warning in c code

knobby67 · 08-10-2007, 09:01 AM

Hi all, hope someone can help me out with this problem. I've written some code which works fine. However I get an warning which I'm unsure how to correct.
Basically I have an array of structures (actually screen set up structures) which I turn off from a kill screen function. However the code produces the following warning

LibaryFilesVer006/SetUpScreen.c: In function ‘killGLWindow’:
LibaryFilesVer006/SetUpScreen.c:170: warning: value computed is not used
LibaryFilesVer006/SetUpScreen.c: In function ‘InitScreenWindows’:
LibaryFilesVer006/SetUpScreen.c:315: warning: value computed is not used

These are called when I step through the array of pointers for each screen connect to my PC so for example the error on line 170 is from the line#

**screenlist++;

in the function

void killGLWindow(GLXScreen **screenlist)
{
while(*screenlist!=EOF_SCREENS)
{

if ((*screenlist)->ctx)
{
if (!glXMakeCurrent(DisplayPointer, None, NULL))
{
printf("Could not release drawing context.\n");
}
glXDestroyContext(DisplayPointer, (*screenlist)->ctx);
(*screenlist)->ctx = NULL;
}
/* switch back to original desktop resolution if we were in fs */
if ((*screenlist)->fs)
{
XF86VidModeSwitchToMode(DisplayPointer, (*screenlist)->screen, &(*screenlist)->deskMode);
XF86VidModeSetViewPort(DisplayPointer, (*screenlist)->screen, 0, 0);
}

**screenlist++;
}

XCloseDisplay(DisplayPointer);
}

Can someone tell me how to get rid of this warning? Thanks

bigearsbilly · 08-10-2007, 09:30 AM

I think it's because you are double de-referencing
**screenlist++
and not using it.

I think you could be lucky to get away with it working

I think you are having trouble understanding how pointers work,
you wouldn't ever de-reference twice a la **

**screenlist is an array of screenlist pointers?
you would typically do *(screenlist +1) or screenlist[1] or something to
use the contents.

get a debugger like ddd and have a root around till you understand
how pointers work.

exvor · 08-10-2007, 09:41 PM

Well you would dereference twice if you have a pointer of a pointer :P

rupertwh · 08-10-2007, 09:53 PM

Quote:

Originally Posted by knobby67

LibaryFilesVer006/SetUpScreen.c:315: warning: value computed is not used

**screenlist++;

The compiler already told you pretty exactly what is wrong.

You are incrementing screenlist, which is also what you probably want to do here, but additionally, you dereference screenlist. Twice. Without using the value you get from dereferencing. Why?

A simple

Code:

screenlist++;

will do.

Matir · 08-10-2007, 11:55 PM

Unless, of course, you mean (**screenlist)++;, but that wouldn't seem to make much sense. I think you want screenlist++ or (*screenlist)++.

bigearsbilly · 08-13-2007, 02:49 AM

deleted duplicate

bigearsbilly · 08-13-2007, 02:50 AM

Quote:

Well you would dereference twice if you have a pointer of a pointer :P

are you sure?

observe:

Code:

#include <stdio.h>

main(int argc, char ** argv)
{
        printf("prog %s got:%s\n", *argv++, *argv);
}

Code:

billym.primadtpdev>1 hello
prog 1 got:hello

larvyde · 08-20-2007, 03:21 AM

Quote:

Originally Posted by bigearsbilly

are you sure?

observe:

Code:

#include <stdio.h>

main(int argc, char ** argv)
{
        printf("prog %s got:%s\n", *argv++, *argv);
}

Code:

billym.primadtpdev>1 hello
prog 1 got:hello

AFAIK this is not portable across compilers, you could instead do

Code:

#include <stdio.h>

main(int argc, char ** argv)
{
        printf("prog %s got:%s\n", argv[0], argv[1]);
}

and get the same result

bigearsbilly · 08-20-2007, 03:34 AM

it should be portable, it's perfectly valid code.
maybe you have a rubbish compiler ?

larvyde · 08-24-2007, 12:58 AM

it is valid but the result is undefined, different compilers give different results.

bigearsbilly · 08-24-2007, 04:14 AM

what exactly are you saying is undefined?

I think the meaning of the postfix and prefix operators is
very clear.

Hivemind · 08-24-2007, 04:36 PM

Code:

#include <stdio.h>

main(int argc, char ** argv)
{
        printf("prog %s got:%s\n", *argv++, *argv);
}

has two problems (well, three as far as the standard is concerned since omitting the return value of main is not valid).

First, the order of the evaluation of the function arguments is undefined. This is true for both C and C++.

Second, and this is the more sinister problem, the code evaulates argv *and* modifies it for a purpose other than generating a new value without an intervening sequence point. This is undefined behaviour.

I was sure about the argument evaluation order but I was unsure about the sequence point error, so I forwarded this question to a group which is known for having several gurus. This is their reply in a nutshell.

bigearsbilly · 08-28-2007, 03:29 AM

Quote:

First, the order of the evaluation of the function arguments is undefined. This is true for both C and C++.

OK you are correct.

Quote:

Second, and this is the more sinister problem, the code evaulates argv *and* modifies it for a purpose other than generating a new value without an intervening sequence point. This is undefined behaviour.

nonsense

in the c-faq

Quote:

If you want to break rule 1, make sure that the several objects being modified are distinctly different, and try to limit yourself to two or at most three modifications, and of a style matching those of the following examples. (Also, make sure that you continue to follow rule 2 for each object modified.) The expression

c = *p++

is allowed under this rule, because the two objects modified (c and p) are distinct. The expression

*p++ = c

is also allowed, because p and *p (i.e. p itself and what it points to) are both modified but are almost certainly distinct.

Hivemind · 08-28-2007, 03:52 AM

No, you are talking nonsense. The problem is not *argv++ by itself but that you also have *argv. There is no intervening sequence point here, hence undefined behaviour. Here's the relevant part from the comp.lang.c FAQ:
http://www.c-faq.com/expr/evalorder1.html

The folks at comp.lang.c agreed totally that this is undefined behaviour, it wasn't even a discussion and I think comp.lang.c is the best place to go if one has questions regarding standard C. No other forum beats the level of expertise exhibited by its regulars. Same goes for comp.lang.c++ when it comes to c++. When one has platform specific questions, like posix, this forum good, however.

bigearsbilly · 08-28-2007, 03:59 AM

agreed,
I was saying there is nothing wrong with *argv++,
you seemed to me to be implying *argv++ is undefined,
I agree with the point is an issue only because of the
function argument evaluation order.
which is the only issue by point of fact.

we won't mention the lack of return