Technical Information Database
TI500C.txt Null Pointer Assignment Errors Explained
Category :General
Platform :All
Product :Borland C++ All
Description:
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 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*)4 char *ds:0004 "Borland C++ - Copyright ..."
*(char*)0,4m 00 00 00 00
Step through your program and monitor theses values in the
watch window. At the point where one of them changes, you have
just 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 either of the following programs in the small
memory model and execute it:
Program 1:
#include
#include
#include
int main(void)
{
char *ptr, *banner;
banner = (char *)
MK_FP(_DS, 4);
printf("banner: %sn", banner);
strcpy(ptr, "Where will this text be copied?!?");
printf("&ptr = %Fpn",(void far*) &ptr[0]);
printf("banner: %sn", banner);
return 0;
}
Program 2:
// The following code illustrates the null pointer
// assignment error. Code using the "bad" pointer will
// cause a null pointer assignment; code using the "good"
// pointer will not.
#include
#include
int main()
{
char good[81];
char *bad;
strcpy (good, "This does not create a"
" null pointer assignment.n");
strcpy (bad, "This creates a null pointer assignment.n");
printf("%s", good);
printf("%s", bad);
return 0;
}
One useful debugging technique is to turn on all warning
compiler messages. If the above programs are compiled with
warnings turned off, no warning messages will be generated.
However, if all warnings are turned on, both the strcpy() and
printf() calls 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, you may wish to use the -wdef switch
to enable the "Possible use of identifier before definition"
warning. In the Borland or Turbo C++ IDE, select "Options /
Compiler / Messages / Display and set Display Warnings to
"All".
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
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.
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pertains.
Reference:
7/2/98 10:39:11 AM
Article originally contributed by Borland Staff