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Its argument <var>location</var> can be any valid location. See <a href="Specify-Location.html#Specify-Location">Specify Location</a>. The <code>trace</code> command defines a tracepoint, which is a point in the target program where the debugger will briefly stop, collect some data, and then allow the program to continue. Setting a tracepoint or changing its actions takes effect immediately if the remote stub supports the ‘<samp>InstallInTrace</samp>’ feature (see <a href="General-Query-Packets.html#install-tracepoint-in-tracing">install tracepoint in tracing</a>). If remote stub doesn’t support the ‘<samp>InstallInTrace</samp>’ feature, all these changes don’t take effect until the next <code>tstart</code> command, and once a trace experiment is running, further changes will not have any effect until the next trace experiment starts. In addition, <small>GDB</small> supports <em>pending tracepoints</em>—tracepoints whose address is not yet resolved. (This is similar to pending breakpoints.) Pending tracepoints are not downloaded to the target and not installed until they are resolved. The resolution of pending tracepoints requires <small>GDB</small> support—when debugging with the remote target, and <small>GDB</small> disconnects from the remote stub (see <a href="Starting-and-Stopping-Trace-Experiments.html#disconnected-tracing">disconnected tracing</a>), pending tracepoints can not be resolved (and downloaded to the remote stub) while <small>GDB</small> is disconnected. </p> <p>Here are some examples of using the <code>trace</code> command: </p> <div class="smallexample"> <pre class="smallexample">(gdb) <b>trace foo.c:121</b> // a source file and line number (gdb) <b>trace +2</b> // 2 lines forward (gdb) <b>trace my_function</b> // first source line of function (gdb) <b>trace *my_function</b> // EXACT start address of function (gdb) <b>trace *0x2117c4</b> // an address </pre></div> <p>You can abbreviate <code>trace</code> as <code>tr</code>. </p> </dd> <dt><code>trace <var>location</var> if <var>cond</var></code></dt> <dd><p>Set a tracepoint with condition <var>cond</var>; evaluate the expression <var>cond</var> each time the tracepoint is reached, and collect data only if the value is nonzero—that is, if <var>cond</var> evaluates as true. See <a href="Tracepoint-Conditions.html#Tracepoint-Conditions">Tracepoint Conditions</a>, for more information on tracepoint conditions. </p> </dd> <dt><code>ftrace <var>location</var> [ if <var>cond</var> ]</code></dt> <dd><a name="index-set-fast-tracepoint"></a> <a name="index-fast-tracepoints_002c-setting"></a> <a name="index-ftrace"></a> <p>The <code>ftrace</code> command sets a fast tracepoint. For targets that support them, fast tracepoints will use a more efficient but possibly less general technique to trigger data collection, such as a jump instruction instead of a trap, or some sort of hardware support. It may not be possible to create a fast tracepoint at the desired location, in which case the command will exit with an explanatory message. </p> <p><small>GDB</small> handles arguments to <code>ftrace</code> exactly as for <code>trace</code>. </p> <p>On 32-bit x86-architecture systems, fast tracepoints normally need to be placed at an instruction that is 5 bytes or longer, but can be placed at 4-byte instructions if the low 64K of memory of the target program is available to install trampolines. Some Unix-type systems, such as <small>GNU</small>/Linux, exclude low addresses from the program’s address space; but for instance with the Linux kernel it is possible to let <small>GDB</small> use this area by doing a <code>sysctl</code> command to set the <code>mmap_min_addr</code> kernel parameter, as in </p> <div class="example"> <pre class="example">sudo sysctl -w vm.mmap_min_addr=32768 </pre></div> <p>which sets the low address to 32K, which leaves plenty of room for trampolines. The minimum address should be set to a page boundary. </p> </dd> <dt><code>strace <var>location</var> [ if <var>cond</var> ]</code></dt> <dd><a name="index-set-static-tracepoint"></a> <a name="index-static-tracepoints_002c-setting"></a> <a name="index-probe-static-tracepoint-marker"></a> <a name="index-strace"></a> <p>The <code>strace</code> command sets a static tracepoint. For targets that support it, setting a static tracepoint probes a static instrumentation point, or marker, found at <var>location</var>. It may not be possible to set a static tracepoint at the desired location, in which case the command will exit with an explanatory message. </p> <p><small>GDB</small> handles arguments to <code>strace</code> exactly as for <code>trace</code>, with the addition that the user can also specify <code>-m <var>marker</var></code> as <var>location</var>. This probes the marker identified by the <var>marker</var> string identifier. This identifier depends on the static tracepoint backend library your program is using. You can find all the marker identifiers in the ‘<samp>ID</samp>’ field of the <code>info static-tracepoint-markers</code> command output. See <a href="Listing-Static-Tracepoint-Markers.html#Listing-Static-Tracepoint-Markers">Listing Static Tracepoint Markers</a>. For example, in the following small program using the UST tracing engine: </p> <div class="smallexample"> <pre class="smallexample">main () { trace_mark(ust, bar33, "str %s", "FOOBAZ"); } </pre></div> <p>the marker id is composed of joining the first two arguments to the <code>trace_mark</code> call with a slash, which translates to: </p> <div class="smallexample"> <pre class="smallexample">(gdb) info static-tracepoint-markers Cnt Enb ID Address What 1 n ust/bar33 0x0000000000400ddc in main at stexample.c:22 Data: "str %s" [etc...] </pre></div> <p>so you may probe the marker above with: </p> <div class="smallexample"> <pre class="smallexample">(gdb) strace -m ust/bar33 </pre></div> <p>Static tracepoints accept an extra collect action — <code>collect $_sdata</code>. This collects arbitrary user data passed in the probe point call to the tracing library. In the UST example above, you’ll see that the third argument to <code>trace_mark</code> is a printf-like format string. The user data is then the result of running that formating string against the following arguments. Note that <code>info static-tracepoint-markers</code> command output lists that format string in the ‘<samp>Data:</samp>’ field. </p> <p>You can inspect this data when analyzing the trace buffer, by printing the $_sdata variable like any other variable available to <small>GDB</small>. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>. </p> <a name="index-_0024tpnum"></a> <a name="index-last-tracepoint-number"></a> <a name="index-recent-tracepoint-number"></a> <a name="index-tracepoint-number"></a> <p>The convenience variable <code>$tpnum</code> records the tracepoint number of the most recently set tracepoint. </p> <a name="index-delete-tracepoint"></a> <a name="index-tracepoint-deletion"></a> </dd> <dt><code>delete tracepoint <span class="roman">[</span><var>num</var><span class="roman">]</span></code></dt> <dd><p>Permanently delete one or more tracepoints. With no argument, the default is to delete all tracepoints. Note that the regular <code>delete</code> command can remove tracepoints also. </p> <p>Examples: </p> <div class="smallexample"> <pre class="smallexample">(gdb) <b>delete trace 1 2 3</b> // remove three tracepoints (gdb) <b>delete trace</b> // remove all tracepoints </pre></div> <p>You can abbreviate this command as <code>del tr</code>. </p></dd> </dl> <hr> <div class="header"> <p> Next: <a href="Enable-and-Disable-Tracepoints.html#Enable-and-Disable-Tracepoints" accesskey="n" rel="next">Enable and Disable Tracepoints</a>, Up: <a href="Set-Tracepoints.html#Set-Tracepoints" accesskey="u" rel="up">Set Tracepoints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> </div> </body> </html>