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<h3 class="section">1.5 Input Files</h3>
<p><a name="index-input-29"></a><a name="index-source-program-30"></a><a name="index-files_002c-input-31"></a>We use the phrase <dfn>source program</dfn>, abbreviated <dfn>source</dfn>, to
describe the program input to one run of <span class="command">as</span>. The program may
be in one or more files; how the source is partitioned into files
doesn't change the meaning of the source.
<!-- I added "con" prefix to "catenation" just to prove I can overcome my -->
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<p>The source program is a concatenation of the text in all the files, in the
order specified.
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<p>Each time you run <span class="command">as</span> it assembles exactly one source
program. The source program is made up of one or more files.
(The standard input is also a file.)
<p>You give <span class="command">as</span> a command line that has zero or more input file
names. The input files are read (from left file name to right). A
command line argument (in any position) that has no special meaning
is taken to be an input file name.
<p>If you give <span class="command">as</span> no file names it attempts to read one input file
from the <span class="command">as</span> standard input, which is normally your terminal. You
may have to type <ctl-D> to tell <span class="command">as</span> there is no more program
to assemble.
<p>Use <span class="samp">--</span> if you need to explicitly name the standard input file
in your command line.
<p>If the source is empty, <span class="command">as</span> produces a small, empty object
file.
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<h4 class="subheading">Filenames and Line-numbers</h4>
<p><a name="index-input-file-linenumbers-32"></a><a name="index-line-numbers_002c-in-input-files-33"></a>There are two ways of locating a line in the input file (or files) and
either may be used in reporting error messages. One way refers to a line
number in a physical file; the other refers to a line number in a
“logical” file. See <a href="Errors.html#Errors">Error and Warning Messages</a>.
<p><dfn>Physical files</dfn> are those files named in the command line given
to <span class="command">as</span>.
<p><dfn>Logical files</dfn> are simply names declared explicitly by assembler
directives; they bear no relation to physical files. Logical file names help
error messages reflect the original source file, when <span class="command">as</span> source
is itself synthesized from other files. <span class="command">as</span> understands the
<span class="samp">#</span> directives emitted by the <code>gcc</code> preprocessor. See also
<a href="File.html#File"><code>.file</code></a>.
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