116 lines
5.9 KiB
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116 lines
5.9 KiB
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<title>Tracepoint Restrictions - Debugging with GDB</title>
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<link rel="up" href="Set-Tracepoints.html#Set-Tracepoints" title="Set Tracepoints">
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<link rel="prev" href="Starting-and-Stopping-Trace-Experiments.html#Starting-and-Stopping-Trace-Experiments" title="Starting and Stopping Trace Experiments">
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<div class="node">
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<a name="Tracepoint-Restrictions"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Starting-and-Stopping-Trace-Experiments.html#Starting-and-Stopping-Trace-Experiments">Starting and Stopping Trace Experiments</a>,
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Up: <a rel="up" accesskey="u" href="Set-Tracepoints.html#Set-Tracepoints">Set Tracepoints</a>
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<hr>
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</div>
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<h4 class="subsection">13.1.10 Tracepoint Restrictions</h4>
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<p><a name="index-tracepoint-restrictions-926"></a>There are a number of restrictions on the use of tracepoints. As
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described above, tracepoint data gathering occurs on the target
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without interaction from <span class="sc">gdb</span>. Thus the full capabilities of
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the debugger are not available during data gathering, and then at data
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examination time, you will be limited by only having what was
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collected. The following items describe some common problems, but it
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is not exhaustive, and you may run into additional difficulties not
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mentioned here.
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<ul>
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<li>Tracepoint expressions are intended to gather objects (lvalues). Thus
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the full flexibility of GDB's expression evaluator is not available.
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You cannot call functions, cast objects to aggregate types, access
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convenience variables or modify values (except by assignment to trace
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state variables). Some language features may implicitly call
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functions (for instance Objective-C fields with accessors), and therefore
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cannot be collected either.
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<li>Collection of local variables, either individually or in bulk with
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<code>$locals</code> or <code>$args</code>, during <code>while-stepping</code> may
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behave erratically. The stepping action may enter a new scope (for
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instance by stepping into a function), or the location of the variable
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may change (for instance it is loaded into a register). The
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tracepoint data recorded uses the location information for the
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variables that is correct for the tracepoint location. When the
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tracepoint is created, it is not possible, in general, to determine
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where the steps of a <code>while-stepping</code> sequence will advance the
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program—particularly if a conditional branch is stepped.
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<li>Collection of an incompletely-initialized or partially-destroyed object
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may result in something that <span class="sc">gdb</span> cannot display, or displays
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in a misleading way.
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<li>When <span class="sc">gdb</span> displays a pointer to character it automatically
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dereferences the pointer to also display characters of the string
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being pointed to. However, collecting the pointer during tracing does
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not automatically collect the string. You need to explicitly
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dereference the pointer and provide size information if you want to
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collect not only the pointer, but the memory pointed to. For example,
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<code>*ptr@50</code> can be used to collect the 50 element array pointed to
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by <code>ptr</code>.
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<li>It is not possible to collect a complete stack backtrace at a
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tracepoint. Instead, you may collect the registers and a few hundred
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bytes from the stack pointer with something like <code>*(unsigned char *)$esp@300</code>
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(adjust to use the name of the actual stack pointer register on your
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target architecture, and the amount of stack you wish to capture).
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Then the <code>backtrace</code> command will show a partial backtrace when
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using a trace frame. The number of stack frames that can be examined
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depends on the sizes of the frames in the collected stack. Note that
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if you ask for a block so large that it goes past the bottom of the
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stack, the target agent may report an error trying to read from an
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invalid address.
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<li>If you do not collect registers at a tracepoint, <span class="sc">gdb</span> can
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infer that the value of <code>$pc</code> must be the same as the address of
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the tracepoint and use that when you are looking at a trace frame
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for that tracepoint. However, this cannot work if the tracepoint has
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multiple locations (for instance if it was set in a function that was
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inlined), or if it has a <code>while-stepping</code> loop. In those cases
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<span class="sc">gdb</span> will warn you that it can't infer <code>$pc</code>, and default
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it to zero.
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</ul>
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