Windows System Programming Third Edition

ERRORS AND EXCEPTIONS

99

This example enables all possible floating-point exceptions except for the

floating-point stack overflow,

. Alternativ ely,

enable specific exceptions by using only selected exception masks, such as

. Program 4–3 uses similar code in the context of a larger example.

Errors and Exceptions

An error can be thought of as a situation that could occur occasionally in known

locations. System call errors, for example, should be detected and reported imme-

diately by logic in the code. Thus, programmers normally include an explicit test

to see, for i nstance, whether a file read operation ha

s fai led. The

function was developed in Chapter 2 to diagnose and respond to errors.

An exception, on the other hand, could occur nearly anywhere, and it is not

possible or practical to test for an exception. Division by zero and memory access

violations are examples.

Nonetheless, the distinction is sometimes blurre

d. Windows will, optionally,

generate exceptions during memory allocation using the

and

functions if memory is insufficient. This is described in Chapter 5 .

Programs can also raise their own exceptions with programmer-defined exception

cod es using the

function, as described next.

Exception handlers provide a convenient mechanism for exiting from inner

blocks or functions under program control without resorting to a

or

100

CHAPTER 4 EXCEPTION HANDLING

to transfer control. This capability is particularly important if the block has

accessed resources, such as open files, memory, or synchronization objects,

because the handler can release them. It is also possible to continue program

execution after the exception handler, rather than terminate the program.

Addit

ionally, a program can restore system state, such as the floating-point mask,

on exiting from a block. Many examples use handlers in this way.

User-Generated Exceptions

It is possible to raise an exception at any point during program execution using

the

function. In this way, your program can detect an error and

treat it as an exception.

Parameters

is the user-defined code. Do not use bit 28, which is reserved

for the system. The error code is encoded in bits 27–0 (all except the most signifi-

cant hex digit). Bit 29 should be set to indicate a “customer” (not Microsoft) excep-

tion. Bits 31–30 encode the severity as follows, where the resulting lead exception

code hex

digit is shown with bit 29 set.

• 0—Success (lead exception code hex digit is 2).

•1—Informational (lead exception code hex digit is 6).

•2—Warning (lead exception code hex digit is A).

• 3—Error (lead exception code hex digit is E).

is normally set to , but setting the value to

indicates that the filter expression should not

generate

; doing so will cause an immediate

exception.

, if not , points to an array of size (the third

parameter) containing 32-bit values to be passed to the filter expression. There is