Volume Number: 16 (2000)
Issue Number: 6
Column Tag: Navigation Services
by Thurman Gillespy III, M.D.
A recent project gave me an excuse to delve seriously into the Navigation Services (NavServices) SDK for the first time. The SDK includes three example projects (Sampler, StExample, and SimpleText) that illustrate the NavServices API. During routine debugging of my code, however, I discovered multiple "write-to-nil" bugs that had originated in the example projects. This article describes the steps you need to fix those errors and to update the projects to work with CodeWarrior Pro 5 and Universal Headers 3.3. For a pointer to the NavServices SDK, see the References section at the end of this article.
Sampler is a small C program that demonstrates to use the NavServices API in a document-based application. It uses NavServices functions to elicit files from the user, save files, and prompt the user to save or discard changes to a document. It also shows how to use NavServices functions to let the user choose a folder, volume, or file-system object. Sampler is a very good place to start learning how to use NavServices. So let's make Sampler usable!
Update Access Paths
In the Access Paths panel of the Sampler.PPC Settings dialog box, remove the 'Shared Libraries' and 'Cincludes' paths from the User Paths list. They are not needed. Then change the System Paths to:
{Compiler}:MacOS Support:
{Compiler}:MSL:
Save the changes to this panel by clicking the Save button.
Implied Int No Longer Allowed
In Templates.c, there is an implied return value of type int to the main function that is no longer legal in C++. Change:
main()
to:
int main(void)
Runtime Library Changes
Change the MWCRuntime.lib library (which is no longer present) to MSL RuntimePPC.Lib.
Add a Header File
Universal Headers 3.3 include the new header file ControlDefinitions.h, which contains some constants previously defined in the file Controls.h. So add the following lines to the file Common.h:
#ifndef __CONTROLDEFINITIONS__ #include <ControlDefinitions.h> #endif
NavCBRec Field Change
Somewhere between "then and now", the fields in the NavCBRec structure changed. In file.c and menu.c, there are a total of 10 NavCBRecPtr variables to update. Change:
callBackParms->eventData.event
to:
callBackParms->eventData.eventDataParms.event
Duplicate cfrg Resource
A 'cfrg' resource is defined in TemplatePPC.r. This definition is no longer needed because the CodeWarrior IDE now generates the correct resource. I simply commented out the definition in TemplatePPC.r.
/*
resource 'cfrg' (0) {
{
kPowerPC,
kFullLib,
kNoVersionNum,kNoVersionNum,
0,0,
kIsApp,kOnDiskFlat,kZeroOffset,kWholeFork,
"Template"
}
};
*/
AEGetDescData Write-to-nil Errors
AEGetDescData (defined in Common.c) is a utility function that extracts a DescType of type typeCode from the AEDesc. In Sampler, it is used to extract an FSSpec from the AEDesc. Listing 1 shows the original function.
Listing 1: Getting data from an AEDesc structure (before)
AEGetDescData
OSErr AEGetDescData(const AEDesc *desc, DescType *typeCode,
void *dataBuffer, ByteCount maximumSize,
ByteCount *actualSize)
{
*typeCode = desc->descriptorType;
Handle h = (Handle)desc->dataHandle;
ByteCount dataSize = GetHandleSize(h);
if (dataSize > maximumSize)
*actualSize = maximumSize;
else
*actualSize = dataSize;
BlockMoveData(*h, dataBuffer, *actualSize);
return noErr;
}
The function is acceptable, except for a complete lack of any error checking (a common practice in developer example code). However, AEGetDescData is called 10 times with two NULL pointers (menus.c).
if ((theErr = AEGetDescData ( &resultDesc, NULL, &finalFSSpec, sizeof ( FSSpec ), NULL )) == noErr)...
This code fragment results in two writes to nil (typeCode and actualSize) and one read from nil (BlockMoveData). Bad form, indeed. For a fix, I simply check for the NULL parameters; I also added a few other error checks for good measure, as shown in Listing 2.
Listing 1: Getting data from an AEDesc structure (after)
AEGetDescData
OSErr AEGetDescData(const AEDesc *desc, DescType *typeCode,
void *dataBuffer, ByteCount maximumSize,
ByteCount *actualSize)
{
Handle h = NULL;
ByteCount dataSize = 0,
bytesToCopy = 0;
OSErr err = -1;
// check for invalid NULL handles or pointers
if (desc == NULL || desc->dataHandle == NULL ||
dataBuffer == NULL)
return nilHandleErr;
// get the size of the object, check for errors
h = (Handle)desc->dataHandle;
dataSize = GetHandleSize(h);
err = MemError();
if (err != noErr)
return err;
// determine how many bytes to copy
if (dataSize > maximumSize)
bytesToCopy = maximumSize;
else
bytesToCopy = dataSize;
// copy object from the handle to the buffer
BlockMoveData(*h, dataBuffer, bytesToCopy);
// return type code and number of bytes copied,
// if pointers are not NULL
if (typeCode != NULL)
*typeCode = desc->descriptorType;
if (actualSize != NULL)
*actualSize = bytesToCopy;
return noErr;
}
AEGetNthDesc Write-to-nil Errors
In Sampler, AEGetNthDesc writes to nil 11 times (files.c, menus.c) because the fourth parameter, a pointer to an AEKeyword structure, is set to NULL.
AEGetNthDesc(&(theReply.selection), index, typeFSS, NULL, &resultDesc)
To correct this error, simply add an appropriate AEKeyword parameter.
AEKeyword aekw; // not used AEGetNthDesc(&(theReply.selection), index, typeFSS, &aekw, &resultDesc)
Sampler CFM68k
For the Sampler.68kCFM project, make the same changes to the access paths, then update the obsolete libraries to the correct versions.
MathLibCFM68k(4i/8d).Lib ==> MathLibCFM68k (4i_8d).Lib MSL C.CFM68kFar(4i/8d).Lib ==> MSL C.CFM68kFa(4i_8d).Lib MWCFM68kRuntime.Lib ==> MSL MWCFM68kRuntime.Lib
The StExample project is a C++ example of using NavServices that is less complete than Sampler. The StExample project is also easier to update.
StNavGetFile gf1( &err, &specs, &numspecs, &rdopen, &navreply, false, true, &rdevt, nil, nil, tlh, 0xaabbccdd);
to
UInt32 usrDat = 0xaabbccdd; StNavGetFile gf1( &err, &specs, &numspecs, &rdopen, &navreply, false, true, &rdevt, nil, nil, tlh, (void *)&usrDat );
The SimpleText project uses the obsolete QuickDrawGX, and a dated MacIncludes.h header. I decided it wasn't worth the effort to update the project.
To see the write-to-nil errors, drop into Macsbug and invoke the EBBE (Even Better Bus Error) dcmd.
EBBE on EBBE is ON (using the value $68F168F1 with a task rate of #17 milliseconds)
Then run the unaltered Sampler application as delivered in the SDK. You should see one or more write-to-nil errors in every call to NavServices. Both QC and Spotlight (Onyx Technology) will also catch the errors.
The EBBE dcmd has detected that location $0000 has been overwritten
After making the corrections described here, run this test again on the compiled example applications and on any of your code based on the SDK sample code. The write-to-nil errors should be eliminated.
Navigation Services SDK
<http://developer.apple.com/sdk/index.html>
Onyx Technology
<http://www.onyx-tech.com/>
Thurman Gillespy III is a radiologist at the Veterans Administration Puget Sound Health Care System in Seattle, Washington. He can be reached at tg3@u.washington.edu
