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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Low-Level Traps (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="Low_002dLevel-Traps">
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Next: <a href="Tracing-Traps.html" accesskey="n" rel="next">Tracing Traps</a>, Previous: <a href="Trap-Interface.html" accesskey="p" rel="prev">Trap Interface</a>, Up: <a href="Traps.html" accesskey="u" rel="up">Traps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<h4 class="subsubsection" id="Low_002dLevel-Traps-1"><span>6.26.4.3 Low-Level Traps<a class="copiable-link" href="#Low_002dLevel-Traps-1"> ¶</a></span></h4>
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<p>To summarize the last sections, traps are enabled or disabled, and when
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they are enabled, they add to various VM hooks.
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</p>
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<p>Note, however, that <em class="emph">traps do not increase the VM trace level</em>. So
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if you create a trap, it will be enabled, but unless something else
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increases the VM’s trace level (see <a class="pxref" href="VM-Hooks.html">VM Hooks</a>), the trap will not
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fire. It turns out that getting the VM trace level right is tricky
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without a global view of what traps are enabled. See <a class="xref" href="Trap-States.html">Trap States</a>,
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for Guile’s answer to this problem.
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</p>
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<p>Traps are created by calling procedures. Most of these procedures share
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a set of common keyword arguments, so rather than document them
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separately, we discuss them all together here:
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</p>
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<dl class="table">
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<dt><code class="code">#:vm</code></dt>
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<dd><p>The VM to instrument. Defaults to the current thread’s VM.
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</p></dd>
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<dt><code class="code">#:current-frame</code></dt>
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<dd><p>For traps that enable more hooks depending on their dynamic context,
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this argument gives the current frame that the trap is running in.
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Defaults to <code class="code">#f</code>.
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</p></dd>
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</dl>
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<p>To have access to these procedures, you’ll need to have imported the
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<code class="code">(system vm traps)</code> module:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (system vm traps))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dat_002dprocedure_002dcall"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-at-procedure-call</strong> <var class="def-var-arguments">proc handler [#:vm]</var><a class="copiable-link" href="#index-trap_002dat_002dprocedure_002dcall"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">handler</var> when <var class="var">proc</var> is applied.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002din_002dprocedure"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-in-procedure</strong> <var class="def-var-arguments">proc enter-handler exit-handler [#:current-frame] [#:vm]</var><a class="copiable-link" href="#index-trap_002din_002dprocedure"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">enter-handler</var> when control enters <var class="var">proc</var>,
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and <var class="var">exit-handler</var> when control leaves <var class="var">proc</var>.
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</p>
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<p>Control can enter a procedure via:
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</p><ul class="itemize mark-bullet">
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<li>A procedure call.
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</li><li>A return to a procedure’s frame on the stack.
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</li><li>A continuation returning directly to an application of this procedure.
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</li></ul>
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<p>Control can leave a procedure via:
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</p><ul class="itemize mark-bullet">
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<li>A normal return from the procedure.
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</li><li>An application of another procedure.
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</li><li>An invocation of a continuation.
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</li><li>An abort.
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</li></ul>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dinstructions_002din_002dprocedure"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-instructions-in-procedure</strong> <var class="def-var-arguments">proc next-handler exit-handler [#:current-frame] [#:vm]</var><a class="copiable-link" href="#index-trap_002dinstructions_002din_002dprocedure"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">next-handler</var> for every instruction executed in
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<var class="var">proc</var>, and <var class="var">exit-handler</var> when execution leaves <var class="var">proc</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dat_002dprocedure_002dip_002din_002drange"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-at-procedure-ip-in-range</strong> <var class="def-var-arguments">proc range handler [#:current-frame] [#:vm]</var><a class="copiable-link" href="#index-trap_002dat_002dprocedure_002dip_002din_002drange"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">handler</var> when execution enters a range of
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instructions in <var class="var">proc</var>. <var class="var">range</var> is a simple of pairs,
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<code class="code">((<var class="var">start</var> . <var class="var">end</var>) ...)</code>. The <var class="var">start</var> addresses are
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inclusive, and <var class="var">end</var> addresses are exclusive.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dat_002dsource_002dlocation"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-at-source-location</strong> <var class="def-var-arguments">file user-line handler [#:current-frame] [#:vm]</var><a class="copiable-link" href="#index-trap_002dat_002dsource_002dlocation"> ¶</a></span></dt>
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<dd><p>A trap that fires when control reaches a given source location. The
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<var class="var">user-line</var> parameter is one-indexed, as a user counts lines,
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instead of zero-indexed, as Guile counts lines.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dframe_002dfinish"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-frame-finish</strong> <var class="def-var-arguments">frame return-handler abort-handler [#:vm]</var><a class="copiable-link" href="#index-trap_002dframe_002dfinish"> ¶</a></span></dt>
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<dd><p>A trap that fires when control leaves the given frame. <var class="var">frame</var>
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should be a live frame in the current continuation. <var class="var">return-handler</var>
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will be called on a normal return, and <var class="var">abort-handler</var> on a nonlocal
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exit.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002din_002ddynamic_002dextent"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-in-dynamic-extent</strong> <var class="def-var-arguments">proc enter-handler return-handler abort-handler [#:vm]</var><a class="copiable-link" href="#index-trap_002din_002ddynamic_002dextent"> ¶</a></span></dt>
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<dd><p>A more traditional dynamic-wind trap, which fires <var class="var">enter-handler</var>
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when control enters <var class="var">proc</var>, <var class="var">return-handler</var> on a normal return,
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and <var class="var">abort-handler</var> on a nonlocal exit.
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</p>
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<p>Note that rewinds are not handled, so there is no rewind handler.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dcalls_002din_002ddynamic_002dextent"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-calls-in-dynamic-extent</strong> <var class="def-var-arguments">proc apply-handler return-handler [#:current-frame] [#:vm]</var><a class="copiable-link" href="#index-trap_002dcalls_002din_002ddynamic_002dextent"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">apply-handler</var> every time a procedure is applied,
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and <var class="var">return-handler</var> for returns, but only during the dynamic extent
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of an application of <var class="var">proc</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dinstructions_002din_002ddynamic_002dextent"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-instructions-in-dynamic-extent</strong> <var class="def-var-arguments">proc next-handler [#:current-frame] [#:vm]</var><a class="copiable-link" href="#index-trap_002dinstructions_002din_002ddynamic_002dextent"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">next-handler</var> for all retired instructions within
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the dynamic extent of a call to <var class="var">proc</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dcalls_002dto_002dprocedure"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-calls-to-procedure</strong> <var class="def-var-arguments">proc apply-handler return-handler [#:vm]</var><a class="copiable-link" href="#index-trap_002dcalls_002dto_002dprocedure"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">apply-handler</var> whenever <var class="var">proc</var> is applied,
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and <var class="var">return-handler</var> when it returns, but with an additional
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argument, the call depth.
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</p>
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<p>That is to say, the handlers will get two arguments: the frame in
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question, and the call depth (a non-negative integer).
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-trap_002dmatching_002dinstructions"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">trap-matching-instructions</strong> <var class="def-var-arguments">frame-pred handler [#:vm]</var><a class="copiable-link" href="#index-trap_002dmatching_002dinstructions"> ¶</a></span></dt>
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<dd><p>A trap that calls <var class="var">frame-pred</var> at every instruction, and if
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<var class="var">frame-pred</var> returns a true value, calls <var class="var">handler</var> on the
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frame.
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</p></dd></dl>
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<p>
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Next: <a href="Tracing-Traps.html">Tracing Traps</a>, Previous: <a href="Trap-Interface.html">Trap Interface</a>, Up: <a href="Traps.html">Traps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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