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| <h4 class="subsection">13.1.1 Create and Delete Tracepoints</h4> |
| |
| |
| <a name="index-set-tracepoint-863"></a> |
| <a name="index-trace-864"></a> |
| <dl><dt><code>trace </code><var>location</var><dd>The <code>trace</code> command is very similar to the <code>break</code> command. |
| 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><span class="samp">InstallInTrace</span></samp>’ feature (see <a href="install-tracepoint-in-tracing.html#install-tracepoint-in-tracing">install tracepoint in tracing</a>). |
| If remote stub doesn't support the ‘<samp><span class="samp">InstallInTrace</span></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, |
| <span class="sc">gdb</span> supports <dfn>pending tracepoints</dfn>—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 |
| <span class="sc">gdb</span> support—when debugging with the remote target, and |
| <span class="sc">gdb</span> disconnects from the remote stub (see <a href="disconnected-tracing.html#disconnected-tracing">disconnected tracing</a>), pending tracepoints can not be resolved (and downloaded to |
| the remote stub) while <span class="sc">gdb</span> is disconnected. |
| |
| <p>Here are some examples of using the <code>trace</code> command: |
| |
| <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> |
| <p class="noindent">You can abbreviate <code>trace</code> as <code>tr</code>. |
| |
| <br><dt><code>trace </code><var>location</var><code> if </code><var>cond</var><dd>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. |
| |
| <br><dt><code>ftrace </code><var>location</var><code> [ if </code><var>cond</var><code> ]</code><dd><a name="index-set-fast-tracepoint-865"></a><a name="index-fast-tracepoints_002c-setting-866"></a><a name="index-ftrace-867"></a>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><span class="sc">gdb</span> handles arguments to <code>ftrace</code> exactly as for |
| <code>trace</code>. |
| |
| <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 <span class="sc">gnu</span>/Linux, exclude low addresses from the program's |
| address space; but for instance with the Linux kernel it is possible |
| to let <span class="sc">gdb</span> use this area by doing a <samp><span class="command">sysctl</span></samp> command |
| to set the <code>mmap_min_addr</code> kernel parameter, as in |
| |
| <pre class="example"> sudo sysctl -w vm.mmap_min_addr=32768 |
| </pre> |
| <p class="noindent">which sets the low address to 32K, which leaves plenty of room for |
| trampolines. The minimum address should be set to a page boundary. |
| |
| <br><dt><code>strace </code><var>location</var><code> [ if </code><var>cond</var><code> ]</code><dd><a name="index-set-static-tracepoint-868"></a><a name="index-static-tracepoints_002c-setting-869"></a><a name="index-probe-static-tracepoint-marker-870"></a><a name="index-strace-871"></a>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><span class="sc">gdb</span> handles arguments to <code>strace</code> exactly as for |
| <code>trace</code>, with the addition that the user can also specify |
| <code>-m </code><var>marker</var> 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><span class="samp">ID</span></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: |
| |
| <pre class="smallexample"> main () |
| { |
| trace_mark(ust, bar33, "str %s", "FOOBAZ"); |
| } |
| </pre> |
| <p class="noindent">the marker id is composed of joining the first two arguments to the |
| <code>trace_mark</code> call with a slash, which translates to: |
| |
| <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> |
| <p class="noindent">so you may probe the marker above with: |
| |
| <pre class="smallexample"> (gdb) strace -m ust/bar33 |
| </pre> |
| <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><span class="samp">Data:</span></samp>’ field. |
| |
| <p>You can inspect this data when analyzing the trace buffer, by printing |
| the $_sdata variable like any other variable available to |
| <span class="sc">gdb</span>. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>. |
| |
| <p><a name="index-g_t_0024tpnum-872"></a><a name="index-last-tracepoint-number-873"></a><a name="index-recent-tracepoint-number-874"></a><a name="index-tracepoint-number-875"></a>The convenience variable <code>$tpnum</code> records the tracepoint number |
| of the most recently set tracepoint. |
| |
| <p><a name="index-delete-tracepoint-876"></a><a name="index-tracepoint-deletion-877"></a><br><dt><code>delete tracepoint </code><span class="roman">[</span><var>num</var><span class="roman">]</span><dd>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>Examples: |
| |
| <pre class="smallexample"> (gdb) <b>delete trace 1 2 3</b> // remove three tracepoints |
| |
| (gdb) <b>delete trace</b> // remove all tracepoints |
| </pre> |
| <p class="noindent">You can abbreviate this command as <code>del tr</code>. |
| </dl> |
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