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<h4 class="subsection">2.6.2 Input for Mapped Applications</h4>

<p>Applications invoked using the <samp><span class="option">--map-to-each-file</span></samp> option
<a name="index-g_t_0040code_007bmap_002dto_002deach_002dfile_007d-command-line-option-219"></a>benefit from a slightly different interface than the one described
above. As the purpose of this option is to save memory by loading only
one file at a time, the application does not have access to all input
files named on the command line simultaneously within the same data
structure. Although the data structure is of the type already described,
the <var>files</var> field is not a list of arbitrary length. Instead, it is
a list containing exactly one item for an input file. If <kbd>-</kbd>
is used as a command line parameter, indicating standard input, then
<var>files</var> will have another item pertaining to standard
input. In no event will it have other than one or two items.

   <p>The mapped application is expected to work by being applied individually
to each of any number of separately constructed data structures, doing
the same in each case as it would if that case were the only one. To make
that possible, copies of the environment variables, the contents of standard
input, and the list of application specific options are contained in the
data structure used for every invocation.

   <p><a name="index-command-line-220"></a>The position numbers in the options are adjusted for each invocation to
reflect the position of the particular input file associated with it. 
For example, in the following command
<pre class="display">     <kbd>avram --map-to-each-file mapster.avm fa.txt --data fb.dat --code fc.o</kbd>
</pre>
   <p class="noindent">the function in the virtual code file <samp><span class="file">mapster.avm</span></samp> would be
applied to each of three data structures, corresponding to the commands

<pre class="display">     <kbd>avram mapster.avm fa.txt --data --code</kbd>
     <kbd>avram mapster.avm --data fb.dat --code</kbd>
     <kbd>avram mapster.avm --data --code fc.o</kbd>
</pre>
   <p class="noindent">If the relative positions of the options and filenames were important to
the application, they could be reliably inferred from the position
numbers. In the first case, they would be 1 and 2, implying that the
file is in position 0. In the second case they would be 0 and 2,
implying that the file is in position 1, and in the third case they
would be 0 and 1, implying that the file is in position 2. (Of course,
nothing compels an application to concern itself with the positions of
its parameters, and the alternative might be preferable.)

   <p>For the most part, any application that can operate on one file at a
time without needing information from any others can be executed more
economically with the <samp><span class="option">--map-to-each-file</span></samp> option and few if any
changes to the code. The effect will normally be analogous to the above
example, subject to a few possible differences.

     <ul>
<li>If an application is supposed to do something by default when
there are no file parameters or only standard input, it won't work as a
mapped application, because if there are no file parameters it won't be
executed at all. 
<li>If a mapped application causes any output files to be generated, they
may be written before other input files are read, possibly causing the
input files to be overwritten if they have the same names, and causing
subsequent invocations to use the overwritten versions. This behavior
differs from that of loading all input files at the outset, which
ensures the application seeing all of the original versions. The latter may be
more convenient for maintaining a group of files in some sort of
consistent state. 
<li>If an application causes standard output to be written along with
output files, normally standard output is written last as a security
measure against malicious code altering the <samp><span class="option">--ask-to-overwrite</span></samp>
prompts by subtly clobbering the console. In a mapped application,
standard output isn't always last because there may be more invocations
to come. 
</ul>

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