c programs

richi · 07-24-2009, 02:31 AM

why some c programs are not run even if there is no error in compile time??????????

Spidey · 07-24-2009, 03:14 AM

Quote:

why some c programs are not run even if there is no error in compile time??????????

Such as ?

Elysia · 07-24-2009, 04:04 AM

Usually because you made a run-time error and not a compile-time error.
For more information, we need to see code!

MK27 · 07-24-2009, 09:38 AM

Quote:

Originally Posted by richi

why some c programs are not run even if there is no error in compile time??????????

You mean like this one:

Code:

#include <stdio.h>
#include <string.h>

int main() {
	char *overflowme;
	strcpy(overflowme,"this will overwrite some memory");
	printf("%s", overflowme);
	return 0;
}

Because the compiler does not check stuff like this. Perhaps it should, and this is a performance trade-off in the compiler.

On the other hand, programming need not be quite so easy as throwing darts blindfolded (which really is easy if you don't care what you hit). You have to know what you are doing. To know you have to learn, partially thru a process of trial and error. It's not nearly as painful as learning to ride a bike, and you already did that (hopefully).

Lesson: Don't expect the compiler to fix everything for you, and be happy when it does.

Brafil · 07-24-2009, 09:44 AM

A compiler doesn't check if you're doing meaningful things. It only checks if you're doing correct things.
I don't think many compilers will complain at this:

Code:

#include <stdio.h>

int main(void)
{
    printf("The mystery is solved: *NULL is %d\n", *(int *)NULL);
    return 0;
}

Even though it is clearly undefined behaviour.

nonoob · 07-24-2009, 04:06 PM

Compilers can't check logic errors or care much whether there are even any output statements. That should be obvious.

Elysia · 07-25-2009, 03:08 AM

They can, and some do.

abachler · 07-25-2009, 04:11 AM

The compiler just checks to make sure you use correct syntax, it doesn't check to make sure your code is coherent.

The warbling of the instantiated was blue.

Perfectly good syntax, with no meaning whatsoever.

Elysia · 07-25-2009, 04:26 AM

No, some do. 99% of the compilers do not, but some, like MSVC can check for problems like this. It's not good to dismiss out of hand. But it's certainly not good to always rely on such a feature.

MK27 · 07-25-2009, 04:29 AM

And if you do this:

Code:

int mostlypointless (int x) {
     int a = 12, i;
     for (i=0; i<10; i++) a = a*i+a;
     return x+1;
}

GCC -O3 will just ignore the statements in red as if they didn't exist, because they are meaningless in the context of the program.

The only way to notice this (I guess) is if you use massive meaningless loops for profiling. Compiled gcc -O3, they won't happen.

**Bubba** · 07-25-2009, 10:29 AM

Newer compilers for other languages are 'attempting' to do this and it comes off to me as 100% annoying. Maybe it's because I don't like my hand being held while I code.

Elysia · 07-25-2009, 10:37 AM

It's extremely helpful to offer such code analysis, because it can find problems in your code.
Naturally, it is also slower, so there should always be an option to turn it off. Otherwise the compiler is flawed.
Or perhaps you don't like the compiler removing certain code in optimizations?

Yarin · 07-25-2009, 12:13 PM

>> They can, and some do.
To a very limited extent, but for the most part they can't.

When it comes to incorrect manipulation of certain things, 'const' does a good job for protecting against this - if for some reason the programmer doesn't want to use it, that's his problem.

Elysia · 07-25-2009, 12:29 PM

Const doesn't help very much at all. It's mostly for for convenience and efficiency.

Mario F. · 07-25-2009, 01:14 PM

Quote:

Originally Posted by Elysia

It's extremely helpful to offer such code analysis, because it can find problems in your code.
Naturally, it is also slower, so there should always be an option to turn it off. Otherwise the compiler is flawed.

Hmm... I disagree. That's definitely not the task of a compiler. Abachler said it best.

That type of code analysis can and should be done by specialized tools only. MSVC offers those tools in its Team System releases. Tools like Code Metrics, C/C++ Code Analysis Tools, Application Verifier, Line Level Sampling and even Code Profiler (and there's even more).

Note that by "compiler", I mean the actual task of code compilation. In this context I don't agree such tasks should be implemented. In fact I suspect most of them couldn't, since better diagnostics can only be achieved after compilation and during program execution with access to debugging symbols, which is what most of the tools above do. On the other hand, if we consider the tasks of a compiler (lexical analysis, pre-processing, parsing and code generation with/without optimization) we can see there is really no place for such analysis as the excellent example provided by Mk27 demonstrates.

Again, go back to abachler statement. There's an underlying truth to it that basically says it's not the task of the compiler to make sure your code works as you want it to.

07-24-2009, 02:31 AM	#1
richi Registered User Join Date: Jul 2009 Posts: 1	c programs why some c programs are not run even if there is no error in compile time??????????
richi is offline

07-24-2009, 09:44 AM	#5
Brafil Making mistakes Join Date: Dec 2008 Posts: 347	A compiler doesn't check if you're doing meaningful things. It only checks if you're doing correct things. I don't think many compilers will complain at this: Code: #include <stdio.h> int main(void) { printf("The mystery is solved: NULL is %d\n", (int *)NULL); return 0; } Even though it is clearly undefined behaviour. __________________ Look at this: Community Project And this: Ascent - Programmer needed
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07-24-2009, 04:06 PM	#6
nonoob Registered User Join Date: Sep 2008 Location: Toronto, Canada Posts: 525	Compilers can't check logic errors or care much whether there are even any output statements. That should be obvious.
nonoob is offline

07-25-2009, 04:11 AM	#8
abachler Malum in se Join Date: Apr 2007 Posts: 3,188	The compiler just checks to make sure you use correct syntax, it doesn't check to make sure your code is coherent. The warbling of the instantiated was blue. Perfectly good syntax, with no meaning whatsoever. __________________ Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.
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07-25-2009, 04:29 AM	#10
MK27 critical genius Join Date: Jul 2008 Location: SE Queens Posts: 5,234	And if you do this: Code: int mostlypointless (int x) { int a = 12, i; for (i=0; i<10; i++) a = ai+a; return x+1; } GCC -O3 will just ignore the statements in red as if they didn't exist, because they are meaningless in the context of the program. The only way to notice this (I guess) is if you use massive meaningless loops for profiling. Compiled gcc -O3, they won't happen. __________________ "A man can't just sit around." --* Larry Walters Last edited by MK27; 07-25-2009 at 04:34 AM.
MK27 is online now

07-25-2009, 10:29 AM	#11
Bubba Super Moderator Join Date: Aug 2001 Posts: 7,819	Newer compilers for other languages are 'attempting' to do this and it comes off to me as 100% annoying. Maybe it's because I don't like my hand being held while I code. __________________ If you aim at everything you will hit something but you won't know what it is.
Bubba is offline

07-25-2009, 12:13 PM	#13
Yarin Unregistered User Join Date: Jul 2007 Posts: 982	>> They can, and some do. To a very limited extent, but for the most part they can't. When it comes to incorrect manipulation of certain things, 'const' does a good job for protecting against this - if for some reason the programmer doesn't want to use it, that's his problem. __________________ May the Source be with you.
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