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| <title>Debugging with GDB: Set Watchpoints</title> |
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| <a name="Set-Watchpoints"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Set-Catchpoints.html#Set-Catchpoints" accesskey="n" rel="next">Set Catchpoints</a>, Previous: <a href="Set-Breaks.html#Set-Breaks" accesskey="p" rel="prev">Set Breaks</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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> |
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| <hr> |
| <a name="Setting-Watchpoints"></a> |
| <h4 class="subsection">5.1.2 Setting Watchpoints</h4> |
| |
| <a name="index-setting-watchpoints"></a> |
| <p>You can use a watchpoint to stop execution whenever the value of an |
| expression changes, without having to predict a particular place where |
| this may happen. (This is sometimes called a <em>data breakpoint</em>.) |
| The expression may be as simple as the value of a single variable, or |
| as complex as many variables combined by operators. Examples include: |
| </p> |
| <ul> |
| <li> A reference to the value of a single variable. |
| |
| </li><li> An address cast to an appropriate data type. For example, |
| ‘<samp>*(int *)0x12345678</samp>’ will watch a 4-byte region at the specified |
| address (assuming an <code>int</code> occupies 4 bytes). |
| |
| </li><li> An arbitrarily complex expression, such as ‘<samp>a*b + c/d</samp>’. The |
| expression can use any operators valid in the program’s native |
| language (see <a href="Languages.html#Languages">Languages</a>). |
| </li></ul> |
| |
| <p>You can set a watchpoint on an expression even if the expression can |
| not be evaluated yet. For instance, you can set a watchpoint on |
| ‘<samp>*global_ptr</samp>’ before ‘<samp>global_ptr</samp>’ is initialized. |
| <small>GDB</small> will stop when your program sets ‘<samp>global_ptr</samp>’ and |
| the expression produces a valid value. If the expression becomes |
| valid in some other way than changing a variable (e.g. if the memory |
| pointed to by ‘<samp>*global_ptr</samp>’ becomes readable as the result of a |
| <code>malloc</code> call), <small>GDB</small> may not stop until the next time |
| the expression changes. |
| </p> |
| <a name="index-software-watchpoints"></a> |
| <a name="index-hardware-watchpoints"></a> |
| <p>Depending on your system, watchpoints may be implemented in software or |
| hardware. <small>GDB</small> does software watchpointing by single-stepping your |
| program and testing the variable’s value each time, which is hundreds of |
| times slower than normal execution. (But this may still be worth it, to |
| catch errors where you have no clue what part of your program is the |
| culprit.) |
| </p> |
| <p>On some systems, such as HP-UX, PowerPC, <small>GNU</small>/Linux and most other |
| x86-based targets, <small>GDB</small> includes support for hardware |
| watchpoints, which do not slow down the running of your program. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-watch"></a> |
| </dd> |
| <dt><code>watch <span class="roman">[</span>-l<span class="roman">|</span>-location<span class="roman">]</span> <var>expr</var> <span class="roman">[</span>thread <var>threadnum</var><span class="roman">]</span> <span class="roman">[</span>mask <var>maskvalue</var><span class="roman">]</span></code></dt> |
| <dd><p>Set a watchpoint for an expression. <small>GDB</small> will break when the |
| expression <var>expr</var> is written into by the program and its value |
| changes. The simplest (and the most popular) use of this command is |
| to watch the value of a single variable: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) watch foo |
| </pre></div> |
| |
| <p>If the command includes a <code><span class="roman">[</span>thread <var>threadnum</var><span class="roman">]</span></code> |
| argument, <small>GDB</small> breaks only when the thread identified by |
| <var>threadnum</var> changes the value of <var>expr</var>. If any other threads |
| change the value of <var>expr</var>, <small>GDB</small> will not break. Note |
| that watchpoints restricted to a single thread in this way only work |
| with Hardware Watchpoints. |
| </p> |
| <p>Ordinarily a watchpoint respects the scope of variables in <var>expr</var> |
| (see below). The <code>-location</code> argument tells <small>GDB</small> to |
| instead watch the memory referred to by <var>expr</var>. In this case, |
| <small>GDB</small> will evaluate <var>expr</var>, take the address of the result, |
| and watch the memory at that address. The type of the result is used |
| to determine the size of the watched memory. If the expression’s |
| result does not have an address, then <small>GDB</small> will print an |
| error. |
| </p> |
| <p>The <code><span class="roman">[</span>mask <var>maskvalue</var><span class="roman">]</span></code> argument allows creation |
| of masked watchpoints, if the current architecture supports this |
| feature (e.g., PowerPC Embedded architecture, see <a href="PowerPC-Embedded.html#PowerPC-Embedded">PowerPC Embedded</a>.) A <em>masked watchpoint</em> specifies a mask in addition |
| to an address to watch. The mask specifies that some bits of an address |
| (the bits which are reset in the mask) should be ignored when matching |
| the address accessed by the inferior against the watchpoint address. |
| Thus, a masked watchpoint watches many addresses simultaneously—those |
| addresses whose unmasked bits are identical to the unmasked bits in the |
| watchpoint address. The <code>mask</code> argument implies <code>-location</code>. |
| Examples: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) watch foo mask 0xffff00ff |
| (gdb) watch *0xdeadbeef mask 0xffffff00 |
| </pre></div> |
| |
| <a name="index-rwatch"></a> |
| </dd> |
| <dt><code>rwatch <span class="roman">[</span>-l<span class="roman">|</span>-location<span class="roman">]</span> <var>expr</var> <span class="roman">[</span>thread <var>threadnum</var><span class="roman">]</span> <span class="roman">[</span>mask <var>maskvalue</var><span class="roman">]</span></code></dt> |
| <dd><p>Set a watchpoint that will break when the value of <var>expr</var> is read |
| by the program. |
| </p> |
| <a name="index-awatch"></a> |
| </dd> |
| <dt><code>awatch <span class="roman">[</span>-l<span class="roman">|</span>-location<span class="roman">]</span> <var>expr</var> <span class="roman">[</span>thread <var>threadnum</var><span class="roman">]</span> <span class="roman">[</span>mask <var>maskvalue</var><span class="roman">]</span></code></dt> |
| <dd><p>Set a watchpoint that will break when <var>expr</var> is either read from |
| or written into by the program. |
| </p> |
| <a name="index-info-watchpoints-_005bn_2026_005d"></a> |
| </dd> |
| <dt><code>info watchpoints <span class="roman">[</span><var>n</var>…<span class="roman">]</span></code></dt> |
| <dd><p>This command prints a list of watchpoints, using the same format as |
| <code>info break</code> (see <a href="Set-Breaks.html#Set-Breaks">Set Breaks</a>). |
| </p></dd> |
| </dl> |
| |
| <p>If you watch for a change in a numerically entered address you need to |
| dereference it, as the address itself is just a constant number which will |
| never change. <small>GDB</small> refuses to create a watchpoint that watches |
| a never-changing value: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) watch 0x600850 |
| Cannot watch constant value 0x600850. |
| (gdb) watch *(int *) 0x600850 |
| Watchpoint 1: *(int *) 6293584 |
| </pre></div> |
| |
| <p><small>GDB</small> sets a <em>hardware watchpoint</em> if possible. Hardware |
| watchpoints execute very quickly, and the debugger reports a change in |
| value at the exact instruction where the change occurs. If <small>GDB</small> |
| cannot set a hardware watchpoint, it sets a software watchpoint, which |
| executes more slowly and reports the change in value at the next |
| <em>statement</em>, not the instruction, after the change occurs. |
| </p> |
| <a name="index-use-only-software-watchpoints"></a> |
| <p>You can force <small>GDB</small> to use only software watchpoints with the |
| <kbd>set can-use-hw-watchpoints 0</kbd> command. With this variable set to |
| zero, <small>GDB</small> will never try to use hardware watchpoints, even if |
| the underlying system supports them. (Note that hardware-assisted |
| watchpoints that were set <em>before</em> setting |
| <code>can-use-hw-watchpoints</code> to zero will still use the hardware |
| mechanism of watching expression values.) |
| </p> |
| <dl compact="compact"> |
| <dt><code>set can-use-hw-watchpoints</code></dt> |
| <dd><a name="index-set-can_002duse_002dhw_002dwatchpoints"></a> |
| <p>Set whether or not to use hardware watchpoints. |
| </p> |
| </dd> |
| <dt><code>show can-use-hw-watchpoints</code></dt> |
| <dd><a name="index-show-can_002duse_002dhw_002dwatchpoints"></a> |
| <p>Show the current mode of using hardware watchpoints. |
| </p></dd> |
| </dl> |
| |
| <p>For remote targets, you can restrict the number of hardware |
| watchpoints <small>GDB</small> will use, see <a href="Remote-Configuration.html#set-remote-hardware_002dbreakpoint_002dlimit">set remote hardware-breakpoint-limit</a>. |
| </p> |
| <p>When you issue the <code>watch</code> command, <small>GDB</small> reports |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">Hardware watchpoint <var>num</var>: <var>expr</var> |
| </pre></div> |
| |
| <p>if it was able to set a hardware watchpoint. |
| </p> |
| <p>Currently, the <code>awatch</code> and <code>rwatch</code> commands can only set |
| hardware watchpoints, because accesses to data that don’t change the |
| value of the watched expression cannot be detected without examining |
| every instruction as it is being executed, and <small>GDB</small> does not do |
| that currently. If <small>GDB</small> finds that it is unable to set a |
| hardware breakpoint with the <code>awatch</code> or <code>rwatch</code> command, it |
| will print a message like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">Expression cannot be implemented with read/access watchpoint. |
| </pre></div> |
| |
| <p>Sometimes, <small>GDB</small> cannot set a hardware watchpoint because the |
| data type of the watched expression is wider than what a hardware |
| watchpoint on the target machine can handle. For example, some systems |
| can only watch regions that are up to 4 bytes wide; on such systems you |
| cannot set hardware watchpoints for an expression that yields a |
| double-precision floating-point number (which is typically 8 bytes |
| wide). As a work-around, it might be possible to break the large region |
| into a series of smaller ones and watch them with separate watchpoints. |
| </p> |
| <p>If you set too many hardware watchpoints, <small>GDB</small> might be unable |
| to insert all of them when you resume the execution of your program. |
| Since the precise number of active watchpoints is unknown until such |
| time as the program is about to be resumed, <small>GDB</small> might not be |
| able to warn you about this when you set the watchpoints, and the |
| warning will be printed only when the program is resumed: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">Hardware watchpoint <var>num</var>: Could not insert watchpoint |
| </pre></div> |
| |
| <p>If this happens, delete or disable some of the watchpoints. |
| </p> |
| <p>Watching complex expressions that reference many variables can also |
| exhaust the resources available for hardware-assisted watchpoints. |
| That’s because <small>GDB</small> needs to watch every variable in the |
| expression with separately allocated resources. |
| </p> |
| <p>If you call a function interactively using <code>print</code> or <code>call</code>, |
| any watchpoints you have set will be inactive until <small>GDB</small> reaches another |
| kind of breakpoint or the call completes. |
| </p> |
| <p><small>GDB</small> automatically deletes watchpoints that watch local |
| (automatic) variables, or expressions that involve such variables, when |
| they go out of scope, that is, when the execution leaves the block in |
| which these variables were defined. In particular, when the program |
| being debugged terminates, <em>all</em> local variables go out of scope, |
| and so only watchpoints that watch global variables remain set. If you |
| rerun the program, you will need to set all such watchpoints again. One |
| way of doing that would be to set a code breakpoint at the entry to the |
| <code>main</code> function and when it breaks, set all the watchpoints. |
| </p> |
| <a name="index-watchpoints-and-threads"></a> |
| <a name="index-threads-and-watchpoints"></a> |
| <p>In multi-threaded programs, watchpoints will detect changes to the |
| watched expression from every thread. |
| </p> |
| <blockquote> |
| <p><em>Warning:</em> In multi-threaded programs, software watchpoints |
| have only limited usefulness. If <small>GDB</small> creates a software |
| watchpoint, it can only watch the value of an expression <em>in a |
| single thread</em>. If you are confident that the expression can only |
| change due to the current thread’s activity (and if you are also |
| confident that no other thread can become current), then you can use |
| software watchpoints as usual. However, <small>GDB</small> may not notice |
| when a non-current thread’s activity changes the expression. (Hardware |
| watchpoints, in contrast, watch an expression in all threads.) |
| </p></blockquote> |
| |
| <p>See <a href="Remote-Configuration.html#set-remote-hardware_002dwatchpoint_002dlimit">set remote hardware-watchpoint-limit</a>. |
| </p> |
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