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<body class="article">
<div id="header">
<h1>trace API</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>The trace API can be used to print debug messages to stderr or a file. Trace
code is inactive unless explicitly enabled by setting <code>GIT_TRACE*</code> environment
variables.</p></div>
<div class="paragraph"><p>The trace implementation automatically adds <code>timestamp file:line ... \n</code> to
all trace messages. E.g.:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>23:59:59.123456 git.c:312               trace: built-in: git 'foo'
00:00:00.000001 builtin/foo.c:99        foo: some message</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_data_structures">Data Structures</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>struct trace_key</code>
</dt>
<dd>
<p>
        Defines a trace key (or category). The default (for API functions that
        don&#8217;t take a key) is <code>GIT_TRACE</code>.
</p>
<div class="paragraph"><p>E.g. to define a trace key controlled by environment variable <code>GIT_TRACE_FOO</code>:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>static struct trace_key trace_foo = TRACE_KEY_INIT(FOO);

static void trace_print_foo(const char *message)
{
        trace_printf_key(&amp;trace_foo, "%s", message);
}</code></pre>
</div></div>
<div class="paragraph"><p>Note: don&#8217;t use <code>const</code> as the trace implementation stores internal state in
the <code>trace_key</code> structure.</p></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_functions">Functions</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>int trace_want(struct trace_key *key)</code>
</dt>
<dd>
<p>
        Checks whether the trace key is enabled. Used to prevent expensive
        string formatting before calling one of the printing APIs.
</p>
</dd>
<dt class="hdlist1">
<code>void trace_disable(struct trace_key *key)</code>
</dt>
<dd>
<p>
        Disables tracing for the specified key, even if the environment
        variable was set.
</p>
</dd>
<dt class="hdlist1">
<code>void trace_printf(const char *format, ...)</code>
</dt>
<dt class="hdlist1">
<code>void trace_printf_key(struct trace_key *key, const char *format, ...)</code>
</dt>
<dd>
<p>
        Prints a formatted message, similar to printf.
</p>
</dd>
<dt class="hdlist1">
<code>void trace_argv_printf(const char **argv, const char *format, ...)`</code>
</dt>
<dd>
<p>
        Prints a formatted message, followed by a quoted list of arguments.
</p>
</dd>
<dt class="hdlist1">
<code>void trace_strbuf(struct trace_key *key, const struct strbuf *data)</code>
</dt>
<dd>
<p>
        Prints the strbuf, without additional formatting (i.e. doesn&#8217;t
        choke on <code>%</code> or even <code>\0</code>).
</p>
</dd>
<dt class="hdlist1">
<code>uint64_t getnanotime(void)</code>
</dt>
<dd>
<p>
        Returns nanoseconds since the epoch (01/01/1970), typically used
        for performance measurements.
</p>
<div class="paragraph"><p>Currently there are high precision timer implementations for Linux (using
<code>clock_gettime(CLOCK_MONOTONIC)</code>) and Windows (<code>QueryPerformanceCounter</code>).
Other platforms use <code>gettimeofday</code> as time source.</p></div>
</dd>
<dt class="hdlist1">
<code>void trace_performance(uint64_t nanos, const char *format, ...)</code>
</dt>
<dt class="hdlist1">
<code>void trace_performance_since(uint64_t start, const char *format, ...)</code>
</dt>
<dd>
<p>
        Prints the elapsed time (in nanoseconds), or elapsed time since
        <code>start</code>, followed by a formatted message. Enabled via environment
        variable <code>GIT_TRACE_PERFORMANCE</code>. Used for manual profiling, e.g.:
</p>
<div class="listingblock">
<div class="content">
<pre><code>uint64_t start = getnanotime();
/* code section to measure */
trace_performance_since(start, "foobar");</code></pre>
</div></div>
<div class="listingblock">
<div class="content">
<pre><code>uint64_t t = 0;
for (;;) {
        /* ignore */
        t -= getnanotime();
        /* code section to measure */
        t += getnanotime();
        /* ignore */
}
trace_performance(t, "frotz");</code></pre>
</div></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_bugs_amp_caveats">Bugs &amp; Caveats</h2>
<div class="sectionbody">
<div class="paragraph"><p>GIT_TRACE_* environment variables can be used to tell Git to show
trace output to its standard error stream. Git can often spawn a pager
internally to run its subcommand and send its standard output and
standard error to it.</p></div>
<div class="paragraph"><p>Because GIT_TRACE_PERFORMANCE trace is generated only at the very end
of the program with atexit(), which happens after the pager exits, it
would not work well if you send its log to the standard error output
and let Git spawn the pager at the same time.</p></div>
<div class="paragraph"><p>As a work around, you can for example use <em>--no-pager</em>, or set
GIT_TRACE_PERFORMANCE to another file descriptor which is redirected
to stderr, or set GIT_TRACE_PERFORMANCE to a file specified by its
absolute path.</p></div>
<div class="paragraph"><p>For example instead of the following command which by default may not
print any performance information:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>GIT_TRACE_PERFORMANCE=2 git log -1</code></pre>
</div></div>
<div class="paragraph"><p>you may want to use:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>GIT_TRACE_PERFORMANCE=2 git --no-pager log -1</code></pre>
</div></div>
<div class="paragraph"><p>or:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>GIT_TRACE_PERFORMANCE=3 3&gt;&amp;2 git log -1</code></pre>
</div></div>
<div class="paragraph"><p>or:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>GIT_TRACE_PERFORMANCE=/path/to/log/file git log -1</code></pre>
</div></div>
</div>
</div>
</div>
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Last updated
 2018-01-27 08:11:04 JST
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