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What's the difference between these two ??

Posted on 2004-10-05
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Last Modified: 2010-04-01
Hi, what is the difference between these two codefragments ??
I don't see any difference...
**** THIS WORKS ****
int16* dst=p1->buffer;  //buffer consists allso out of int16's
for (int i = 0; i < strlen(aa); i++)
      {
      val = *aa;
      SSVAL(dst, 0, val);
      dst++;
      aa++;
      if (val == 0)
            break;
      }
**** END ****

**** THIS DOESN'T WORKS ****

for (int i = 0; i < strlen(aa); i++)
      {
      val = *aa;
      SSVAL(p1->buffer, 0, val);
      p1->(buffer++);
      aa++;
      if (val == 0)
            break;
      }
****END****

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Question by:pimpernel
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7 Comments
 
LVL 45

Expert Comment

by:Kent Olsen
ID: 12226086

From your comment and the code fragments, it appears that p1 is defined as:

typedef struct
{
...
  int16 buffer[SomeLength];
...
} p1_type;

p1_type *p1;

In your first example you assign the address of p1->buffer to dst and then increment dst each time through your loop.

In your second example you increment p1->buffer[0] each time through the loop.


Kent
0
 

Author Comment

by:pimpernel
ID: 12226155
Thanks. But how should I increment it then correct (like in the first example...) ??

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LVL 45

Accepted Solution

by:
Kent Olsen earned 100 total points
ID: 12226262

You need a temporary variable that you can increment, like in your first example.

The second fragment is actually an error because you're incrementing your data instead of a pointer.  Even if p1->buffer is defined as an 'int16 *' you don't want to do what the second fragment does because you're now incrementing the pointer to your buffer.  In all likelihood this is the base pointer so this coding style causes you to lose the base pointer to your buffer.

Stick with something close to example 1.  It works because it's correct.


Good Luck,
Kent
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LVL 2

Assisted Solution

by:guntherothk
guntherothk earned 25 total points
ID: 12227342
I'm not sure this loop does what you think it does. As you increment i in the loop, you are also incrementing aa, so strlen(aa) is decreasing each time through the loop. C++ evaluates the expression fully each time through. I'm betting you only read half the things you want to read. Unless this is the behavior you want, you need to assign strlen(aa) to a variable outside the loop

    int slen = strlen(aa);
    for (int i = 0; i < slen; i++) ...

or you can be fancy and put both into the for loop initializer

    for (int slen = strlen(aa), i = 0; i < slen; i++) ...
0
 

Author Comment

by:pimpernel
ID: 12228106
OK, Kdo, I'll keep the first sollution...
Thanks guntherothk for the tip. I'll put the strlen out of the "for".

0
 
LVL 39

Expert Comment

by:itsmeandnobodyelse
ID: 12229591
Yesterday i answered to that question

     http:Q_21154939.html

It was using a ToUnic function that had identical code fragments than your code, but hadn't the bugs kdo and gunther already pointed out. Obviously you are trying to change working code, what isn't recommended even if you would exactly know what you are doing.

As you made no response to the comments of kdo and gunther, i'll explain again the errors you made:

With p1->(buffer++) you are changing the pointer value of the data member 'buffer' in the struct _UNICODE_STRING. The instance 'p1' of  that struct has an invalid buffer member then (it points to the terminating zero and *not* to the begin of the previously allocated storage).

The main difference to the old working code is, that it uses a local pointer variable, thus leaving the value of p1->buffer intact.

The second error was similar: Instead of using a local variable 'len' as upper bound of the for loop, you are using strlen function. However, the argument of strlen is the char pointer 'aa' that get changed in the loop when that statement executes: aa++; . So, strlen(aa) decrements and i++ increments, e. g. with a string length of 10 only 5 bytes were copied to the UNICODE string.

Regards, Alex








0
 

Author Comment

by:pimpernel
ID: 12331500
Hello,
sorry for my very late response, but I don't check my mail very often...
Thanks for your extra explanation.
I allreasy understood the remark of Kdo, but indeed didn't entirely understood guntherothk's remark (since the first code with the strlen also worked).

Now it's entirely clear (I think), and since my strings allways have a fixed length I just replaced the strlen part with th length.

Greetz, pimpernel
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