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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<html>
<head>
<title>API Tracing</title>
</head>
<body>
<h1>API Tracing</h1>
<h2>Introduction</h2>
<p>The HDF5 library is now able to trace API calls by printing the
function name, the argument names and their values, and the
return value. Some people like to see lots of output during
program execution instead of using a good symbolic debugger, and
this feature is intended for their consumption. For example,
the output from <code>h5ls foo</code> after turning on tracing,
includes:
<p>
<center>
<table border align=center width="100%">
<caption align=top><b>Sample Output</b></caption>
<tr>
<td>
<code><pre>
H5Tcopy(type=184549388) = 184549419 (type);
H5Tcopy(type=184549392) = 184549424 (type);
H5Tlock(type=184549424) = SUCCEED;
H5Tcopy(type=184549393) = 184549425 (type);
H5Tlock(type=184549425) = SUCCEED;
H5Fopen(filename="foo", flags=0, access=H5P_DEFAULT) = FAIL;
HDF5-DIAG: Error detected in thread 0. Back trace follows.
#000: H5F.c line 1245 in H5Fopen(): unable to open file
major(04): File interface
minor(10): Unable to open file
#001: H5F.c line 846 in H5F_open(): file does not exist
major(04): File interface
minor(10): Unable to open file
</pre></code>
</td>
</tr>
</table>
</center>
<h2>Configuation</h2>
<p>This all happens with some magic in the configuration script,
the makefiles, and macros. First, from the end-user point of
view, the library must be configured with the
<code>--enable-trace</code> switch (the default;
`--disable-trace' is the alternative). This causes the library
to include the support necessary for API tracing.
<p>
<center>
<table border align=center width="100%">
<caption align=top><b>Configuration</b></caption>
<tr>
<td>
<code><pre>
$ make distclean
$ sh configure --enable-trace
$ make
</pre></code>
</td>
</tr>
</table>
</center>
<h2>Execution</h2>
<p>In order to actually get tracing output one must turn tracing
on and specify a file descriptor where the tracing output should
be written. This is done by assigning a file descriptor number
to the <code>HDF5_TRACE</code> environment variable.
<p>
<center>
<table border align=center width="100%">
<caption align=top><b>Execution Examples</b></caption>
<tr>
<td>To display the trace on the standard error stream:
<code><pre>
$ export HDF5_TRACE=2
$ a.out
</pre></code>
</td>
</tr>
<tr>
<td>To send the trace to a file:
<code><pre>
$ export HDF5_TRACE=255
$ a.out 255>trace-output
</pre></code>
</td>
</tr>
</table>
</center>
<h2>Performance</h2>
<p>If the library was not configured for tracing then there is no
unnecessary overhead since all tracing code is
excluded.
<p>However, if tracing is enabled but not used there is a
small penalty. First, code size is larger because of extra
statically-declared character strings used to store argument
types and names and extra auto variable pointer in each
function. Also, execution is slower because each function sets
and tests a local variable and each API function calls the
<code>H5_trace()</code> function.
<p>If tracing is enabled and turned on then the penalties from the
previous paragraph apply plus the time required to format each
line of tracing information. There is also an extra call to
H5_trace() for each API function to print the return value.
<h2>Safety</h2>
<p>The tracing mechanism is invoked for each API function before
arguments are checked for validity. If bad arguments are passed
to an API function it could result in a segmentation fault.
However, the tracing output is line-buffered so all previous
output will appear.
<h2>Completeness</h2>
<p>There are two API functions that don't participate in
tracing. They are <code>H5Eprint()</code> and
<code>H5Eprint_cb()</code> because their participation would
mess up output during automatic error reporting.
<p>On the other hand, a number of API functions are called during
library initialization and they print tracing information.
<h2>Implementation</h2>
<p>For those interested in the implementation here is a
description. Each API function should have a call to one of the
<code>H5TRACE()</code> macros immediately after the
<code>FUNC_ENTER()</code> macro. The first argument is the
return type encoded as a string. The second argument is the
types of all the function arguments encoded as a string. The
remaining arguments are the function arguments. This macro was
designed to be as terse and unobtrousive as possible.
<p>In order to keep the <code>H5TRACE()</code> calls synchronized
with the source code we've written a perl script which gets
called automatically just before Makefile dependencies are
calculated for the file. However, this only works when one is
using GNU make. To reinstrument the tracing explicitly, invoke
the <code>trace</code> program from the hdf5 bin directory with
the names of the source files that need to be updated. If any
file needs to be modified then a backup is created by appending
a tilde to the file name.
<p>
<center>
<table border align=center width="100%">
<caption align=top><b>Explicit Instrumentation</b></caption>
<tr>
<td>
<code><pre>
$ ../bin/trace *.c
H5E.c: in function `H5Ewalk_cb':
H5E.c:336: warning: trace info was not inserted
</pre></code>
</td>
</tr>
</table>
</center>
<p>Note: The warning message is the result of a comment of the
form <code>/*NO TRACE*/</code> somewhere in the function
body. Tracing information will not be updated or inserted if
such a comment exists.
<p>Error messages have the same format as a compiler so that they
can be parsed from program development environments like
Emacs. Any function which generates an error will not be
modified.
<hr>
<address><a href="mailto:matzke@llnl.gov">Robb Matzke</a></address>
<!-- Created: Wed Jun 17 12:29:12 EDT 1998 -->
<!-- hhmts start -->
Last modified: Wed Jul 8 14:07:23 EDT 1998
<!-- hhmts end -->
</body>
</html>
|
