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This chapter is from the book

As our previous examples have shown, the default logging sink for ACE's logging facility is the standard error stream. In this section, we discuss output to the standard error stream, as well as two other common and useful targets:

  • The system logger (UNIX syslog or NT Event Log)

  • A programmer-specified output stream, such as a file

3.4.1 Standard Error Stream

Output to the standard error stream (STDERR) is so common that it is, in fact, the default sink for all ACE logging messages. Our examples so far have taken advantage of this. Sometimes, you may want to direct your output not only to STDERR but also to one of the other targets available to you. In these cases, you will have to explicitly include STDERR in your choices:

int ACE_TMAIN (int, ACE_TCHAR *argv[])
{
  // open() requires the name of the application
  // (e.g. -- argv[0]) because the underlying
  // implementation may use it in the log output.
  ACE_LOG_MSG->open (argv[0], ACE_Log_Msg::STDERR);

or

ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("%IHi Mom\n")));
ACE_LOG_MSG->set_flags (ACE_Log_Msg::STDERR);
foo ();

If you choose the second approach, it may be necessary to invoke clr_flags() to disable any other output destinations. Everything after the set_flags() will be directed to STDERR until you invoke clr_flags() to prevent it. The complete signatures of these methods are:

// Enable the bits in the logger's options flags.
void set_flags (unsigned long f);

// Disable the bits in the logger's options flags.
void clr_flags (unsigned long f);

The set of defined flag values are listed in Table 3.5.

Table 3.5. Valid ACE_Log_Msg Flags Values

Flag

Meaning

STDERR

Write messages to STDERR

LOGGER

Write messages to the local client logger daemon (see Section 3.6)

OSTREAM

Write messages to the assigned output stream

MSG_CALLBACK

Write messages to the callback object (see Section 3.5)

VERBOSE

Prepends program name, timestamp, host name, process ID, and message priority to each message

VERBOSE_LITE

Prepends timestamp and message priority to each message

SILENT

Do not print messages at all

SYSLOG

Write messages to the system's event log

CUSTOM

Write messages to the user-provided back end: an advanced usage topic not discussed in this book

3.4.2 System Logger

Most modern operating systems support the notion of a system logger. The implementation details range from a library of function calls to a network daemon. The general idea is that all applications direct their logging activity to the system logger, which will, in turn, direct it to the correct file(s) or other configurable destination(s). For example, UNIX system administrators can configure the UNIX syslog facility so that different classes and levels of logging get directed to different destinations. Such an approach provides a good combination of scalability and configurability.

To use the system logger, you would do something like this:

int ACE_TMAIN (int, ACE_TCHAR *argv[])
{
  ACE_LOG_MSG->open
    (argv[0], ACE_Log_Msg::SYSLOG, ACE_TEXT ("syslogTest"));

Although one would think that we could use the set_flags() method to enable syslog output after the ACE_Log_Msg instance has been opened, that isn't the case, unfortunately. Likewise, if you want to quit sending output to syslog, a simple clr_flags() won't do the trick.

In order to communicate with the system logger, ACE_Log_Msg must perform a set of initialization procedures that are done only in the open() method. Part of the initialization requires the program name that will be recorded in syslog: (the third argument). If we don't do this when our program starts, we will have to do it later, in order to get the behavior we expect from invoking set_flags(). Similarly, the open() method will properly close down any existing connection to the system logger if invoked without the ACE_Log_Msg::SYSLOG flag:

#include "ace/Log_Msg.h"

void foo (void);

int ACE_TMAIN (int, ACE_TCHAR *argv[])
{
  // This will be directed to stderr (the default ACE_Log_Msg
  // behavior).
  ACE_TRACE (ACE_TEXT ("main"));

  ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("%IHi Mom\n")));
  // Everything from foo() will be directed to the system logger
  ACE_LOG_MSG->open
    (argv[0], ACE_Log_Msg::SYSLOG, ACE_TEXT ("syslogTest"));
  foo ();

  // Now we reset the log output to default (stderr)
  ACE_LOG_MSG->open (argv[0]);
  ACE_DEBUG ((LM_INFO, ACE_TEXT ("%IGoodnight\n")));

  return 0;
}

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

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

Although it may seem strange to invoke ACE_LOG_MSG->open() more than once in your application, nothing is wrong with it. Think of it as more of a reopen. Before we end this chapter, we will create a simple LogManager class to help hide some of these kinds of details.

Directing logging output to SYSLOG means different things on different platforms, according to what the platform's native "system logger" is and what it is capable of. If the runtime platform doesn't support any type of system logger, directing output to SYSLOG has no effect. The following platforms have SYSLOG support in ACE:

  • Windows NT 4 and newer, such as Windows 2000 and XP: ACE directs SYSLOG output to the system's Event Log. The third argument to ACE_Log_Msg::open() is an ACE_TCHAR* character string. It is optional; if supplied, it replaces the program name as the event source name for recording events in the system's event log. The ACE message severities are mapped to Event Log severities, as shown in Table 3.6.

  • UNIX/Linux: ACE directs SYSLOG output to the syslog facility. The syslog facility has its own associated configuration details about logging facilities, which are different from ACE's logging severity levels. ACE's syslog back end specifies the LOG_USER syslog facility by default. This value can be changed at compile time by changing the config.h setting ACE_DEFAULT_SYSLOG_FACILITY. Please consult the syslog man page for details on how to configure the logging destination for the specified facility.

Table 3.6. Mapping ACE Logging Severity to Windows Event Log Severity

ACE Severity

Event Log Severity

LM_STARTUP
LM_SHUTDOWN
LM_TRACE
LM_DEBUG
LM_INFO

EVENTLOG_INFORMATION_TYPE

LM_NOTICE
LM_WARNING

EVENTLOG_WARNING_TYPE

LM_ERROR
LM_CRITICAL
LM_ALERT
LM_EMERGENCY

EVENTLOG_ERROR_TYPE

3.4.3 Output Streams

The preferred way to handle output to files and other targets in C++ is output streams (C++ ostream objects). They provide enhanced functionality over the printf() family of functions and usually result in more readable code. The ACE_Log_Msg::msg_ostream() method lets us provide an output stream on which the logger will write our information:

ACE_OSTREAM_TYPE *output =
  new std::ofstream ("ostream.output.test");
ACE_LOG_MSG->msg_ostream (output, 1);
ACE_LOG_MSG->set_flags (ACE_Log_Msg::OSTREAM);
ACE_LOG_MSG->clr_flags (ACE_Log_Msg::STDERR);

Note that it's perfectly safe to select OSTREAM as output—via either open() or set_flags()—and then generate logging output before you invoke msg_ostream(). If you do so, the output will simply disappear, because no ostream is assigned. Also note that we have used the two-argument version of msg_ostream(). This not only sets the ostream for the ACE_Log_Msg instance to use but also tells ACE_Log_Msg that it should assume ownership and delete the ostream instance when the ACE_Log_Msg object is deleted. The single-argument version of msg_ostream() doesn't specify its default behavior with regard to ownership, so it pays to be explicit in your wishes.

You may wonder why the stream type is ACE_OSTREAM_TYPE instead of simply std::ostream. This is another aspect of ACE that helps its portability to platforms of all sizes and capabilities. ACE_OSTREAM_TYPE can be defined with or without the std namespace declaration, and it can also be defined as FILE for platforms without any C++ iostream support at all, such as some embedded environments.

3.4.4 Combined Techniques

We can now easily combine all these techniques and distribute our logging information among all three choices:

#include "ace/Log_Msg.h"
#include "ace/streams.h"

int ACE_TMAIN (int, ACE_TCHAR *argv[])
{
  // Output to default destination (stderr)
  ACE_LOG_MSG->open (argv[0]);

  ACE_TRACE (ACE_TEXT ("main"));

  ACE_OSTREAM_TYPE *output =
        new std::ofstream ("ostream.output.test");

  ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("%IThis will go to STDERR\n")));

  ACE_LOG_MSG->open
    (argv[0], ACE_Log_Msg::SYSLOG, ACE_TEXT ("syslogTest"));
  ACE_LOG_MSG->set_flags (ACE_Log_Msg::STDERR);
  ACE_DEBUG
    ((LM_DEBUG, ACE_TEXT ("%IThis goes to STDERR & syslog\n")));

  ACE_LOG_MSG->msg_ostream (output, 0);
  ACE_LOG_MSG->set_flags (ACE_Log_Msg::OSTREAM);
  ACE_DEBUG ((LM_DEBUG,
              ACE_TEXT ("%IThis will go to STDERR, ")
              ACE_TEXT ("syslog & an ostream\n")));

  ACE_LOG_MSG->clr_flags (ACE_Log_Msg::OSTREAM);
  delete output;

  return 0;
}

Beware of a subtle bug waiting to get you when you use an ostream. Note that before we deleted the ostream instance output, we first cleared the OSTREAM flag on the ACE_Log_Msg instance. Remember that the ACE_TRACE for main still has to write its final message when the trace instance goes out of scope at the end of main(). If we delete the ostream without removing the OSTREAM flag, ACE_Log_Msg will dutifully attempt to write that final message on a deleted ostream instance, and your program will most likely crash.

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