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This file documents the GNU Assembler "as".
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<a name="Symbol-Names"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Dot.html#Dot">Dot</a>,
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Previous: <a rel="previous" accesskey="p" href="Setting-Symbols.html#Setting-Symbols">Setting Symbols</a>,
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Up: <a rel="up" accesskey="u" href="Symbols.html#Symbols">Symbols</a>
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</div>
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<h3 class="section">5.3 Symbol Names</h3>
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<p><a name="index-symbol-names-213"></a><a name="index-names_002c-symbol-214"></a>Symbol names begin with a letter or with one of ‘<samp><span class="samp">._</span></samp>’. On most
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machines, you can also use <code>$</code> in symbol names; exceptions are
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noted in <a href="Machine-Dependencies.html#Machine-Dependencies">Machine Dependencies</a>. That character may be followed by any
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string of digits, letters, dollar signs (unless otherwise noted for a
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particular target machine), and underscores.
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<p>Case of letters is significant: <code>foo</code> is a different symbol name
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than <code>Foo</code>.
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<p>Symbol names do not start with a digit. An exception to this rule is made for
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Local Labels. See below.
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<p>Multibyte characters are supported. To generate a symbol name containing
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multibyte characters enclose it within double quotes and use escape codes. cf
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See <a href="Strings.html#Strings">Strings</a>. Generating a multibyte symbol name from a label is not
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currently supported.
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<p>Each symbol has exactly one name. Each name in an assembly language program
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refers to exactly one symbol. You may use that symbol name any number of times
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in a program.
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<h4 class="subheading">Local Symbol Names</h4>
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<p><a name="index-local-symbol-names-215"></a><a name="index-symbol-names_002c-local-216"></a>A local symbol is any symbol beginning with certain local label prefixes.
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By default, the local label prefix is ‘<samp><span class="samp">.L</span></samp>’ for ELF systems or
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‘<samp><span class="samp">L</span></samp>’ for traditional a.out systems, but each target may have its own
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set of local label prefixes.
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On the HPPA local symbols begin with ‘<samp><span class="samp">L$</span></samp>’.
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<p>Local symbols are defined and used within the assembler, but they are
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normally not saved in object files. Thus, they are not visible when debugging.
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You may use the ‘<samp><span class="samp">-L</span></samp>’ option (see <a href="L.html#L">Include Local Symbols</a>)
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to retain the local symbols in the object files.
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<h4 class="subheading">Local Labels</h4>
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<p><a name="index-local-labels-217"></a><a name="index-temporary-symbol-names-218"></a><a name="index-symbol-names_002c-temporary-219"></a>Local labels are different from local symbols. Local labels help compilers and
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programmers use names temporarily. They create symbols which are guaranteed to
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be unique over the entire scope of the input source code and which can be
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referred to by a simple notation. To define a local label, write a label of
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the form ‘<samp><b>N</b><span class="samp">:</span></samp>’ (where <b>N</b> represents any non-negative integer).
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To refer to the most recent previous definition of that label write
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‘<samp><b>N</b><span class="samp">b</span></samp>’, using the same number as when you defined the label. To refer
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to the next definition of a local label, write ‘<samp><b>N</b><span class="samp">f</span></samp>’. The ‘<samp><span class="samp">b</span></samp>’
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stands for “backwards” and the ‘<samp><span class="samp">f</span></samp>’ stands for “forwards”.
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<p>There is no restriction on how you can use these labels, and you can reuse them
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too. So that it is possible to repeatedly define the same local label (using
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the same number ‘<samp><b>N</b></samp>’), although you can only refer to the most recently
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defined local label of that number (for a backwards reference) or the next
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definition of a specific local label for a forward reference. It is also worth
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noting that the first 10 local labels (‘<samp><b>0:</b></samp>’<small class="dots">...</small>‘<samp><b>9:</b></samp>’) are
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implemented in a slightly more efficient manner than the others.
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<p>Here is an example:
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<pre class="smallexample"> 1: branch 1f
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2: branch 1b
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1: branch 2f
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2: branch 1b
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</pre>
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<p>Which is the equivalent of:
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<pre class="smallexample"> label_1: branch label_3
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label_2: branch label_1
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label_3: branch label_4
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label_4: branch label_3
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</pre>
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<p>Local label names are only a notational device. They are immediately
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transformed into more conventional symbol names before the assembler uses them.
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The symbol names are stored in the symbol table, appear in error messages, and
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are optionally emitted to the object file. The names are constructed using
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these parts:
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<dl>
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<dt><em>local label prefix</em><dd>All local symbols begin with the system-specific local label prefix.
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Normally both <samp><span class="command">as</span></samp> and <code>ld</code> forget symbols
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that start with the local label prefix. These labels are
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used for symbols you are never intended to see. If you use the
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‘<samp><span class="samp">-L</span></samp>’ option then <samp><span class="command">as</span></samp> retains these symbols in the
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object file. If you also instruct <code>ld</code> to retain these symbols,
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you may use them in debugging.
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<br><dt><var>number</var><dd>This is the number that was used in the local label definition. So if the
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label is written ‘<samp><span class="samp">55:</span></samp>’ then the number is ‘<samp><span class="samp">55</span></samp>’.
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<br><dt><kbd>C-B</kbd><dd>This unusual character is included so you do not accidentally invent a symbol
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of the same name. The character has ASCII value of ‘<samp><span class="samp">\002</span></samp>’ (control-B).
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<br><dt><em>ordinal number</em><dd>This is a serial number to keep the labels distinct. The first definition of
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‘<samp><span class="samp">0:</span></samp>’ gets the number ‘<samp><span class="samp">1</span></samp>’. The 15th definition of ‘<samp><span class="samp">0:</span></samp>’ gets the
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number ‘<samp><span class="samp">15</span></samp>’, and so on. Likewise the first definition of ‘<samp><span class="samp">1:</span></samp>’ gets
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the number ‘<samp><span class="samp">1</span></samp>’ and its 15th definition gets ‘<samp><span class="samp">15</span></samp>’ as well.
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</dl>
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<p>So for example, the first <code>1:</code> may be named <code>.L1</code><kbd>C-B</kbd><code>1</code>, and
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the 44th <code>3:</code> may be named <code>.L3</code><kbd>C-B</kbd><code>44</code>.
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<h4 class="subheading">Dollar Local Labels</h4>
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<p><a name="index-dollar-local-symbols-220"></a>
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On some targets <code>as</code> also supports an even more local form of
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local labels called dollar labels. These labels go out of scope (i.e., they
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become undefined) as soon as a non-local label is defined. Thus they remain
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valid for only a small region of the input source code. Normal local labels,
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by contrast, remain in scope for the entire file, or until they are redefined
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by another occurrence of the same local label.
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<p>Dollar labels are defined in exactly the same way as ordinary local labels,
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except that they have a dollar sign suffix to their numeric value, e.g.,
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‘<samp><b>55$:</b></samp>’.
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<p>They can also be distinguished from ordinary local labels by their transformed
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names which use ASCII character ‘<samp><span class="samp">\001</span></samp>’ (control-A) as the magic character
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to distinguish them from ordinary labels. For example, the fifth definition of
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‘<samp><span class="samp">6$</span></samp>’ may be named ‘<samp><span class="samp">.L6</span><kbd>C-A</kbd><span class="samp">5</span></samp>’.
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</body></html>
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