Thread: 1st post by 1st time C coder

Hello! I'm Servo Wizard.

I'm an accomplished VB6 programmer who is attempting to learn the C-C++ programming language. This project is my very first DLL and it is also the extent of my C programming experience. I proto-typed the functions in a console project and then moved on to the DLL.

1st major obstacle was figuring out how to return something that VB could digest. There was no way that I know of to export 21 digits other than a string. After 3 days of extensive research I learned how to export a string that VB would accept.

OK! End of 1st major obstacle, start of 2nd MAJOR OBSTACLE. When called from VB app it returns a string of 21 digits, all being the correct value when split into 7 individual values. I'm calling the DLL from a timer sub in VB and I noticed that the values were not varying as they occasionally should. It did not take to long to find that my string of 21 digits returned as 42 digits, 84 digits, 168 digits and so on until the MS error message arrived. It's quite obvious that the DLL does not close between calls.

I have spent the entire Labor day weekend trying to learn how to erase, free or whatever method is used to unload the 2 arrays in my DLL so as to prevent the compounding of the export string. Now, I sure that I have looked right at the solution several times but did not recognize it due to my inexperience with the C language.

It's not my style to have some one else do my learning and thinking for me but I would really appreciate some help with this problem.

Servo

Code:

#include "myheader.h"

char NxtBt[22];
char Bytes[22];

int  Something;

HANDLE h;

BOOL APIENTRY DllMain
(
HANDLE hModule, 
DWORD  ul_reason_for_call, 
LPVOID lpReserved
)

{
return TRUE;
}

BSTR __stdcall This()
{
if(Something)
  {
  strcat(Bytes, "0");
  }
  itoa(_inp(Blank),NxtBt,10); // decimal value of 4Bh as string "075"
  strcat(Bytes,NxtBt); // array[0]="075"

// repeats the following conditional 5 times with different value (Blank) each time.

  if(_inp(Blank)<100)
    {
    strcat(Bytes, "0");
    }
    itoa(_inp(Blank),NxtBt,10);
    strcat(Bytes,NxtBt); // Bytes[0-2]="075";Bytes[3-5]="0??" ; and so on to Bytes[18-21]="0??"
  }
return SysAllocString((OLECHAR*)Bytes);
SysFreeString((OLECHAR*)Bytes);
}

Some seemingly random code attempting to make a point:

Code:

#include <stdio.h>

void foo(int *array, size_t size)
{
   char text[22];
   size_t i, j = 0;
   for ( i = 0; i < size; ++i )
   {
      j += sprintf(&text[j], "&#37;03d", array[i]);
   }
   puts(text);
}

int main(void)
{
   int a[] = {0x4B,1,2,3,4,5,6}, b[] = {1,2,3,4,5,6,7};
   foo(a, sizeof a / sizeof *a); 
   foo(b, sizeof b / sizeof *b); 
   return 0;
}

/* my output
075001002003004005006
001002003004005006007
*/

Basically I think I'm saying there sounds like an issue with concatenation.

I think I was merely recommending to tack a different way in regard to your string creation using C-style arrays. My implementation is not quite the same as yours, so I went a standard route.

Please don't say "C-C++" there are two distinct languages, C and C++ (use a slash instead of a hyphen).
btw What you have written is entirely C language code, no C++ at all.

Don't use a variable with the same name as the function please.

First bug is using strcat on an uninitialised variable. Use strcpy(Blank, "0"); instead.
Second bug: SysFreeString wont do anything if you put it after the return statement because the function exits before reaching that.
Third bug: Don't just cast a char* to an OLECHAR* they are not the same. You may need to switch to WCHAR or TCHAR instead.

It works because the variable is initialized to zero before the first use - but if you call the function again, it will add the zero at the end of the previous string - do this a few times and you start overwriting the next string, etc, etc.

--
Mats

Originally Posted by Servo Wizard

I learned VB the same way that I'm attempting to learn C, the internet is worlds best text book.

Usually yes, but remember not to be unfair but there are a lot of unqualified people who find it "fun" to write tutorials which may not be to the standard of a well written book - by a well regarded publisher, K&R for example.

But it is true the internet is often the most convenient for reference.

Originally Posted by xp5

I'm reading this programming book and it gives a sample code:

Code:

int get_maxpos(int x[], int eff_size)
{
  int i, maxpos = 0;

  for (i = 0; i < eff_size; i++)
    maxpos = x[i] > x[maxpos] ? i: maxpos;
  return maxpos;

}

Books can also be badly written. Who in their right mind reccomends leaving out braces in a for loop? /me shakes head sadly

QuantumPete

Originally Posted by QuantumPete

Books can also be badly written. Who in their right mind reccomends leaving out braces in a for loop? /me shakes head sadly

QuantumPete

There's nothing wrong with that...

Originally Posted by zacs7

There's nothing wrong with that...

Just because the language supports it, doesn't mean it should be done!

Code:

if (false == result) 
    action_on_false (arg1, arg2);

Then you want to print an error message as well:

Code:

if (false == result)
   printf ("result is false\n");
   action_on_false (arg1, arg2);

Now action_on_false will always be executed and it's a hard bug to spot if result *usually* is false (such as when you're checking the return code of fopen). Defensive programming saves hours of debugging!