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<chapter>
  <header>
    <copyright>
      <year>2001</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    </legalnotice>

    <title>Common Caveats</title>
    <prepared>Bjorn Gustavsson</prepared>
    <docno></docno>
    <date>2001-08-08</date>
    <rev></rev>
    <file>commoncaveats.xml</file>
  </header>

  <p>This section lists a few modules and BIFs to watch out for, not only
  from a performance point of view.</p>

  <section>
     <title>Timer Module</title>

     <p>Creating timers using <seealso
     marker="erts:erlang#erlang:send_after/3">erlang:send_after/3</seealso>
     and
     <seealso marker="erts:erlang#erlang:start_timer/3">erlang:start_timer/3</seealso>
,
     is much more efficient than using the timers provided by the
     <seealso marker="stdlib:timer">timer</seealso> module in STDLIB.
     The <c>timer</c> module uses a separate process to manage the timers.
     That process can easily become overloaded if many processes
     create and cancel timers frequently (especially when using the
     SMP emulator).</p>

     <p>The functions in the <c>timer</c> module that do not manage timers
     (such as <c>timer:tc/3</c> or <c>timer:sleep/1</c>), do not call the
     timer-server process and are therefore harmless.</p>
  </section>

  <section>
    <title>list_to_atom/1</title>

    <p>Atoms are not garbage-collected. Once an atom is created, it is never
    removed. The emulator terminates if the limit for the number
    of atoms (1,048,576 by default) is reached.</p>

    <p>Therefore, converting arbitrary input strings to atoms can be
    dangerous in a system that runs continuously.
    If only certain well-defined atoms are allowed as input,
    <seealso marker="erts:erlang#list_to_existing_atom/1">list_to_existing_atom/1</seealso>
    can be used to
    to guard against a denial-of-service attack. (All atoms that are allowed
    must have been created earlier, for example, by simply using all of them
    in a module and loading that module.)</p>

    <p>Using <c>list_to_atom/1</c> to construct an atom that is passed to
    <c>apply/3</c> as follows, is quite expensive and not recommended
    in time-critical code:</p>
    <code type="erl">
apply(list_to_atom("some_prefix"++Var), foo, Args)</code>
  </section>

  <section>
    <title>length/1</title>

    <p>The time for calculating the length of a list is proportional to the
    length of the list, as opposed to <c>tuple_size/1</c>, <c>byte_size/1</c>,
    and <c>bit_size/1</c>, which all execute in constant time.</p>

    <p>Normally, there is no need to worry about the speed of <c>length/1</c>,
    because it is efficiently implemented in C. In time-critical code,
    you might want to avoid it if the input list could potentially be very
    long.</p>

    <p>Some uses of <c>length/1</c> can be replaced by matching.
    For example, the following code:</p>
    <code type="erl">
foo(L) when length(L) >= 3 ->
    ...</code>    

    <p>can be rewritten to:</p>
    <code type="erl">
foo([_,_,_|_]=L) ->
   ...</code>    

   <p>One slight difference is that <c>length(L)</c> fails if <c>L</c>
   is an improper list, while the pattern in the second code fragment
   accepts an improper list.</p>
  </section>

  <section>
    <title>setelement/3</title>

    <p><seealso marker="erts:erlang#setelement/3">setelement/3</seealso>
    copies the tuple it modifies. Therefore, updating a tuple in a loop
    using <c>setelement/3</c> creates a new copy of the tuple every time.</p>
    
    <p>There is one exception to the rule that the tuple is copied.
    If the compiler clearly can see that destructively updating the tuple would
    give the same result as if the tuple was copied, the call to
    <c>setelement/3</c> is replaced with a special destructive <c>setelement</c>
    instruction. In the following code sequence, the first <c>setelement/3</c>
    call copies the tuple and modifies the ninth element:</p>
    <code type="erl">
multiple_setelement(T0) ->
    T1 = setelement(9, T0, bar),
    T2 = setelement(7, T1, foobar),
    setelement(5, T2, new_value).</code>    

    <p>The two following <c>setelement/3</c> calls modify
    the tuple in place.</p>

    <p>For the optimization to be applied, <em>all</em> the followings conditions
    must be true:</p>

    <list type="bulleted">
    <item>The indices must be integer literals, not variables or expressions.</item>
    <item>The indices must be given in descending order.</item>
    <item>There must be no calls to another function in between the calls to
    <c>setelement/3</c>.</item>
    <item>The tuple returned from one <c>setelement/3</c> call must only be used
    in the subsequent call to <c>setelement/3</c>.</item>
    </list>

    <p>If the code cannot be structured as in the <c>multiple_setelement/1</c>
    example, the best way to modify multiple elements in a large tuple is to
    convert the tuple to a list, modify the list, and convert it back to
    a tuple.</p>
  </section>

  <section>
    <title>size/1</title>

    <p><c>size/1</c> returns the size for both tuples and binaries.</p>

    <p>Using the BIFs <c>tuple_size/1</c> and <c>byte_size/1</c>
    gives the compiler and the runtime system more opportunities for
    optimization. Another advantage is that the BIFs give Dialyzer more
    type information.</p>
  </section>

  <section>
    <title>split_binary/2</title>
      <p>It is usually more efficient to split a binary using matching
      instead of calling the <c>split_binary/2</c> function.
      Furthermore, mixing bit syntax matching and <c>split_binary/2</c>
      can prevent some optimizations of bit syntax matching.</p>

        <p><em>DO</em></p>
        <code type="none"><![CDATA[
        <<Bin1:Num/binary,Bin2/binary>> = Bin,]]></code>
        <p><em>DO NOT</em></p>
        <code type="none">
        {Bin1,Bin2} = split_binary(Bin, Num)</code>
   </section>

  <section>
    <title>Operator "--"</title>
     <p>The "<c>--</c>" operator has a complexity
     proportional to the product of the length of its operands.
     This means that the operator is very slow if both of its operands
     are long lists:</p>

        <p><em>DO NOT</em></p>
        <code type="none"><![CDATA[
        HugeList1 -- HugeList2]]></code>

     <p>Instead use the <seealso marker="stdlib:ordsets">ordsets</seealso>
     module in STDLIB:</p>

        <p><em>DO</em></p>
        <code type="none">
        HugeSet1 = ordsets:from_list(HugeList1),
        HugeSet2 = ordsets:from_list(HugeList2),
        ordsets:subtract(HugeSet1, HugeSet2)</code>

     <p>Obviously, that code does not work if the original order
     of the list is important. If the order of the list must be
     preserved, do as follows:</p>

        <p><em>DO</em></p>
        <code type="none"><![CDATA[
        Set = gb_sets:from_list(HugeList2),
        [E || E <- HugeList1, not gb_sets:is_element(E, Set)]]]></code>

     <note><p>This code behaves differently from "<c>--</c>"
     if the lists contain duplicate elements (one occurrence
     of an element in HugeList2 removes <em>all</em>
     occurrences in HugeList1.)</p>
     <p>Also, this code compares lists elements using the
     "<c>==</c>" operator, while "<c>--</c>" uses the "<c>=:=</c>" operator.
     If that difference is important, <c>sets</c> can be used instead of
     <c>gb_sets</c>, but <c>sets:from_list/1</c> is much
     slower than <c>gb_sets:from_list/1</c> for long lists.</p></note>

     <p>Using the "<c>--</c>" operator to delete an element
     from a list is not a performance problem:</p>

        <p><em>OK</em></p>
        <code type="none">
        HugeList1 -- [Element]</code>

   </section>

</chapter>

