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148 lines
7.5 KiB
HTML
148 lines
7.5 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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<head>
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<title>Tracepoint Restrictions (Debugging with GDB)</title>
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<meta name="description" content="Tracepoint Restrictions (Debugging with GDB)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Set-Tracepoints.html#Set-Tracepoints" rel="up" title="Set Tracepoints">
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<link href="Analyze-Collected-Data.html#Analyze-Collected-Data" rel="next" title="Analyze Collected Data">
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<link href="Starting-and-Stopping-Trace-Experiments.html#Starting-and-Stopping-Trace-Experiments" rel="prev" title="Starting and Stopping Trace Experiments">
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</head>
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<body lang="en">
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<a name="Tracepoint-Restrictions"></a>
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<div class="header">
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<p>
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Previous: <a href="Starting-and-Stopping-Trace-Experiments.html#Starting-and-Stopping-Trace-Experiments" accesskey="p" rel="prev">Starting and Stopping Trace Experiments</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>
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</div>
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<hr>
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<a name="Tracepoint-Restrictions-1"></a>
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<h4 class="subsection">13.1.10 Tracepoint Restrictions</h4>
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<a name="index-tracepoint-restrictions"></a>
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<p>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 <small>GDB</small>. 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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</p>
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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><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><li> Collection of an incompletely-initialized or partially-destroyed object
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may result in something that <small>GDB</small> cannot display, or displays
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in a misleading way.
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</li><li> When <small>GDB</small> 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><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 *)$esp@300</code>
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(adjust to use the name of the actual stack pointer register on your
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target architecture, and the amount of stack you wish to capture).
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Then the <code>backtrace</code> command will show a partial backtrace when
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using a trace frame. The number of stack frames that can be examined
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depends on the sizes of the frames in the collected stack. Note that
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if you ask for a block so large that it goes past the bottom of the
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stack, the target agent may report an error trying to read from an
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invalid address.
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</li><li> If you do not collect registers at a tracepoint, <small>GDB</small> can
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infer that the value of <code>$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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<small>GDB</small> will warn you that it can’t infer <code>$pc</code>, and default
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it to zero.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Starting-and-Stopping-Trace-Experiments.html#Starting-and-Stopping-Trace-Experiments" accesskey="p" rel="prev">Starting and Stopping Trace Experiments</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>
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</div>
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</body>
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</html>
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