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