ID Number: Q71252
1.x 2.x 3.00 3.10 3.11 3.14 | 2.x 3.00 3.10 3.11 3.12 3.50
MS-DOS | OS/2
Summary:
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When a program produces the error "R6001: null pointer assignment" at
run time, the Microsoft CodeView Debugger may be used to track down
the errant pointer. To locate the source of an R6001 error, bring the
program up in CodeView and change focus to the command window. Next,
type the following four commands into the command window (make sure
you use the correct commands for the version of CodeView in use):
With CodeView Versions With CodeView Versions
3.00 and Later 2.35 and Earlier
---------------------- ----------------------
g main g main
n16 n16
bp=DS:0#42 tpb DS:0 DS:42
g g
The first command above executes to the beginning of the main()
function. The second command, "n16", sets the radix for subsequent
numeric entries to hexadecimal. The third command sets a breakpoint
that will halt execution if any of the first 66 (0x42) bytes of the
data segment change because this is the range of the null data
segment. The fourth command begins execution of the program.
CodeView may run slowly because it must check the null segment for
changes between every instruction. When the instruction that writes to
the null segment is executed, CodeView will stop the program. When
execution halts, the highlight will be on the line after the last
statement to be executed. Thus, the line above the highlighted line
will be the line where the bad pointer assignment occurred.
More Information:
The location of the null segment can be observed in a link map. It
starts at DS:0 and is 42H bytes long. The Microsoft copyright notice
is written there at program startup and if this area is written to
during the course of the program, the run-time error R6001 will be
generated upon program termination.
The most common cause of this error is using a pointer that has not
been initialized to point to a memory area. Pointers that have not had
space allocated for them [e.g., with malloc()] or that have not been
assigned to point to a specific data element (e.g., arrays or
structures) are considered uninitialized. Writing to one of these
pointers generally results in an overwrite of the null data segment.
With CodeView versions 1.00, 2.00, 2.10, 2.20, 2.30, and 2.35, the
procedure described above should be used for locating the source of an
R6001 error. With CodeView versions 3.00 and above, you may also use
the following menu and keyboard commands to set the breakpoint for
tracking down a null pointer assignment:
1. Press the F8 or F10 key to step into main().
2. Select Set Breakpoint from the Watch menu.
3. Choose "Break When Expression Has Changed" from the Set Breakpoint
dialog box.
4. In the Expression field, enter DS:0.
5. In the Length field, enter 66.
6. Choose OK, or press ENTER.
7. Press the F5 key to run the program.
As described above, the program will then break immediately after the
errant pointer has written to the null segment.
Note that it may be necessary to clear any breakpoints on the null
data segment and restart the program before entering the above
commands in order for these procedures to work correctly.
In addition, any breakpoint on DS:0 may stop execution if DS changes.
For example, if the above breakpoint is set and the program is
restarted under CodeView, the program will break during the C start-up
code [before reaching the main() function], since the C start-up sets
DS to the default data segment. [This is also the reason you must step
into main() before setting the breakpoint in the first place,
otherwise you would not be setting the breakpoint on the correct
segment.]
Additional reference words: s_c s_quickc