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This chapter is from the book

Consider this slightly modified code:

#include "ace/Log_Msg.h"

void foo(void);

int ACE_TMAIN (int, ACE_TCHAR *[])
{
  ACE_TRACE (ACE_TEXT ("main"));

  ACE_LOG_MSG->priority_mask (LM_DEBUG | LM_NOTICE,
                              ACE_Log_Msg::PROCESS);
  ACE_DEBUG ((LM_INFO, ACE_TEXT ("%IHi Mom\n")));
  foo ();
  ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("%IGoodnight\n")));

  return 0;
}

void foo(void)
{
  ACE_TRACE (ACE_TEXT ("foo"));

  ACE_DEBUG ((LM_NOTICE, ACE_TEXT ("%IHowdy Pardner\n")));
}

The following output is produced:

(1024) calling main in file `Simple2.cpp' on line 7
      Howdy Pardner
   Goodnight

In this example, we changed the logging level at runtime so that only messages logged with LM_DEBUG and LM_NOTICE priority are displayed; all others are ignored. The LM_INFO "Hi Mom" message is not displayed, and there is no ACE_TRACE output.

We've also revealed a little more about how ACE's logging facility works. The ACE_Log_Msg class implements the log message formatting capabilities in ACE. ACE automatically maintains a thread-specific singleton instance of the ACE_Log_Msg class for each spawned thread, as well as the main thread. ACE_LOG_MSG is a shortcut for obtaining the pointer to the thread's singleton instance. All the ACE logging macros use ACE_LOG_MSG to make method calls on the correct ACE_Log_Msg instance. There is seldom a reason to instantiate an ACE_Log_Msg object directly. ACE automatically creates a new instance for each thread spawned, keeping each thread's logging output separate.

Table 3.3. ACE Logging Macros

Macro

Function

Disabled by

ACE_ASSERT(test)

Much like the assert() library call. If the test fails, an assertion message including the file name and line number, along with the test itself, will be printed and the application aborted.

ACE_NDEBUG

ACE_HEX_DUMP
((level, buffer, size [,text]))

Dumps the buffer as a string of hex digits. If provided, the optional text parameter will be printed prior to the hex string. The op_statusa is set to 0.

ACE_NLOGGING

ACE_RETURN(value)

No message is printed, the calling function returns with value; op_status is set to value.

ACE_NLOGGING

ACE_ERROR_RETURN
((level, string, ...), value)

Logs the string at the requested level. The calling function then returns with value; op_status is set to value.

ACE_NLOGGING

ACE_ERROR((level, string, ...))

Sets the op_status to –1 and logs the string at the requested level.

ACE_NLOGGING

ACE_DEBUG((level, string, ...))

Sets the op_status to 0 and logs the string at the requested level.

ACE_NLOGGING

ACE_ERROR_INIT( value, flags )

Sets the op_status to value and the logger's option flags to flags. Valid flags values are defined in Table 3.5.

ACE_NLOGGING

ACE_ERROR_BREAK
((level, string, ...))

Invokes ACE_ERROR() followed by a break. Use this to display an error message and exit a while or for loop, for instance.

ACE_NLOGGING

ACE_TRACE(string)

Displays the file name, line number, and string where ACE_TRACE appears. Displays "Leaving 'string'" when the ACE_TRACE-enclosing scope exits.

ACE_NTRACE

We can use the ACE_Log_Msg::priority_mask() method to set the logging severity levels we desire output to be produced for: All the available logging levels are listed in Table 3.1. Each level is represented by a mask, so the levels can be combined. Let's look at the complete signature of the priority_mask() methods:

/// Get the current ACE_Log_Priority mask.
u_long priority_mask (MASK_TYPE = THREAD);

/// Set the ACE_Log_Priority mask, returns original mask.
u_long priority_mask (u_long, MASK_TYPE = THREAD);

The first version is used to read the severity mask; the second changes it and returns the original mask so it can be restored later. The second argument must be one of two values, reflecting two different scopes of severity mask setting:

  1. ACE_Log_Msg::PROCESS: Specifying PROCESS retrieves or sets the processwide mask affecting logging severity for all ACE_Log_Msg instances.

  2. ACE_Log_Msg::THREAD: Each ACE_Log_Msg instance also has its own severity mask, and this value retrieves or sets it. THREAD is technically a misnomer, as it refers to the ACE_Log_Msg instance the method is invoked on, and you can create ACE_Log_Msg instances in addition to those that ACE creates for each thread. However, that is a relatively rare thing to do, so we usually simply refer to ACE_Log_Msg instances as thread specific.

When evaluating a log message's severity, ACE_Log_Msg examines both the processwide and per instance severity masks. If either of them has the message's severity enabled, the message is logged. By default, all bits are set at the process level and none at the instance level, so all message severities are logged. To make each thread decide for itself which severity levels will be logged, set the processwide mask to 0 and allow each thread set its own per instance mask. For example, the following code disables all logging severities at the process level and enables LM_DEBUG and LM_NOTICE severities in the current thread only:

ACE_LOG_MSG->priority_mask (0, ACE_Log_Msg::PROCESS);
ACE_LOG_MSG->priority_mask (LM_DEBUG | LM_NOTICE,
                            ACE_Log_Msg::THREAD);

A third mask maintained by ACE_Log_Msg is important when you start setting individual severity masks on ACE_Log_Msg instances. The per instance default mask is used to initialize each ACE_Log_Msg instance's severity mask. The per instance default mask is initially 0 (no severities are enabled). Because each ACE_Log_Msg instance's severity mask is set from the default value when the instance is created, you can change the default for groups of threads before spawning them. This puts the logging policy into the thread-spawning part of your application, alleviating the need for the threads to set their own level, although each thread can change its ACE_Log_Msg instance's mask at any time. Consider this example:

ACE_LOG_MSG->priority_mask (0, ACE_Log_Msg::PROCESS);
ACE_Log_Msg::enable_debug_messages ();
ACE_Thread_Manager::instance ()->spawn (service);
ACE_Log_Msg::disable_debug_messages ();
ACE_Thread_Manager::instance ()->spawn_n (3, worker);

We'll learn about thread management in Chapter 13. For now, all you need to know is that ACE_Thread_Manager::spawn() spawns one thread and that ACE_Thread_Manager::spawn_n() spawns multiple threads. In the preceding example, the processwide severity mask is set to 0 (all disabled). This means that each ACE_Log_Msg instance's mask controls its enabled severities totally. The thread executing the service() function will have the LM_DEBUG severity enabled, but the threads executing the worker() function will not.

The complete method signatures for changing the per instance default mask are:

static void disable_debug_messages
    (ACE_Log_Priority priority = LM_DEBUG);

static void enable_debug_messages
    (ACE_Log_Priority priority = LM_DEBUG);

Our example used the default argument, LM_DEBUG, in both cases. Even though the method names imply that LM_DEBUG is the only severity that can be changed, you can also supply any set of legal severity masks to either method. Unlike the priority_mask() method, which replaces the specified mask, the enable_debug_messages() and disable_debug_messages() methods add and subtract, respectively, the specified severity bits in both the calling thread's per instance mask and the per instance default severity mask.

Of course, you can use any message severity level at any time. However, take care to specify a reasonable level for each of your messages; then at runtime, you can use the priority_mask() method to enable or disable messages you're interested in. This allows you to easily overinstrument your code and then enable only the things that are useful at any particular time.

ACE_Log_Msg has a rich set of methods for recording the current state of your application. Table 3.4 summarizes the more commonly used functions. Most methods have both accessor and mutator signatures. For example, there are two op_status() methods:

int op_status(void);
void op_status(int status));

Although the method calls are most often made indirectly via the ACE logging macros, they are also available for direct use.

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