To aid in debugging asynchronous programs, Boost.Asio provides support
for handler tracking. When enabled by defining
Boost.Asio writes debugging output to the standard error stream. The output
records asynchronous operations and the relationships between their handlers.
This feature is useful when debugging and you need to know how your asynchronous operations are chained together, or what the pending asynchronous operations are. As an illustration, here is the output when you run the HTTP Server example, handle a single request, then shut down via Ctrl+C:
@email@example.com_wait @firstname.lastname@example.org_accept @email@example.com @asio|1298160118.075438|>2|ec=asio.system:0 @firstname.lastname@example.org_receive @email@example.com_accept @asio|1298160118.075527|<2| @asio|1298160118.075540|>3|ec=asio.system:0,bytes_transferred=122 @firstname.lastname@example.org_send @asio|1298160118.075778|<3| @asio|1298160118.075793|>5|ec=asio.system:0,bytes_transferred=156 @email@example.com @asio|1298160118.075855|<5| @asio|1298160122.827317|>1|ec=asio.system:0,signal_number=2 @firstname.lastname@example.org @asio|1298160122.827359|<1| @asio|1298160122.827370|>4|ec=asio.system:125 @asio|1298160122.827378|<4| @email@example.com
Each line is of the form:
<tag> is always
@asio, and is used
to identify and extract the handler tracking messages from the program
<timestamp> is seconds and microseconds from 1 Jan
<action> takes one of the following forms:
The program entered the handler number
shows the arguments to the handler.
The program left handler number
The program left handler number n due to an exception.
The handler number
n was destroyed without having been
invoked. This is usually the case for any unfinished asynchronous
operations when the
io_service is destroyed.
The handler number
n created a new asynchronous operation
with completion handler number
shows what asynchronous operation was started.
The handler number n performed some other operation. The
shows what function was called. Currently only
cancel() operations are logged, as these may affect
the state of pending asynchronous operations.
<description> shows a synchronous or asynchronous
operation, the format is
For handler entry, it shows a comma-separated list of arguments and their
As shown above, Each handler is assigned a numeric identifier. Where the handler tracking output shows a handler number of 0, it means that the action was performed outside of any handler.
The handler tracking output may be post-processed using the included
tool to create a visual representation of the handlers (requires the GraphViz