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unreferenced local variable

 
 
Alzane
Guest
Posts: n/a
 
      11-29-2004
I'm new to C++ programming. I have an exercise that I have written
code for but getting warnings. Can I get some help?


#include "stdafx.h"
#include <string.h>
#include <stdio.h>
#include <stdlib.h>


int main(int argc, char* argv[])

{
char *StrNam1;
char *StrNam2;

StrNam2 = (char *) malloc (strlen (StrNam2) + 1);
sprintf("%s%s", strcat
("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FS ID%3D",
"A3D6F20D-7091"));


return 0;
}

Compiling...
Concatenate.cpp
C:\Program Files\Microsoft Visual
Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
'StrNam1' : unreferenced local variable
C:\Program Files\Microsoft Visual
Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
variable 'StrNam2' used without having been initialized
Linking...

Concatenate.exe - 0 error(s), 2 warning(s)
 
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Ron Natalie
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Posts: n/a
 
      11-29-2004
Alzane wrote:
> I'm new to C++ programming. I have an exercise that I have written
> code for but getting warnings. Can I get some help?
>
>
> #include "stdafx.h"


BOGUS include file.


>
> int main(int argc, char* argv[])
>
> {
> char *StrNam1;

You indeed don't use this anywhere, causing the first warning.

> char *StrNam2;
>
> StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


At this point StrNam2 is set to some indeterminate value. What do
you think passing it to strlen is going to do? The length of WHAT
string?

> sprintf("%s%s", strcat
> ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FS ID%3D",
> "A3D6F20D-7091"));
>

This isn't how sprintf works at all. The first arg to sprintf has
to be a char* that points to some allcoated memory that the printf()
results will be written too.

The second arg is the format string. And you strcat doesn't work
the way you think it should either.

At this point, you probably should stay away from C's hiddeous attempt
to implement strings as character arrays and just use C++'s string type.

#include <string>
int main() {
std::string StrNam2 = "..%2F..dkafjkjdflasdkjf";
StrNam2 += "A3D6F20D-7091";

}
 
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Victor Bazarov
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Posts: n/a
 
      11-29-2004
Alzane wrote:
> I'm new to C++ programming. I have an exercise that I have written
> code for but getting warnings. Can I get some help?
>
>
> #include "stdafx.h"
> #include <string.h>
> #include <stdio.h>
> #include <stdlib.h>
>
>
> int main(int argc, char* argv[])
>
> {
> char *StrNam1;


You declared the pointer 'StrNam1' but never use it in the program.
That's why the compiler warns you.

> char *StrNam2;
>
> StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


What are you trying to do here? 'StrNam2' has no value given to it.
You're trying to find its length by calling 'strlen'. That's the
second warning you get. Anyway, that's not the right way to manage
your memory. If you want to allocate some memory and store the pointer
to it in 'StrNam2', you need to give it some meaningful size. What is
the purpose of 'StrNam2'?

> sprintf("%s%s", strcat
> ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FS ID%3D",
> "A3D6F20D-7091"));


'sprintf' is supposed to have the string where you want the result as
the very first argument. The format is the _second_ argument. RTFM.

'strcat' is used incorrectly. If you just want to have two literals
live on two different lines of code but represent the same string, you
can simply leave whitespace between them:

"..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSI D%3D"
"A3D6F20D-7091"

No commas, no 'strcat' calls. 'strcat' requires the first argument to
be the _resulting_ buffer. If you pass a literal as the first argument,
you're asking for trouble. RTFM.

>
>
> return 0;
> }
>
> Compiling...
> Concatenate.cpp
> C:\Program Files\Microsoft Visual
> Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
> 'StrNam1' : unreferenced local variable
> C:\Program Files\Microsoft Visual
> Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
> variable 'StrNam2' used without having been initialized
> Linking...
>
> Concatenate.exe - 0 error(s), 2 warning(s)


V
 
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Mike Wahler
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Posts: n/a
 
      11-29-2004

"Alzane" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> I'm new to C++ programming. I have an exercise that I have written
> code for but getting warnings. Can I get some help?
>
>
> #include "stdafx.h"


This is a nonstandard header. Please omit such from
code posted here. You don't need this anyway.

> #include <string.h>
> #include <stdio.h>
> #include <stdlib.h>
>
>
> int main(int argc, char* argv[])
>
> {
> char *StrNam1;
> char *StrNam2;


You've just defined two pointers. Since you did not initialize them,
their values are indetermindate, unknown. I.e. these pointers don't
point anywhere.

>
> StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


Here you pass an ininitalizaed pointer ('StrNam2') to a function
('strlen()'), which expects a valid pointer to a zero-terminated
array of characters. The resulting behavior is undefined. If you
want to compute the length of a string, first you need a string.

You also need to check 'malloc()'s return value to see if it
failed (returns NULL in that case).

You never subsequently use 'StrNam2' in your program anyway,
so I'm not sure what your trying to do. Perhaps if you
explain that, I could offer more specific advice.

> sprintf("%s%s", strcat
> ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FS ID%3D",
> "A3D6F20D-7091"));


Here's another case of undefined behavior. 'strcat()' will attempt
to modify the string pointed to by its first parameter. But you give
a pointer to a string literal, which the language prohibits modifying.

Another 'undefined' aspect of this is that you've given 'printf()'
two format specifiers ('%s'), but only one corresponding argument.

If you want to concatentate two strings, you need a place to store
the resulting string. (This could be the string being lengthened,
if sufficient storage has been allocated for it).

>
>
> return 0;
> }
>
> Compiling...
> Concatenate.cpp
> C:\Program Files\Microsoft Visual
> Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
> 'StrNam1' : unreferenced local variable


This is harmless. It simply means you've created an object an
never used it. Use modern C++ and forget that low-level memory
management you've tried above.


-Mike

> C:\Program Files\Microsoft Visual
> Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
> variable 'StrNam2' used without having been initialized


This is the real problem.


Since none of what you wrote above enough sense for me to
try to guess what you're trying to do, I can only offer this
simple C++ program which outputs the result of concatenating
two strings.


#include <iostream>
#include <string>

int main()
{
std::string StrNam1("Hello ");
std::string StrNam2("world");
std::cout << StrNam1 + StrNam2 << '\n';
return 0;
}

-Mike


 
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Default User
Guest
Posts: n/a
 
      11-29-2004
Alzane wrote:

> I'm new to C++ programming. I have an exercise that I have written
> code for but getting warnings. Can I get some help?
>
>
> #include "stdafx.h"


The above header is non-standard.

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


While technically C++ headers, they are provided mostly for
compatibility with C. You should probably not be using them.

>
> int main(int argc, char* argv[])


As you don't use the args in main(), you should leave them out.

> {
> char *StrNam1;
> char *StrNam2;


It's a poor idea for a beginner to use pointers in learning programs.

> StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


StrNam2 was not initialized. What did you think the result of getting
the string length of it would be? What it is in Undefined Behavior, a
Very Bad Thing.

Also, malloc() is generally not used in C++ programming.

> sprintf("%s%s", strcat
> ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FS ID%3D",
> "A3D6F20D-7091"));


You don't know how to use sprintf() at all. The arg count is wrong.
Also, your use of strcat() modifies a string literal. Undefined
Behavior again.

>
> return 0;


You got this right.

> }
>
> Compiling...
> Concatenate.cpp
> C:\Program Files\Microsoft Visual
> Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
> 'StrNam1' : unreferenced local variable


You didn't use StrNam1.

> C:\Program Files\Microsoft Visual
> Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
> variable 'StrNam2' used without having been initialized


This is the far more dangerous problem.


What book are you using? This is a mishmashed, dangerous piece of
essentially C code.

Get a good book, one using modern C++, then read it. Otherwise, you're
wasting your time and ours.



Brian

 
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Method Man
Guest
Posts: n/a
 
      11-30-2004

"Alzane" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> I'm new to C++ programming. I have an exercise that I have written
> code for but getting warnings. Can I get some help?
>
>
> #include "stdafx.h"
> #include <string.h>
> #include <stdio.h>
> #include <stdlib.h>
>
>
> int main(int argc, char* argv[])
>
> {
> char *StrNam1;
> char *StrNam2;
>
> StrNam2 = (char *) malloc (strlen (StrNam2) + 1);
> sprintf("%s%s", strcat
> ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FS ID%3D",
> "A3D6F20D-7091"));
>
>
> return 0;
> }



A couple more suggestions:

1. Don't cast the result of malloc, it can be unsafe.
2. For every malloc(), make sure to have a corresponding free() call. It's
better to be safe and manage your own memory.

Note: The above looks like C code. A good book on C or C++ would help you
with your exercise much more.


 
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Ron Natalie
Guest
Posts: n/a
 
      11-30-2004
Method Man wrote:
=
>
> A couple more suggestions:
>
> 1. Don't cast the result of malloc, it can be unsafe.


You have to cast the result of malloc. There is no implicit
conversion from void* in C++.


> 2. For every malloc(), make sure to have a corresponding free() call. It's
> better to be safe and manage your own memory.


True, but it would be better to avoid user-managed dynamic allocations
here entirely. That would have avoided 90% of the problems.
 
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Alzane
Guest
Posts: n/a
 
      11-30-2004
Thanks to you all for you explanations. I'm going through this book
called "C Programming for the Absolute Beginner", which as you can see
is definitely me, but I have this person who trying to get me up to
speed with some exercises. I agree I should study more. If you have
any suggestion of a good C Programming book, please advise.
 
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Default User
Guest
Posts: n/a
 
      11-30-2004
Ron Natalie wrote:

> Method Man wrote:
> =
> >
> > A couple more suggestions:
> >
> > 1. Don't cast the result of malloc, it can be unsafe.

>
> You have to cast the result of malloc. There is no implicit
> conversion from void* in C++.


Yep, MM is thinking of C. In C++, not only is the cast required but the
"C problem" of implicit declaration of undeclared functions doesn't
exist either.

> > 2. For every malloc(), make sure to have a corresponding free()
> > call. It's better to be safe and manage your own memory.

>
> True, but it would be better to avoid user-managed dynamic allocations
> here entirely. That would have avoided 90% of the problems.


I agree. Newbies to C++ should be starting out with containers. Once a
firm grasp of programming is established, then dynamic memory can be
tackled.



Brian
 
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Default User
Guest
Posts: n/a
 
      11-30-2004
Alzane wrote:

> Thanks to you all for you explanations. I'm going through this book
> called "C Programming for the Absolute Beginner", which as you can see
> is definitely me, but I have this person who trying to get me up to
> speed with some exercises. I agree I should study more. If you have
> any suggestion of a good C Programming book, please advise.


Why are you posting to a C++ newsgroup, when you are attempting to
learn C? Many of the comments you received were inappropriate in the
context of a review of a C program (although many others were the same).

You should be using comp.lang.c.




Brian
 
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