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NSExceptionHandler Class Reference

Overview

The NSExceptionHandler class provides facilities for monitoring and debugging exceptional conditions in Objective-C programs. It works by installing a special uncaught exception handler via the NSSetUncaughtExceptionHandler function. Consequently, to use the services of NSExceptionHandler, you must not install your own custom uncaught exception handler.

To use these services, you set a bit mask in the singleton NSExceptionHandler instance and, optionally, a delegate. The constants comprising the bit mask indicate the type of exception to be monitored and the behavior of the NSExceptionHandler object (or, simply, the exception handler). The delegate is asked to approve the logging and handling of each monitored NSException object.

The constants for configuring exception handler behavior can be categorized in several ways:

• Uncaught exceptions versus caught exceptions—or, more accurately, exceptions that would be caught (for example, by the top-level handler)

• Exception type or cause: system exceptions (such as invalid memory accesses), Objective-C runtime errors (such as messages sent to freed objects), and other exceptions

• Exception handler behavior: logging the exception (including a stack trace) to the console, handling the exception, and suspending program execution so the debugger can be attached

The way the exception handler handles an exception depends on the type of exception; the exception handler converts system exceptions and runtime errors into NSException objects with a stack trace embedded in their userInfo dictionary; for all other uncaught exceptions, it terminates the thread on which they occur . The constants used to configure an NSExceptionHandler object are described in Logging and Handling Constants and System Hang Constants.

The defaults command-line system also allows you to set values corresponding to the enum constants used to configure the exception handler; see “Controlling Application Response to Exceptions" for details.

Tasks

Getting the Default Exception Handler

• + defaultExceptionHandler

Getting and Setting Exception Masks

• – exceptionHandlingMask
• – exceptionHangingMask
• – setExceptionHandlingMask:
• – setExceptionHangingMask:

Getting and Setting the Delegate

• – delegate
• – setDelegate:

Logging and handling exceptions

• – exceptionHandler:shouldHandleException:mask:  delegate method
• – exceptionHandler:shouldLogException:mask:  delegate method

Class Methods

defaultExceptionHandler

Returns the singleton NSExceptionHandler instance.

+ (NSExceptionHandler *)defaultExceptionHandler

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

Instance Methods

delegate

Returns the delegate of the NSExceptionHandler object.

- (id)delegate

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

exceptionHandlingMask

Returns a bit mask representing the types of exceptions monitored by the receiver and its handling and logging behavior.

- (unsigned int)exceptionHandlingMask

Return Value

A bit mask composed of one or more constants specifying the types of exceptions monitored and whether they are handled or logged (or both). See Logging and Handling Constants for information about the constants.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

exceptionHangingMask

Returns a bit mask representing the types of exceptions that will halt execution for debugging.

- (unsigned int)exceptionHangingMask

Return Value

A bit mask composed of one or more constants specifying the types of exceptions that will halt execution for debugging. See System Hang Constants for information about the constants.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

setDelegate:

Sets the delegate of the NSExceptionHandler object.

- (void)setDelegate:(id)anObject

Parameters

anObject

The object to receive the delegation messages described in “Logging and handling exceptions”

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

setExceptionHandlingMask:

Sets the bit mask of constants specifying the types of exceptions monitored by the receiver and its handling and logging behavior.

- (void)setExceptionHandlingMask:(unsigned int)aMask

Parameters

aMask

A bit mask composed of one or more constants specifying the types of exceptions monitored and whether they are handled or logged (or both). You specify multiple constants by performing a bitwise-OR operation. See Logging and Handling Constants for information about the constants.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

setExceptionHangingMask:

Sets the bit mask of constants specifying the types of exceptions that will halt execution for debugging.

- (void)setExceptionHangingMask:(unsigned int)aMask

Parameters

aMask

A bit mask composed of one or more constants specifying the types of exceptions that will halt execution for debugging. You specify multiple constants by performing a bitwise-OR operation. See System Hang Constants for information about the constants.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

Delegate Methods

exceptionHandler:shouldHandleException:mask:

Implemented by the delegate to evaluate whether the delegating NSExceptionHandler instance should handle a given exception.

- (BOOL)exceptionHandler:(NSExceptionHandler *)sender shouldHandleException:(NSException *)exception mask:(unsigned int)aMask

Parameters

sender

The NSExceptionHandler object sending the message.

exception

An NSException object describing the exception to be evaluated.

aMask

The bit mask indicating the types of exceptions handled by the NSExceptionHandler object. See Logging and Handling Constants and System Hang Constants for descriptions of the possible enum constants.

Return Value

YES to have the NSExceptionHandler object handle the exception, NO otherwise.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

exceptionHandler:shouldLogException:mask:

Implemented by the delegate to evaluate whether the delegating NSExceptionHandler instance should log a given exception.

- (BOOL)exceptionHandler:(NSExceptionHandler *)sender shouldLogException:(NSException *)exception mask:(unsigned int)aMask

Parameters

sender

The NSExceptionHandler object sending the message.

exception

An NSException object describing the exception to be evaluated.

aMask

The bit mask indicating the types of exceptions logged by the NSExceptionHandler object. See Logging and Handling Constants and System Hang Constants for descriptions of the possible enum constants.

Return Value

YES to have the NSExceptionHandler object log the exception, NO otherwise.

Availability

• Available in Mac OS X v10.0 and later.

Declared In

NSExceptionHandler.h

Constants

Logging and Handling Constants

Use one or more of the following constants in the parameter of setExceptionHandlingMask: to specify the types of exceptions that the exception handler should monitor and whether it should handle or log them.

enum {
   NSLogUncaughtExceptionMask            = 1 << 0,
   NSHandleUncaughtExceptionMask         = 1 << 1,
   NSLogUncaughtSystemExceptionMask      = 1 << 2,
   NSHandleUncaughtSystemExceptionMask   = 1 << 3,
   NSLogUncaughtRuntimeErrorMask         = 1 << 4,
   NSHandleUncaughtRuntimeErrorMask      = 1 << 5,
   NSLogTopLevelExceptionMask            = 1 << 6,
   NSHandleTopLevelExceptionMask         = 1 << 7,
   NSLogOtherExceptionMask               = 1 << 8,
   NSHandleOtherExceptionMask            = 1 << 9
};

Constants

NSLogUncaughtExceptionMask

The exception handler logs uncaught exceptions.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHandleUncaughtExceptionMask

The exception handler handles uncaught exceptions by terminating the thread in which they occur.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSLogUncaughtSystemExceptionMask

The exception handler logs uncaught system exceptions.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHandleUncaughtSystemExceptionMask

The exception handler handles uncaught system exceptions by converting them to NSException objects containing a stack trace.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSLogUncaughtRuntimeErrorMask

The exception handler logs uncaught runtime errors.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHandleUncaughtRuntimeErrorMask

The exception handler handles uncaught runtime errors by converting them to NSException objects containing a stack trace.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSLogTopLevelExceptionMask

The exception handler logs exceptions that would be caught by the top-level handler.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHandleTopLevelExceptionMask

The exception handler handles exceptions caught by the top-level handler by converting them to NSException objects containing a stack trace.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSLogOtherExceptionMask

The exception handler logs exceptions caught by handlers lower than the top-level handler.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHandleOtherExceptionMask

The exception handler handles exceptions caught by handlers lower than the top-level handler by converting them to NSException objects containing a stack trace.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

Discussion

When exception-handling domains are nested, NSLogTopLevelExceptionMask and NSHandleTopLevelExceptionMask deal with exceptions that would make it to the top two levels of exception handlers. In the main thread of a Cocoa application, the top-level handler is the global NSApplication instance.

Declared In

ExceptionHandling/ExceptionHandler.h

System Hang Constants

Use one or more of the following constants in the parameter of setExceptionHangingMask: to specify the types of exceptions that cause the exception to halt execution so a debugger can be attached.

enum {
   NSHangOnUncaughtExceptionMask        = 1 << 0,
   NSHangOnUncaughtSystemExceptionMask        = 1 << 1,
   NSHangOnUncaughtRuntimeErrorMask        = 1 << 2,
   NSHangOnTopLevelExceptionMask        = 1 << 3,
   NSHangOnOtherExceptionMask            = 1 << 4
};

Constants

NSHangOnUncaughtExceptionMask

The exception handler suspends execution when it detects an uncaught exception (other than a system exception or runtime error).

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHangOnUncaughtSystemExceptionMask

The exception handler suspends execution when it detects an uncaught system exception.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHangOnUncaughtRuntimeErrorMask

The exception handler suspends execution when it detects an uncaught runtime error.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHangOnTopLevelExceptionMask

The exception handler suspends execution when it detects an exception that would be handled by the top-level handler.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSHangOnOtherExceptionMask

The exception handler suspends execution when it detects an exception that would be handled by an object other than the top-level handler.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

Discussion

When exception-handling domains are nested, NSHangOnTopLevelExceptionMask deals with exceptions that would make it to the top two levels of exception handlers. In the main thread of a Cocoa application, the top-level handler is the global NSApplication instance.

Declared In

ExceptionHandling/ExceptionHandler.h

Mask Definitions

The following #define constants are conveniences for specifying complete sets of exception-handling enum constants.

NSHangOnEveryExceptionMask NSLogAndHandleEveryExceptionMask

Constants

NSHangOnEveryExceptionMask

Combines via bitwise-OR all the constants listed in System Hang Constants.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSLogAndHandleEveryExceptionMask

Combines via bitwise-OR all the constants listed in Logging and Handling Constants.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

Declared In

ExceptionHandling/ExceptionHandler.h

Exception Global String Constants

Two of the following global string constants identify exceptions generated by the framework for Objective-C runtime errors and system exceptions such as invalid memory accesses. The other constant is used as a key to access the stack trace in the userInfo dictionary of an NSException object, when requested.

EXCEPTIONHANDLING_EXPORT NSString *NSUncaughtSystemExceptionException;
EXCEPTIONHANDLING_EXPORT NSString *NSUncaughtRuntimeErrorException;
EXCEPTIONHANDLING_EXPORT NSString *NSStackTraceKey;

Constants

NSUncaughtSystemExceptionException

Identifies an uncaught system exception.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSUncaughtRuntimeErrorException

Identifies an Objective-C runtime error.

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

NSStackTraceKey

The key for fetching the stack trace (an NSString object) in the userInfo dictionary of the NSException object passed into one of the delegate methods described in “Logging and handling exceptions.”

Available in Mac OS X v10.0 and later.

Declared in NSExceptionHandler.h.

Declared In

ExceptionHandling/ExceptionHandler.h

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