Windows System Programming Third Edition [Electronic resources] نسخه متنی

A CRITICAL_SECTION for Protecting Shared Variables

Using CRITICAL_SECTIONs is simple, and one common use is to allow threads to access global shared variables. For example, consider a threaded server (as in Figure 7-1) in which there might be a need to maintain usage statistics such as:

• The total number of requests received

• The total number of responses sent

• The number of requests currently being processed by server threads

Because the count variables are global to the process, two threads must not modify the counts simultaneously. CRITICAL_SECTION objects provide one means of ensuring this, as shown by the code sequence below and in Program 8-1, much simpler than the server system, illustrates this CRITICAL_SECTION usage.

CSs can be used to solve problems such as the one shown in Figure 8-1, in which two threads increment the same variable. The following code segment will do more than increment the variable because simple incrementing is possible with the interlocked functions. Notice the use of volatile so that an optimizing compiler will not leave the current variable value in a register rather than store it back into the variable. This example also uses an intermediate variable; this unnecessary inefficiency more clearly illustrates how the problem in Figure 8-1 is solved.

CRITICAL_SECTION cs1;
volatile DWORD N = 0, M;
/* N is a global variable, shared by all threads. */
InitializeCriticalSection (&cs1);
...
EnterCriticalSection (&cs1);
if (N < N_MAX) { M = N; M += 1; N = M; }
LeaveCriticalSection (&cs1);
...
DeleteCriticalSection (&cs1);

Figure 8-1 example and illustrates how CSs can solve synchronization problems.