Category : C Source Code
Archive   : C-ALL.ZIP
Filename : TI726.ASC

 
Output of file : TI726.ASC contained in archive : C-ALL.ZIP







PRODUCT : Borland C++ NUMBER : 726
VERSION : 2.0
OS : DOS
DATE : September 18, 1991 PAGE : 1/3

TITLE : "Null Pointer Assignment" Errors Explained




Null Pointer Assignment

1. What is a Null Pointer Assignment error?

The Null Pointer Assignment error is generated in programs that
corrupt the bottom of the data segment in such a fashion as to
indicate a high probability that an improperly-initialized
pointer has been used. The error can only be generated in the
small and medium memory models.

2. What causes a Null Pointer Assignment error?

Borland places four zero bytes at the bottom of the data segment,
followed by the Borland copyright notice. In the small and
medium memory models, a null pointer points to DS:0000. Thus
assigning a value to the memory referenced by this pointer will
overwrite the first zero byte in the data segment. At program
termination, the four zeros and the copyright banner are checked.
If either has been modified, then the Null Pointer Assignment
error is generated. Note that the pointer may not truly be null,
but may be a wild pointer that references these key areas in the
data segment.

3. How can I debug a Null Pointer Assignment error?

In either the Integrated Development Environment (except BCX) or
in Turbo Debugger, set two watches on these key memory locations.
These watches, and what they should display in the watch window,
are:

*(char*)0,4m "Borland C++ - Copyright 1991 Borland Intl."
(char *)0 00 00 00 00

Of course, the copyright banner will vary depending on your
version of the Borland C/C++ compiler.

Caution: The first of these watches--*(char *)0,4m--cannot be
used in BCX or a general protection fault will be generated;
however, the second watch--(char *)0--will perform properly.

Step through your program and monitor theses values in the watch
window. At the point where one of them changes, you have just














PRODUCT : Borland C++ NUMBER : 726
VERSION : 2.0
OS : DOS
DATE : September 18, 1991 PAGE : 2/3

TITLE : "Null Pointer Assignment" Errors Explained




executed a statement that uses a pointer that has not been
properly initialized.

The most common cause of this error is probably declaring a
pointer and then using it before allocating memory for it. For
example, compile this program in the small memory model and
execute it:

#include
#include
#include

int main(void)
{
char *ptr, *banner;
banner = (char *) MK_FP(_DS, 4);
printf("banner: %s\n", banner);
strcpy(ptr, "Where will this text be copied?!?");
printf("&ptr = %Fp\n", (void far*) &ptr[0]);
printf("banner: %s\n", banner); return 0;
}

One of the best debugging techniques is to turn on all warning
compiler messages. If the above program is compiled with
warnings turned off, no warning messages will be generated.
However, if all warnings are turned on, both the strcpy() and
printf() calls using the buffer variable will generate warnings.
Be particularly suspicious of any warnings that a variable might
be used before being initialized, or of a suspicious pointer
assignment. With the command-line compiler, just include the -w
switch. In the IDE, select the appropriate menu option for
compiler messages and work through all the menu windows, ensuring
all items are checked. In Turbo C++ and Borland C++, some of the
option windows have sub-menus indicated by a 'more' selection box
in the lower left corner of the menu box.

4. Why is a Null Pointer Assignment error not generated in all
models?

In the compact, large and huge memory models, far pointers are
used for data. Therefore, a null pointer will reference
0000:0000, or the base of system memory, and using it will not
cause a corruption of the key values at the base of the data













PRODUCT : Borland C++ NUMBER : 726
VERSION : 2.0
OS : DOS
DATE : September 18, 1991 PAGE : 3/3

TITLE : "Null Pointer Assignment" Errors Explained




segment. Modifying the base of system memory usually causes a
system crash, however. Although it would be possible that a wild
pointer would overwrite the key values, it would not indicate a
null pointer.

In the tiny memory model, DS = CS = SS. Therefore, using a null
pointer will overwrite the beginning of the code segment.

5. Can anything else generate a Null Pointer Assignment error?

Using a wild pointer that happens to reference the base area of
the data segment--thus causing a corruption of the zeros or the
copyright banner--will generate this error. Since data
corruption or stack corruption could cause an otherwise-valid
pointer to be corrupted and point to the base of the data
segment, any memory corruption could result in this error being
generated. If the pointer used in the program statement which
corrupts the key values appears to have been properly
initialized, place a watch on that pointer. Step through your
program again and watch for its value (address) to change.































  3 Responses to “Category : C Source Code
Archive   : C-ALL.ZIP
Filename : TI726.ASC

  1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

  2. This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.

  3. But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: http://www.os2museum.com/wp/mtswslnk/