Wednesday, December 29, 2004

PODS and variables or standards vs. comfort

I recently ported a small, but working OnboardC project over to PODS 1.0 to benefit from the comfort of the large screen standing on my desktop. According to friendly advice gathered from the developer newsgroup, getting sources and XRD files to the PC was a painless process. After having faced a few minor difficulties at the Windows 2000 command prompt, everything was set. I now expected a painless build cycle - but the app didn't even survive the parser. Parse errors at almost every variable definition. An Update to the latest and greatest version didnt help. Treat this as an example:

//Useless code
case foo:
int bar;
//More useless code

A parse error occurs immediately when the code above is compiled-and the variable is not accepted. A short post at the user group brought enlightment to the coder-ANSI C requires that variable definitions must be the first code in a block-and this requirement isn't met above.
While some compilers(at least OnBoardC) accept variable definitions anywhere, the PODS(Ben Combee says that this is GCC's fault, as we all known PODS is based on GCC) is picky.
However, ANSI C has a comfortable way of combatting this problem-the creating of instruction blocks. Many C programmers treat the {} as a fixed part of the syntax-but actually they only group instructions together. Each block gets its own stack space-and can thus have lots of local variables. So, just put your code into a block-and alas, the PODS accepts it. The corrected example would look like this:

//Useless code
case foo:
int bar;
//More useless code

A possible second way is to change the C file uinto a C++ file by changing its filename to .cpp and then reinserting the file into the project-C++ dors not impose this limitation.

Altough it is difficult to call this 'pedantery' shortcoming or even bug, it can still pull nerves and cause headaches.
Feel free to comment!

P.S.This article isnt published to bash Palmsource. It should just save developer's time. PODS is a cool product in general, but knowing possible traps is always good.
Full disclosure: This article was sent to Ben Combee for correcting before it was published here!


Blogger Steven Fisher said...

Two comments:

"While most compilers accept variable definitions anywhere..."

Are you sure about that? I've worked with a variety of C compilers, and while C++ compilers will accept variables anywhere, I have never seen this in a C compiler.

"Each block gets its own stack space-and can thus have lots of local variables."

I'm not sure about this, either. Although that's the way it seems to work, I think that at least in Codewarrior only the main function actually adjusts the stack. But it puts space on the stack for subvariables.

Still, you've got my curious about this, and I'm going to see if I can confirm one way or the other!

12:16 AM  
Blogger Steven Fisher said...

I should add:

The primary use of variables within { } is (as far as I know) scope limitation. The variables are not available outside of those brackets. This allows a few clever things.

For instance:

void Foo( int a, int b )
  if ( a < b ) {
    int i;
    for ( i = a; i<b; i++ )
    { /* do something here */ }
  } else {
    int j;
    for ( j = b; j<a; j++ )
    { /* do something here */ }

In this case, i is not accessible outside of its block. And j is not accessible outside of its block. However, since the scope of i and j are so limited, the compiler is able to actually use the same space on the stack. Instead of this function taking 4 bytes for local variable storage, it will only take 2.

In fact, a compiler with a good optimizer could very likely reuse stack space/registers for i and j regardless of whether they were put in a limited scope like that, so long as the life of the variables is short. Variables in sub-blocks are more useful in C++ than in C, where constructors/destructors get called.

It's really mostly just a tool for making your code easier to understand, like naming your function something other than foo and your parameters something other than a and b.

3:14 AM  
Blogger Tam Hanna said...

Hi Steven,
thanks yet again for the good and informative comments! A C beginner like me can always learn something new here!
However, lets face it:
I didn't want to discuss scope, etc here. I just wanted to get the program working and compiling under PODS. And inserting blocks into each switch statement and moving the variables to the top of the block solved my problem!
BTW, if you want a compiler that CAN run such code, go for OnBoardC! It can definitely do it.
Merry 2005 and thanks for the comments!
Tam Hanna

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