Pointer Incrementing

[zoowho]

This is from the ENVIRON.C program in Petzold's win32 book.

void FillListBox (HWND hwndList) 
{
     int     iLength ;
     TCHAR * pVarBlock, * pVarBeg, * pVarEnd, * pVarName ;

     pVarBlock = GetEnvironmentStrings () ;  // Get pointer to environment block

     while (*pVarBlock)
     {
          if (*pVarBlock != '=')   // Skip variable names beginning with `=`
          {
               pVarBeg = pVarBlock ;              // Beginning of variable name
               while (*pVarBlock++ != '=') ;      // Scan until `=`
               pVarEnd = pVarBlock - 1 ;          // Points to `=` sign
               iLength = pVarEnd - pVarBeg ;      // Length of variable name

                    // Allocate memory for the variable name and terminating
                    // zero. Copy the variable name and append a zero.

               pVarName = (TCHAR *) calloc (iLength + 1, sizeof (TCHAR)) ;
               CopyMemory (pVarName, pVarBeg, iLength * sizeof (TCHAR)) ;
               pVarName[iLength] = '\0' ;

                    // Put the variable name in the list box and free memory.
               SendMessage (hwndList, LB_ADDSTRING, 0, (LPARAM) pVarName) ;
               free (pVarName) ;
          }
          while (*pVarBlock++ != '\0') ;     // Scan until terminating zero
     }
     FreeEnvironmentStrings (pVarBlock) ;
}

When does pVarBlock become =0 and therefore false in the first while loop? It seems to me that this code will cause pVarBlock to keep increasing (>0) until it points to the end of the computers memory. The program shouldn't ever exit the while loop.

Since the program works I am obviously missing something and it's probably pretty basic. Can someone explain?

[Salem]

GetEnvironmentStrings Function (Windows)
Look carefully at what is stored at the end of the strings.

> while (*pVarBlock++ != '\0') ;
This leaves pVarBlock pointing to the char after the \0

[MK27]

while (*pVarBlock++ != '\0') ;     // Scan until terminating zero

This leaves pVarBlock pointing to the char after the \0

Probably this memory was calloc'd so will be all zeros after the last string.

[zoowho]

GetEnvironmentStrings Function (Windows)
Look carefully at what is stored at the end of the strings.

> while (*pVarBlock++ != '\0') ;
This leaves pVarBlock pointing to the char after the \0

Uhmm, yea. But that just positions it to the start of the next environment string so that it can process the next environment variable and get it's name. Even if it did increment pVarBlock until the end of all of the environment variable names and values, wouldn't pVarBlock still be nonzero and therefore true?

[MK27]

Even if it did increment pVarBlock until the end of all of the environment variable names and values, wouldn't pVarBlock still be nonzero and therefore true?

The value of pVarBlock is not the same as the value of *pVarBlock.

Try this:

	char str[]="adsfa\0x", *ptr=str;
	while (*ptr++);

	printf("%c\n",*ptr);

while (*ptr++) is identical to while(*ptr++ != 0)