariable == NULL
if you want to check wether it is an empty string, you'd do
*charVariable == '\0'
or perhaps charVariable[0] == 0

||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{
//do something
}

When charVariable is null, I expect the construct not to attempt at
evaluating the next sub-expression. It seems to me that it does,
resulting in a segmentation fault.

Is it possible to control this behavior?

Michael

[michaelp]

On 25.10.2006 08:47, Nils O. Selåsdal wrote:

*charVariable == '\0'

This is what I am actually doing. I expressed myself clumsily in the
message. *charVariable == "\0" is rubbish, of course.

M.

[michaelp]

On 25.10.2006 08:30, Conor Hughes wrote:

"michaelp" <mi******@hio.nowrote in message

Are you trying to test if charVariable points to the null character '\0' or
if it is a null reference? Your code insinuates that you're trying the
former, your explanation insinuates you're trying the latter.

'\0', of course. I expressed myself clumsily in the message. Sorry for that.

[Keith Thompson]

michaelp <mi******@hio.nowrites:

I am running gcc 2.95 on linux debian, and have come across some
strange behavior.

I have an expression like this:

if (
*charVariable == "\0"
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{
//do something
}

What type is charVariable?

"\0" is not the same thing as '\0'.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[michaelp]

On 25.10.2006 10:18, Keith Thompson wrote:

michaelp <mi******@hio.nowrites:

>I am running gcc 2.95 on linux debian, and have come across some
strange behavior.

I have an expression like this:

if (
*charVariable == "\0"
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{
//do something
}

What type is charVariable?

"\0" is not the same thing as '\0'.

It is of course the latter I mean: '\0' . Hastily and clumsily written
message. Sorry for that

>

[Keith Thompson]

michaelp <mi******@hio.nowrites:

On 25.10.2006 08:47, Nils O. Selåsdal wrote:

>*charVariable == '\0'

This is what I am actually doing. I expressed myself clumsily in the
message. *charVariable == "\0" is rubbish, of course.

Then post a complete, compilable program that illustrates the problem.
We can't tell what's going on from the fragment you posted.

Keep in mind that null pointers, null charaters, and empty strings are
different things.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Rachael]

Presumably the segfault is because you're trying to dereference
charVariable when it's NULL. Try charVariable instead of *charVariable.

[Lew Pitcher]

michaelp wrote:

Hello!

I am running gcc 2.95 on linux debian, and have come across some strange
behavior.

I have an expression like this:

if (
*charVariable == "\0"
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{

[snip]

To summarize the discussion, your code probably should look like some
variant of

if ( charVariable != NULL)
{
if (*charVariable == '\0' ||
strncmp(charVariable,anotherCharVariable,2) == 0)
{
/* do something */
}
}

[Kenneth Brody]

michaelp wrote:

>
Hello!

I am running gcc 2.95 on linux debian, and have come across some strange
behavior.

I have an expression like this:

if (
*charVariable == "\0"
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
{
//do something
}

When charVariable is null, I expect the construct not to attempt at
evaluating the next sub-expression. It seems to me that it does,
resulting in a segmentation fault.

Is it possible to control this behavior?

You have already posted that you meant:

*charVariable == '\0'

rather than "\0".

However, your text says "charVariable is null", and given the usage
in the following strncmp, I assume you really mean "charVariable is
NULL". (I'm also assuming that charVariable is really a "char*" and
not a "char".)

In that case, you don't want the dereference. In other words, you
want to check is charVariable is a NULL pointer, rather than if it
points to a NUL ('\0') character.

if ( charVariable == NULL
||
strncmp(charVariable, anotherCharVariable, 2) == 0
)
...
--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
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+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th*************@gmail.com>