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| |
| <h2 class="chapter">7 Recording Inferior's Execution and Replaying It</h2> |
| |
| <p><a name="index-process-record-and-replay-456"></a><a name="index-recording-inferior_0027s-execution-and-replaying-it-457"></a> |
| On some platforms, <span class="sc">gdb</span> provides a special <dfn>process record |
| and replay</dfn> target that can record a log of the process execution, and |
| replay it later with both forward and reverse execution commands. |
| |
| <p><a name="index-replay-mode-458"></a>When this target is in use, if the execution log includes the record |
| for the next instruction, <span class="sc">gdb</span> will debug in <dfn>replay |
| mode</dfn>. In the replay mode, the inferior does not really execute code |
| instructions. Instead, all the events that normally happen during |
| code execution are taken from the execution log. While code is not |
| really executed in replay mode, the values of registers (including the |
| program counter register) and the memory of the inferior are still |
| changed as they normally would. Their contents are taken from the |
| execution log. |
| |
| <p><a name="index-record-mode-459"></a>If the record for the next instruction is not in the execution log, |
| <span class="sc">gdb</span> will debug in <dfn>record mode</dfn>. In this mode, the |
| inferior executes normally, and <span class="sc">gdb</span> records the execution log |
| for future replay. |
| |
| <p>The process record and replay target supports reverse execution |
| (see <a href="Reverse-Execution.html#Reverse-Execution">Reverse Execution</a>), even if the platform on which the |
| inferior runs does not. However, the reverse execution is limited in |
| this case by the range of the instructions recorded in the execution |
| log. In other words, reverse execution on platforms that don't |
| support it directly can only be done in the replay mode. |
| |
| <p>When debugging in the reverse direction, <span class="sc">gdb</span> will work in |
| replay mode as long as the execution log includes the record for the |
| previous instruction; otherwise, it will work in record mode, if the |
| platform supports reverse execution, or stop if not. |
| |
| <p>For architecture environments that support process record and replay, |
| <span class="sc">gdb</span> provides the following commands: |
| |
| |
| <a name="index-target-record-460"></a> |
| <a name="index-target-record_002dfull-461"></a> |
| <a name="index-target-record_002dbtrace-462"></a> |
| <a name="index-record-463"></a> |
| <a name="index-record-full-464"></a> |
| <a name="index-record-btrace-465"></a> |
| <a name="index-record-btrace-bts-466"></a> |
| <a name="index-record-btrace-pt-467"></a> |
| <a name="index-record-bts-468"></a> |
| <a name="index-record-pt-469"></a> |
| <a name="index-rec-470"></a> |
| <a name="index-rec-full-471"></a> |
| <a name="index-rec-btrace-472"></a> |
| <a name="index-rec-btrace-bts-473"></a> |
| <a name="index-rec-btrace-pt-474"></a> |
| <a name="index-rec-bts-475"></a> |
| <a name="index-rec-pt-476"></a> |
| <dl><dt><code>record </code><var>method</var><dd>This command starts the process record and replay target. The |
| recording method can be specified as parameter. Without a parameter |
| the command uses the <code>full</code> recording method. The following |
| recording methods are available: |
| |
| <dl> |
| <dt><code>full</code><dd>Full record/replay recording using <span class="sc">gdb</span>'s software record and |
| replay implementation. This method allows replaying and reverse |
| execution. |
| |
| <br><dt><code>btrace </code><var>format</var><dd>Hardware-supported instruction recording. This method does not record |
| data. Further, the data is collected in a ring buffer so old data will |
| be overwritten when the buffer is full. It allows limited reverse |
| execution. Variables and registers are not available during reverse |
| execution. In remote debugging, recording continues on disconnect. |
| Recorded data can be inspected after reconnecting. The recording may |
| be stopped using <code>record stop</code>. |
| |
| <p>The recording format can be specified as parameter. Without a parameter |
| the command chooses the recording format. The following recording |
| formats are available: |
| |
| <dl> |
| <dt><code>bts</code><dd><a name="index-branch-trace-store-477"></a>Use the <dfn>Branch Trace Store</dfn> (<acronym>BTS</acronym>) recording format. In |
| this format, the processor stores a from/to record for each executed |
| branch in the btrace ring buffer. |
| |
| <br><dt><code>pt</code><dd><a name="index-Intel-Processor-Trace-478"></a>Use the <dfn>Intel Processor Trace</dfn> recording format. In this |
| format, the processor stores the execution trace in a compressed form |
| that is afterwards decoded by <span class="sc">gdb</span>. |
| |
| <p>The trace can be recorded with very low overhead. The compressed |
| trace format also allows small trace buffers to already contain a big |
| number of instructions compared to <acronym>BTS</acronym>. |
| |
| <p>Decoding the recorded execution trace, on the other hand, is more |
| expensive than decoding <acronym>BTS</acronym> trace. This is mostly due to the |
| increased number of instructions to process. You should increase the |
| buffer-size with care. |
| </dl> |
| |
| <p>Not all recording formats may be available on all processors. |
| </dl> |
| |
| <p>The process record and replay target can only debug a process that is |
| already running. Therefore, you need first to start the process with |
| the <kbd>run</kbd> or <kbd>start</kbd> commands, and then start the recording |
| with the <kbd>record </kbd><var>method</var> command. |
| |
| <p><a name="index-displaced-stepping_002c-and-process-record-and-replay-479"></a>Displaced stepping (see <a href="Maintenance-Commands.html#Maintenance-Commands">displaced stepping</a>) |
| will be automatically disabled when process record and replay target |
| is started. That's because the process record and replay target |
| doesn't support displaced stepping. |
| |
| <p><a name="index-non_002dstop-mode_002c-and-process-record-and-replay-480"></a><a name="index-asynchronous-execution_002c-and-process-record-and-replay-481"></a>If the inferior is in the non-stop mode (see <a href="Non_002dStop-Mode.html#Non_002dStop-Mode">Non-Stop Mode</a>) or in |
| the asynchronous execution mode (see <a href="Background-Execution.html#Background-Execution">Background Execution</a>), not |
| all recording methods are available. The <code>full</code> recording method |
| does not support these two modes. |
| |
| <p><a name="index-record-stop-482"></a><a name="index-rec-s-483"></a><br><dt><code>record stop</code><dd>Stop the process record and replay target. When process record and |
| replay target stops, the entire execution log will be deleted and the |
| inferior will either be terminated, or will remain in its final state. |
| |
| <p>When you stop the process record and replay target in record mode (at |
| the end of the execution log), the inferior will be stopped at the |
| next instruction that would have been recorded. In other words, if |
| you record for a while and then stop recording, the inferior process |
| will be left in the same state as if the recording never happened. |
| |
| <p>On the other hand, if the process record and replay target is stopped |
| while in replay mode (that is, not at the end of the execution log, |
| but at some earlier point), the inferior process will become “live” |
| at that earlier state, and it will then be possible to continue the |
| usual “live” debugging of the process from that state. |
| |
| <p>When the inferior process exits, or <span class="sc">gdb</span> detaches from it, |
| process record and replay target will automatically stop itself. |
| |
| <p><a name="index-record-goto-484"></a><br><dt><code>record goto</code><dd>Go to a specific location in the execution log. There are several |
| ways to specify the location to go to: |
| |
| <dl> |
| <dt><code>record goto begin</code><dt><code>record goto start</code><dd>Go to the beginning of the execution log. |
| |
| <br><dt><code>record goto end</code><dd>Go to the end of the execution log. |
| |
| <br><dt><code>record goto </code><var>n</var><dd>Go to instruction number <var>n</var> in the execution log. |
| </dl> |
| |
| <p><a name="index-record-save-485"></a><br><dt><code>record save </code><var>filename</var><dd>Save the execution log to a file <samp><var>filename</var></samp>. |
| Default filename is <samp><span class="file">gdb_record.</span><var>process_id</var></samp>, where |
| <var>process_id</var> is the process ID of the inferior. |
| |
| <p>This command may not be available for all recording methods. |
| |
| <p><a name="index-record-restore-486"></a><br><dt><code>record restore </code><var>filename</var><dd>Restore the execution log from a file <samp><var>filename</var></samp>. |
| File must have been created with <code>record save</code>. |
| |
| <p><a name="index-set-record-full-487"></a><br><dt><code>set record full insn-number-max </code><var>limit</var><dt><code>set record full insn-number-max unlimited</code><dd>Set the limit of instructions to be recorded for the <code>full</code> |
| recording method. Default value is 200000. |
| |
| <p>If <var>limit</var> is a positive number, then <span class="sc">gdb</span> will start |
| deleting instructions from the log once the number of the record |
| instructions becomes greater than <var>limit</var>. For every new recorded |
| instruction, <span class="sc">gdb</span> will delete the earliest recorded |
| instruction to keep the number of recorded instructions at the limit. |
| (Since deleting recorded instructions loses information, <span class="sc">gdb</span> |
| lets you control what happens when the limit is reached, by means of |
| the <code>stop-at-limit</code> option, described below.) |
| |
| <p>If <var>limit</var> is <code>unlimited</code> or zero, <span class="sc">gdb</span> will never |
| delete recorded instructions from the execution log. The number of |
| recorded instructions is limited only by the available memory. |
| |
| <p><a name="index-show-record-full-488"></a><br><dt><code>show record full insn-number-max</code><dd>Show the limit of instructions to be recorded with the <code>full</code> |
| recording method. |
| |
| <br><dt><code>set record full stop-at-limit</code><dd>Control the behavior of the <code>full</code> recording method when the |
| number of recorded instructions reaches the limit. If ON (the |
| default), <span class="sc">gdb</span> will stop when the limit is reached for the |
| first time and ask you whether you want to stop the inferior or |
| continue running it and recording the execution log. If you decide |
| to continue recording, each new recorded instruction will cause the |
| oldest one to be deleted. |
| |
| <p>If this option is OFF, <span class="sc">gdb</span> will automatically delete the |
| oldest record to make room for each new one, without asking. |
| |
| <br><dt><code>show record full stop-at-limit</code><dd>Show the current setting of <code>stop-at-limit</code>. |
| |
| <br><dt><code>set record full memory-query</code><dd>Control the behavior when <span class="sc">gdb</span> is unable to record memory |
| changes caused by an instruction for the <code>full</code> recording method. |
| If ON, <span class="sc">gdb</span> will query whether to stop the inferior in that |
| case. |
| |
| <p>If this option is OFF (the default), <span class="sc">gdb</span> will automatically |
| ignore the effect of such instructions on memory. Later, when |
| <span class="sc">gdb</span> replays this execution log, it will mark the log of this |
| instruction as not accessible, and it will not affect the replay |
| results. |
| |
| <br><dt><code>show record full memory-query</code><dd>Show the current setting of <code>memory-query</code>. |
| |
| <p><a name="index-set-record-btrace-489"></a>The <code>btrace</code> record target does not trace data. As a |
| convenience, when replaying, <span class="sc">gdb</span> reads read-only memory off |
| the live program directly, assuming that the addresses of the |
| read-only areas don't change. This for example makes it possible to |
| disassemble code while replaying, but not to print variables. |
| In some cases, being able to inspect variables might be useful. |
| You can use the following command for that: |
| |
| <br><dt><code>set record btrace replay-memory-access</code><dd>Control the behavior of the <code>btrace</code> recording method when |
| accessing memory during replay. If <code>read-only</code> (the default), |
| <span class="sc">gdb</span> will only allow accesses to read-only memory. |
| If <code>read-write</code>, <span class="sc">gdb</span> will allow accesses to read-only |
| and to read-write memory. Beware that the accessed memory corresponds |
| to the live target and not necessarily to the current replay |
| position. |
| |
| <br><dt><code>set record btrace cpu </code><var>identifier</var><dd>Set the processor to be used for enabling workarounds for processor |
| errata when decoding the trace. |
| |
| <p>Processor errata are defects in processor operation, caused by its |
| design or manufacture. They can cause a trace not to match the |
| specification. This, in turn, may cause trace decode to fail. |
| <span class="sc">gdb</span> can detect erroneous trace packets and correct them, thus |
| avoiding the decoding failures. These corrections are known as |
| <dfn>errata workarounds</dfn>, and are enabled based on the processor on |
| which the trace was recorded. |
| |
| <p>By default, <span class="sc">gdb</span> attempts to detect the processor |
| automatically, and apply the necessary workarounds for it. However, |
| you may need to specify the processor if <span class="sc">gdb</span> does not yet |
| support it. This command allows you to do that, and also allows to |
| disable the workarounds. |
| |
| <p>The argument <var>identifier</var> identifies the <span class="sc">cpu</span> and is of the |
| form: <var>vendor</var><code>:</code><var>procesor identifier</var>. In addition, |
| there are two special identifiers, <code>none</code> and <code>auto</code> |
| (default). |
| |
| <p>The following vendor identifiers and corresponding processor |
| identifiers are currently supported: |
| |
| <p><table summary=""> |
| <tr align="left"><td valign="top" width="10%"><code>intel</code> |
| </td><td valign="top" width="90%"><var>family</var>/<var>model</var>[/<var>stepping</var>] |
| |
| <br></td></tr></table> |
| |
| <p>On GNU/Linux systems, the processor <var>family</var>, <var>model</var>, and |
| <var>stepping</var> can be obtained from <code>/proc/cpuinfo</code>. |
| |
| <p>If <var>identifier</var> is <code>auto</code>, enable errata workarounds for the |
| processor on which the trace was recorded. If <var>identifier</var> is |
| <code>none</code>, errata workarounds are disabled. |
| |
| <p>For example, when using an old <span class="sc">gdb</span> on a new system, decode |
| may fail because <span class="sc">gdb</span> does not support the new processor. It |
| often suffices to specify an older processor that <span class="sc">gdb</span> |
| supports. |
| |
| <pre class="smallexample"> (gdb) info record |
| Active record target: record-btrace |
| Recording format: Intel Processor Trace. |
| Buffer size: 16kB. |
| Failed to configure the Intel Processor Trace decoder: unknown cpu. |
| (gdb) set record btrace cpu intel:6/158 |
| (gdb) info record |
| Active record target: record-btrace |
| Recording format: Intel Processor Trace. |
| Buffer size: 16kB. |
| Recorded 84872 instructions in 3189 functions (0 gaps) for thread 1 (...). |
| </pre> |
| <p><a name="index-show-record-btrace-490"></a><br><dt><code>show record btrace replay-memory-access</code><dd>Show the current setting of <code>replay-memory-access</code>. |
| |
| <br><dt><code>show record btrace cpu</code><dd>Show the processor to be used for enabling trace decode errata |
| workarounds. |
| |
| <p><a name="index-set-record-btrace-bts-491"></a><br><dt><code>set record btrace bts buffer-size </code><var>size</var><dt><code>set record btrace bts buffer-size unlimited</code><dd>Set the requested ring buffer size for branch tracing in <acronym>BTS</acronym> |
| format. Default is 64KB. |
| |
| <p>If <var>size</var> is a positive number, then <span class="sc">gdb</span> will try to |
| allocate a buffer of at least <var>size</var> bytes for each new thread |
| that uses the btrace recording method and the <acronym>BTS</acronym> format. |
| The actually obtained buffer size may differ from the requested |
| <var>size</var>. Use the <code>info record</code> command to see the actual |
| buffer size for each thread that uses the btrace recording method and |
| the <acronym>BTS</acronym> format. |
| |
| <p>If <var>limit</var> is <code>unlimited</code> or zero, <span class="sc">gdb</span> will try to |
| allocate a buffer of 4MB. |
| |
| <p>Bigger buffers mean longer traces. On the other hand, <span class="sc">gdb</span> will |
| also need longer to process the branch trace data before it can be used. |
| |
| <br><dt><code>show record btrace bts buffer-size </code><var>size</var><dd>Show the current setting of the requested ring buffer size for branch |
| tracing in <acronym>BTS</acronym> format. |
| |
| <p><a name="index-set-record-btrace-pt-492"></a><br><dt><code>set record btrace pt buffer-size </code><var>size</var><dt><code>set record btrace pt buffer-size unlimited</code><dd>Set the requested ring buffer size for branch tracing in Intel |
| Processor Trace format. Default is 16KB. |
| |
| <p>If <var>size</var> is a positive number, then <span class="sc">gdb</span> will try to |
| allocate a buffer of at least <var>size</var> bytes for each new thread |
| that uses the btrace recording method and the Intel Processor Trace |
| format. The actually obtained buffer size may differ from the |
| requested <var>size</var>. Use the <code>info record</code> command to see the |
| actual buffer size for each thread. |
| |
| <p>If <var>limit</var> is <code>unlimited</code> or zero, <span class="sc">gdb</span> will try to |
| allocate a buffer of 4MB. |
| |
| <p>Bigger buffers mean longer traces. On the other hand, <span class="sc">gdb</span> will |
| also need longer to process the branch trace data before it can be used. |
| |
| <br><dt><code>show record btrace pt buffer-size </code><var>size</var><dd>Show the current setting of the requested ring buffer size for branch |
| tracing in Intel Processor Trace format. |
| |
| <p><a name="index-info-record-493"></a><br><dt><code>info record</code><dd>Show various statistics about the recording depending on the recording |
| method: |
| |
| <dl> |
| <dt><code>full</code><dd>For the <code>full</code> recording method, it shows the state of process |
| record and its in-memory execution log buffer, including: |
| |
| <ul> |
| <li>Whether in record mode or replay mode. |
| <li>Lowest recorded instruction number (counting from when the current execution log started recording instructions). |
| <li>Highest recorded instruction number. |
| <li>Current instruction about to be replayed (if in replay mode). |
| <li>Number of instructions contained in the execution log. |
| <li>Maximum number of instructions that may be contained in the execution log. |
| </ul> |
| |
| <br><dt><code>btrace</code><dd>For the <code>btrace</code> recording method, it shows: |
| |
| <ul> |
| <li>Recording format. |
| <li>Number of instructions that have been recorded. |
| <li>Number of blocks of sequential control-flow formed by the recorded |
| instructions. |
| <li>Whether in record mode or replay mode. |
| </ul> |
| |
| <p>For the <code>bts</code> recording format, it also shows: |
| <ul> |
| <li>Size of the perf ring buffer. |
| </ul> |
| |
| <p>For the <code>pt</code> recording format, it also shows: |
| <ul> |
| <li>Size of the perf ring buffer. |
| </ul> |
| </dl> |
| |
| <p><a name="index-record-delete-494"></a><a name="index-rec-del-495"></a><br><dt><code>record delete</code><dd>When record target runs in replay mode (“in the past”), delete the |
| subsequent execution log and begin to record a new execution log starting |
| from the current address. This means you will abandon the previously |
| recorded “future” and begin recording a new “future”. |
| |
| <p><a name="index-record-instruction_002dhistory-496"></a><a name="index-rec-instruction_002dhistory-497"></a><br><dt><code>record instruction-history</code><dd>Disassembles instructions from the recorded execution log. By |
| default, ten instructions are disassembled. This can be changed using |
| the <code>set record instruction-history-size</code> command. Instructions |
| are printed in execution order. |
| |
| <p>It can also print mixed source+disassembly if you specify the the |
| <code>/m</code> or <code>/s</code> modifier, and print the raw instructions in hex |
| as well as in symbolic form by specifying the <code>/r</code> modifier. |
| |
| <p>The current position marker is printed for the instruction at the |
| current program counter value. This instruction can appear multiple |
| times in the trace and the current position marker will be printed |
| every time. To omit the current position marker, specify the |
| <code>/p</code> modifier. |
| |
| <p>To better align the printed instructions when the trace contains |
| instructions from more than one function, the function name may be |
| omitted by specifying the <code>/f</code> modifier. |
| |
| <p>Speculatively executed instructions are prefixed with ‘<samp><span class="samp">?</span></samp>’. This |
| feature is not available for all recording formats. |
| |
| <p>There are several ways to specify what part of the execution log to |
| disassemble: |
| |
| <dl> |
| <dt><code>record instruction-history </code><var>insn</var><dd>Disassembles ten instructions starting from instruction number |
| <var>insn</var>. |
| |
| <br><dt><code>record instruction-history </code><var>insn</var><code>, +/-</code><var>n</var><dd>Disassembles <var>n</var> instructions around instruction number |
| <var>insn</var>. If <var>n</var> is preceded with <code>+</code>, disassembles |
| <var>n</var> instructions after instruction number <var>insn</var>. If |
| <var>n</var> is preceded with <code>-</code>, disassembles <var>n</var> |
| instructions before instruction number <var>insn</var>. |
| |
| <br><dt><code>record instruction-history</code><dd>Disassembles ten more instructions after the last disassembly. |
| |
| <br><dt><code>record instruction-history -</code><dd>Disassembles ten more instructions before the last disassembly. |
| |
| <br><dt><code>record instruction-history </code><var>begin</var><code>, </code><var>end</var><dd>Disassembles instructions beginning with instruction number |
| <var>begin</var> until instruction number <var>end</var>. The instruction |
| number <var>end</var> is included. |
| </dl> |
| |
| <p>This command may not be available for all recording methods. |
| |
| <p><a name="index-set-record-498"></a><br><dt><code>set record instruction-history-size </code><var>size</var><dt><code>set record instruction-history-size unlimited</code><dd>Define how many instructions to disassemble in the <code>record |
| instruction-history</code> command. The default value is 10. |
| A <var>size</var> of <code>unlimited</code> means unlimited instructions. |
| |
| <p><a name="index-show-record-499"></a><br><dt><code>show record instruction-history-size</code><dd>Show how many instructions to disassemble in the <code>record |
| instruction-history</code> command. |
| |
| <p><a name="index-record-function_002dcall_002dhistory-500"></a><a name="index-rec-function_002dcall_002dhistory-501"></a><br><dt><code>record function-call-history</code><dd>Prints the execution history at function granularity. It prints one |
| line for each sequence of instructions that belong to the same |
| function giving the name of that function, the source lines |
| for this instruction sequence (if the <code>/l</code> modifier is |
| specified), and the instructions numbers that form the sequence (if |
| the <code>/i</code> modifier is specified). The function names are indented |
| to reflect the call stack depth if the <code>/c</code> modifier is |
| specified. The <code>/l</code>, <code>/i</code>, and <code>/c</code> modifiers can be |
| given together. |
| |
| <pre class="smallexample"> (gdb) <b>list 1, 10</b> |
| 1 void foo (void) |
| 2 { |
| 3 } |
| 4 |
| 5 void bar (void) |
| 6 { |
| 7 ... |
| 8 foo (); |
| 9 ... |
| 10 } |
| (gdb) <b>record function-call-history /ilc</b> |
| 1 bar inst 1,4 at foo.c:6,8 |
| 2 foo inst 5,10 at foo.c:2,3 |
| 3 bar inst 11,13 at foo.c:9,10 |
| </pre> |
| <p>By default, ten lines are printed. This can be changed using the |
| <code>set record function-call-history-size</code> command. Functions are |
| printed in execution order. There are several ways to specify what |
| to print: |
| |
| <dl> |
| <dt><code>record function-call-history </code><var>func</var><dd>Prints ten functions starting from function number <var>func</var>. |
| |
| <br><dt><code>record function-call-history </code><var>func</var><code>, +/-</code><var>n</var><dd>Prints <var>n</var> functions around function number <var>func</var>. If |
| <var>n</var> is preceded with <code>+</code>, prints <var>n</var> functions after |
| function number <var>func</var>. If <var>n</var> is preceded with <code>-</code>, |
| prints <var>n</var> functions before function number <var>func</var>. |
| |
| <br><dt><code>record function-call-history</code><dd>Prints ten more functions after the last ten-line print. |
| |
| <br><dt><code>record function-call-history -</code><dd>Prints ten more functions before the last ten-line print. |
| |
| <br><dt><code>record function-call-history </code><var>begin</var><code>, </code><var>end</var><dd>Prints functions beginning with function number <var>begin</var> until |
| function number <var>end</var>. The function number <var>end</var> is included. |
| </dl> |
| |
| <p>This command may not be available for all recording methods. |
| |
| <br><dt><code>set record function-call-history-size </code><var>size</var><dt><code>set record function-call-history-size unlimited</code><dd>Define how many lines to print in the |
| <code>record function-call-history</code> command. The default value is 10. |
| A size of <code>unlimited</code> means unlimited lines. |
| |
| <br><dt><code>show record function-call-history-size</code><dd>Show how many lines to print in the |
| <code>record function-call-history</code> command. |
| </dl> |
| |
| </body></html> |
| |
