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6. Exception Handling, Logging, and Assertions

No matter how well a program is designed, if it has to interact with a user or other aspect of the outside world in any way, the code is bound to occasionally meet with cases that are either invalid or just plain unexpected. A very simple example is when a program asks the user to enter a filename, and the user enters the name of a file that does not exist, or does not enter a name at all. Perhaps a valid filename is entered, but, due to a previous disk write error the contents are garbled. Any number of things can go wrong. In addition, programmer error inevitably occurs and needs to be taken account of. Internal functions may be called with invalid arguments, either due to unexpected paths being taken through the code, or silly things like typos using the wrong variable for something. When these problems happen (and they will happen), it is better to handle them gracefully than for the program to crash, or worse, to continue processing but in an erroneous way.

To allow for this, many computer languages provide two types of facilities. The first is referred to as exception handling or sometimes error trapping. The second is referred to as assertion checking. Exceptions allow the program to catch errors when they occur and react to them explicitly. Assertions allow a programmer to establish that certain conditions hold before attempting to execute a particular operation. GNUstep provides both of these facilities, and we will cover each in turn. The assertion facility is tied in with the GNUstep logging facilities, so we describe those as well.

To use any of the facilities described in this chapter requires that you include Foundation/NSException.h.


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6.1 Exceptions

GNUstep exception handling provides for two things:

  1. When an error condition is detected during execution, control is passed to a special error-handling routine, which is given information on the error that occurred.
  2. This routine may itself, if it chooses, pass this information up the function call stack to the next higher level of control. Often higher level code is more aware of the context in which the error is occurring, and can therefore make a better decision as to how to react.

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6.1.1 Catching and Handling Exceptions

GNUstep exception handling is implemented through the macros NS_DURING, NS_HANDLER, and NS_ENDHANDLER in conjunction with the NSException class. The following illustrates the pattern:

 
NS_DURING
  {
    // do something risky ...
  }
NS_HANDLER
  {
    // a problem occurred; inform user or take another tack ...
  }
NS_ENDHANDLER
  // back to normal code...

For instance:

 
- (DataTree *) readDataFile: (String *)filename
{
  ParseTree *parse = nil;
  NS_DURING
    {
      FileHandle *handle = [self getFileHandle: filename];
      parse = [parser parseFile: handle];
      if (parse == nil)
        {
          NS_VALUERETURN(nil);
        }
    }
  NS_HANDLER
    {
      if ([[localException name] isEqualToString: MyFileNotFoundException])
        {
          return [self readDataFile: fallbackFilename];
        }
      else if ([[localException name] isEqualToString: NSParseErrorException])
        {
          return [self readDataFileInOldFormat: filename];
        }
      else
        {
          [localException raise];
        }
    }
  NS_ENDHANDLER
  return [[DataTree alloc] initFromParseTree: parse];
}

Here, a file is parsed, with the possibility of at least two different errors: not finding the file and the file being misformatted. If a problem does occur, the code in the NS_HANDLER block is jumped to. Information on the error is passed to this code in the localException variable, which is an instance of NSException. The handler code examines the name of the exception to determine if it can implement a work-around. In the first two cases, an alternative approach is available, and so an alternative value is returned.

If the file is found but the parse simply produces a nil parse tree, the NS_VALUERETURN macro is used to return nil to the readDataFile: caller. Note that it is not allowed to simply write "return nil;" inside the NS_DURING block, owing to the nature of the behind-the-scenes C constructs implementing the mechanism (the setjmp() and longjmp() functions). If you are in a void function not returning a value, you may use simply "NS_VOIDRETURN" instead.

Finally, notice that in the third case above the handler does not recognize the exception type, so it passes it one level up to the caller by calling -raise on the exception object.


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6.1.2 Passing Exceptions Up the Call Stack

If the caller of -readDataFile: has enclosed the call inside its own NS_DURINGNS_HANDLERNS_ENDHANDLER block, it will be able to catch this exception and react to it in the same way as we saw here. Being at a higher level of execution, it may be able to take actions more appropriate than the -readDataFile: method could have.

If, on the other hand, the caller had not enclosed the call, it would not get a chance to react, but the exception would be passed up to the caller of this code. This is repeated until the top control level is reached, and then as a last resort NSUncaughtExceptionHandler is called. This is a built-in function that will print an error message to the console and exit the program immediately. If you don't want this to happen it is possible to override this function by calling NSSetUncaughtExceptionHandler(fn_ptr). Here, fn_ptr should be the name of a function with this signature (defined in NSException.h):

 
void NSUncaughtExceptionHandler(NSException *exception);

One possibility would be to use this to save files or any other unsaved state before an application exits because of an unexpected error.


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6.1.3 Where do Exceptions Originate?

You may be wondering at this point where exceptions come from in the first place. There are two main possibilities. The first is from the Base library; many of its classes raise exceptions when they run into error conditions. The second is that application code itself raises them, as described in the next section. Exceptions do not arise automatically from C-style error conditions generated by C libraries. Thus, if you for example call the strtod() function to convert a C string to a double value, you still need to check errno yourself in standard C fashion.

Another case that exceptions are not raised in is in the course of messaging. If a message is sent to nil, it is silently ignored without error. If a message is sent to an object that does not implement it, the forwardInvocation method is called instead, as discussed in Advanced Messaging.


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6.1.4 Creating Exceptions

If you want to explicitly create an exception for passing a particular error condition upwards to calling code, you may simply create an NSException object and raise it:

 
NSException myException = [[NSException alloc]
                              initWithName: @"My Exception"
                                    reason: @"[Description of the cause...]"
                                  userInfo: nil];
[myException raise];
 // code in block after here is unreachable..

The userInfo argument here is a NSDictionary of key-value pairs containing application-specific additional information about the error. You may use this to pass arbitrary arguments within your application. (Because this is a convenience for developers, it should have been called developerInfo..)

Alternatively, you can create the exception and raise it in one call with +raise:

 
[NSException raise: @"My Exception"
            format: @"Parse error occurred at line %d.",lineNumber];

Here, the format argument takes a printf-like format analogous to [NSString -stringWithFormat:] discussed Strings in GNUstep. In general, you should not use arbitrary names for exceptions as shown here but constants that will be recognized throughout your application. In fact, GNUstep defines some standard constants for this purpose in NSException.h:

NSCharacterConversionException

An exception when character set conversion fails.

NSGenericException

A generic exception for general purpose usage.

NSInternalInconsistencyException

An exception for cases where unexpected state is detected within an object.

NSInvalidArgumentException

An exception used when an invalid argument is passed to a method or function.

NSMallocException

An exception used when the system fails to allocate required memory.

NSParseErrorException

An exception used when some form of parsing fails.

NSRangeException

An exception used when an out-of-range value is encountered.

Also, some Foundation classes define their own more specialized exceptions:

NSFileHandleOperationException (NSFileHandle.h)

An exception used when a file error occurs.

NSInvalidArchiveOperationException (NSKeyedArchiver.h)

An archiving error has occurred.

NSInvalidUnarchiveOperationException (NSKeyedUnarchiver.h)

An unarchiving error has occurred.

NSPortTimeoutException (NSPort.h)

Exception raised if a timeout occurs during a port send or receive operation.

NSUnknownKeyException (NSKeyValueCoding.h)

An exception for an unknown key.


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6.1.5 When to Use Exceptions

As might be evident from the -readDataFile: example above, if a certain exception can be anticipated, it can also be checked for, so you don't necessarily need the exception mechanism. You may want to use exceptions anyway if it simplifies the code paths. It is also good practice to catch exceptions when it can be seen that an unexpected problem might arise, as any time file, network, or database operations are undertaken, for instance.

Another important case where exceptions are useful is when you need to pass detailed information up to the calling method so that it can react appropriately. Without the ability to raise an exception, you are limited to the standard C mechanism of returning a value that will hopefully be recognized as invalid, and perhaps using an errno-like strategy where the caller knows to examine the value of a certain global variable. This is inelegant, difficult to enforce, and leads to the need, with void methods, to document that "the caller should check errno to see if any problems arose".


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6.2 Logging

GNUstep provides several distinct logging facilities best suited for different purposes.


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6.2.1 NSLog

The simplest of these is the NSLog(NSString *format, ...) function. For example:

 
NSLog(@"Error occurred reading file at line %d.", lineNumber);

This would produce, on the console (stderr) of the application calling it, something like:

 
2004-05-08 22:46:14.294 SomeApp[15495] Error occurred reading file at line 20.

The behavior of this function may be controlled in two ways. First, the user default GSLogSyslog can be set to "YES", which will send these messages to the syslog on systems that support that (Unix variants). Second, the function GNUstep uses to write the log messages can be overridden, or the file descriptor the existing function writes to can be overridden:

 
  // these changes must be enclosed within a lock for thread safety
NSLock *logLock = GSLogLock();
[logLock lock];

  // to change the file descriptor:
_NSLogDescriptor = <fileDescriptor>;
  // to change the function itself:
_NSLog_printf_handler = <functionName>;

[logLock unlock];

Due to locking mechanisms used by the logging facility, you should protect these changes using the lock provided by GSLogLock() (see Threads and Run Control on locking).

The NSLog function was defined in OpenStep and is also available in Mac OS X Cocoa, although the overrides described above may not be. The next set of logging facilities to be described are only available under GNUstep.


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6.2.2 NSDebugLog, NSWarnLog

The facilities provided by the NSDebugLog and NSWarnLog families of functions support source code method name and line-number reporting and allow compile- and run-time control over logging level.

The NSDebugLog functions are enabled at compile time by default. To turn them off, set 'diagnose = no' in your makefile, or undefine GSDIAGNOSE in your code before including NSDebug.h. To turn them off at runtime, call [[NSProcessInfo processInfo] setDebugLoggingEnabled: NO]. (An NSProcessInfo instance is automatically instantiated in a running GNUstep application and may be obtained by invoking [NSProcessInfo processInfo].)

At runtime, whether or not logging is enabled, a debug log method is called like this:

 
NSDebugLLog(@"ParseError", @"Error parsing file at line %d.", lineNumber);

Here, the first argument to NSDebugLog, "ParseError", is a string key that specifies the category of message. The message will only actually be logged (through a call to NSLog()) if this key is in the set of active debug categories maintained by the NSProcessInfo object for the application. Normally, this list is empty. There are three ways for string keys to make it onto this list:

While any string can be used as a debug key, conventionally three types of keys are commonly used. The first type expresses a "level of importance" for the message, for example, "Debug", "Info", "Warn", or "Error". The second type of key that is used is class name. The GNUstep Base classes used this approach. For example if you want to activate debug messages for the NSBundle" class, simply add 'NSBundle' to the list of keys. The third category of key is the default key, 'dflt'. This key can be used whenever the specificity of the other key types is not required. Note that it still needs to be turned on like any other logging key before messasges will actually be logged.

There is a family of NSDebugLog functions with slightly differing behaviors:

NSDebugLLog(key, format, args,...)

Basic debug log function already discussed.

NSDebugLog(format, args,...)

Equivalent to NSDebugLLog with key "dflt" (for default).

NSDebugMLLog(level, format, args,...)

Equivalent to NSDebugLLog but includes information on which method the logging call was made from in the message.

NSDebugMLog(format, args,...)

Same, but use 'dflt' log key.

NSDebugFLLog(level, format, args,...)

As NSDebugMLLog but includes information on a function rather than a method.

NSDebugFLog(format, args,...)

As previous but using 'dflt' log key.

The implementations of the NSDebugLog functions are optimized so that they consume little time when logging is turned off. In particular, if debug logging is deactivated at compile time, there is NO performance cost, and if it is completely deactivated at runtime, each call entails only a boolean test. Thus, they can be left in production code.

There is also a family of NSWarn functions. They are similar to the NSDebug functions except that they do not take a key. Instead, warning messages are shown by default unless they are disabled at compile time by setting 'warn = no' or undefining GSWARN, or at runtime by adding "NoWarn" to [NSProcessInfo debugSet]. (Command-line argument --GNU-Debug=NoWarn and adding "NoWarn" to the GNU-Debug user default will also work.) NSWarnLog(), NSWarnLLog(), NSWarnMLLog, NSWarnMLog, NSWarnFLLog, and NSWarnFLog are all similar to their NSDebugLog counterparts.


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6.2.3 Last Resorts: GSPrintf and fprintf

Both the NSDebugLog and the simpler NSLog facilities utilize a fair amount of machinery - they provide locking and timestamping for example. Sometimes this is not appropriate, or might be too heavyweight in a case where you are logging an error which might involve the application being in some semi-undefined state with corrupted memory or worse. You can use the GSPrintf() function, which simply converts a format string to UTF-8 and writes it to a given file:

 
GSPrintf(stderr, "Error at line %d.", n);

If even this might be too much (it uses the NSString and NSData classes), you can always use the C function fprintf():

 
fprintf(stderr, "Error at line %d.", n);

Except under extreme circumstances, the preferred logging approach is either NSDebugLog/NSWarnLog, due the the compile- and run-time configurability they offer, or NSLog.


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6.2.4 Profiling Facilities

GNUstep supports optional programmatic access to object allocation statistics. To initiate collection of statistics, call the function GSDebugAllocationActive(BOOL active) with an argument of "YES". To turn it off, call it with "NO". The overhead of statistics collection is only incurred when it is active. To access the statistics, use the set of GSDebugAllocation...() functions defined in NSDebug.h.


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6.3 Assertions

Assertions provide a way for the developer to state that certain conditions must hold at a certain point in source code execution. If the conditions do not hold, an exception is automatically raised (and succeeding code in the block is not executed). This avoids an operation from taking place with illegal inputs that may lead to worse problems later.

The use of assertions is generally accepted to be an efficient means of improving code quality, for, like unit testing, they can help rapidly uncover a developer's implicit or mistaken assumptions about program behavior. However this is only true to the extent that you carefully design the nature and placement of your assertions. There is an excellent discussion of this issue bundled in the documentation with Sun's Java distribution.


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6.3.1 Assertions and their Handling

Assertions allow the developer to establish that certain conditions hold before undertaking an operation. In GNUstep, the standard means to make an assertion is to use one of a collection of NSAssert macros. The general form of these macros is:

 
NSAssert(<boolean test>, <formatString>, <argumentsToFormat>);

For instance:

 
NSAssert1(x == 10, "X should have been 10, but it was %d.", x);

If the test 'x == 10' evaluates to true, NSLog() is called with information on the method and line number of the failure, together with the format string and argument. The resulting console message will look like this:

 
Foo.m:126  Assertion failed in Foo(instance), method Bar.  X should have been
10, but it was 5.

After this is logged, an exception is raised of type 'NSInternalInconsistencyException', with this string as its description.

In order to provide the method and line number information, the NSAssert() routine must be implemented as a macro, and therefore to handle different numbers of arguments to the format string, there are 5 assertion macros for methods: NSAssert(condition, description), NSAssert1(condition, format, arg1), NSAssert2(condition, format, arg1, arg2), ..., NSAssert5(...).

If you need to make an assertion inside a regular C function (not an Objective-C method), use the equivalent macros NSCAssert(), etc..

Note, you can completely disable assertions (saving the time for the boolean test and avoiding the exception if fails) by putting #define NS_BLOCK_ASSERTIONS before you include NSException.h.


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6.3.2 Custom Assertion Handling

The aforementioned behavior of logging an assertion failure and raising an exception can be overridden if desired. You need to create a subclass of NSAssertionHandler and register an instance in each thread in which you wish the handler to be used. This is done by calling:

 
[[[NSThread currentThread] threadDictionary]
    setObject:myAssertionHandlerInstance forKey:N"SAssertionHandler"];

See Threads and Run Control for more information on what this is doing.


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6.4 Comparison with Java

GNUstep's exception handling facilities are, modulo syntax, equivalent to those in Java in all but three respects:

The logging facilities provided by NSDebugLog and company are similar to but a bit more flexible than those provided in the Java/JDK 1.4 logging APIs, which were based on the IBM/Apache Log4J project.

The assertion facilities are similar to but a bit more flexible than those in Java/JDK 1.4 since you can override the assertion handler.


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