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| <a name="String-Field"></a> |
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| Next: <a href="C-Example.html#C-Example" accesskey="n" rel="next">C Example</a>, Previous: <a href="Stabs-Format.html#Stabs-Format" accesskey="p" rel="prev">Stabs Format</a>, Up: <a href="Overview.html#Overview" accesskey="u" rel="up">Overview</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="The-String-Field"></a> |
| <h3 class="section">1.3 The String Field</h3> |
| |
| <p>For most stabs the string field holds the meat of the |
| debugging information. The flexible nature of this field |
| is what makes stabs extensible. For some stab types the string field |
| contains only a name. For other stab types the contents can be a great |
| deal more complex. |
| </p> |
| <p>The overall format of the string field for most stab types is: |
| </p> |
| <div class="example"> |
| <pre class="example">"<var>name</var>:<var>symbol-descriptor</var> <var>type-information</var>" |
| </pre></div> |
| |
| <p><var>name</var> is the name of the symbol represented by the stab; it can |
| contain a pair of colons (see <a href="Nested-Symbols.html#Nested-Symbols">Nested Symbols</a>). <var>name</var> can be |
| omitted, which means the stab represents an unnamed object. For |
| example, ‘<samp>:t10=*2</samp>’ defines type 10 as a pointer to type 2, but does |
| not give the type a name. Omitting the <var>name</var> field is supported by |
| AIX dbx and GDB after about version 4.8, but not other debuggers. GCC |
| sometimes uses a single space as the name instead of omitting the name |
| altogether; apparently that is supported by most debuggers. |
| </p> |
| <p>The <var>symbol-descriptor</var> following the ‘<samp>:</samp>’ is an alphabetic |
| character that tells more specifically what kind of symbol the stab |
| represents. If the <var>symbol-descriptor</var> is omitted, but type |
| information follows, then the stab represents a local variable. For a |
| list of symbol descriptors, see <a href="Symbol-Descriptors.html#Symbol-Descriptors">Symbol Descriptors</a>. The ‘<samp>c</samp>’ |
| symbol descriptor is an exception in that it is not followed by type |
| information. See <a href="Constants.html#Constants">Constants</a>. |
| </p> |
| <p><var>type-information</var> is either a <var>type-number</var>, or |
| ‘<samp><var>type-number</var>=</samp>’. A <var>type-number</var> alone is a type |
| reference, referring directly to a type that has already been defined. |
| </p> |
| <p>The ‘<samp><var>type-number</var>=</samp>’ form is a type definition, where the |
| number represents a new type which is about to be defined. The type |
| definition may refer to other types by number, and those type numbers |
| may be followed by ‘<samp>=</samp>’ and nested definitions. Also, the Lucid |
| compiler will repeat ‘<samp><var>type-number</var>=</samp>’ more than once if it |
| wants to define several type numbers at once. |
| </p> |
| <p>In a type definition, if the character that follows the equals sign is |
| non-numeric then it is a <var>type-descriptor</var>, and tells what kind of |
| type is about to be defined. Any other values following the |
| <var>type-descriptor</var> vary, depending on the <var>type-descriptor</var>. |
| See <a href="Type-Descriptors.html#Type-Descriptors">Type Descriptors</a>, for a list of <var>type-descriptor</var> values. If |
| a number follows the ‘<samp>=</samp>’ then the number is a <var>type-reference</var>. |
| For a full description of types, <a href="Types.html#Types">Types</a>. |
| </p> |
| <p>A <var>type-number</var> is often a single number. The GNU and Sun tools |
| additionally permit a <var>type-number</var> to be a pair |
| (<var>file-number</var>,<var>filetype-number</var>) (the parentheses appear in the |
| string, and serve to distinguish the two cases). The <var>file-number</var> |
| is 0 for the base source file, 1 for the first included file, 2 for the |
| next, and so on. The <var>filetype-number</var> is a number starting with |
| 1 which is incremented for each new type defined in the file. |
| (Separating the file number and the type number permits the |
| <code>N_BINCL</code> optimization to succeed more often; see <a href="Include-Files.html#Include-Files">Include Files</a>). |
| </p> |
| <p>There is an AIX extension for type attributes. Following the ‘<samp>=</samp>’ |
| are any number of type attributes. Each one starts with ‘<samp>@</samp>’ and |
| ends with ‘<samp>;</samp>’. Debuggers, including AIX’s dbx and GDB 4.10, skip |
| any type attributes they do not recognize. GDB 4.9 and other versions |
| of dbx may not do this. Because of a conflict with C<tt>++</tt> |
| (see <a href="Cplusplus.html#Cplusplus">Cplusplus</a>), new attributes should not be defined which begin |
| with a digit, ‘<samp>(</samp>’, or ‘<samp>-</samp>’; GDB may be unable to distinguish |
| those from the C<tt>++</tt> type descriptor ‘<samp>@</samp>’. The attributes are: |
| </p> |
| <dl compact="compact"> |
| <dt><code>a<var>boundary</var></code></dt> |
| <dd><p><var>boundary</var> is an integer specifying the alignment. I assume it |
| applies to all variables of this type. |
| </p> |
| </dd> |
| <dt><code>p<var>integer</var></code></dt> |
| <dd><p>Pointer class (for checking). Not sure what this means, or how |
| <var>integer</var> is interpreted. |
| </p> |
| </dd> |
| <dt><code>P</code></dt> |
| <dd><p>Indicate this is a packed type, meaning that structure fields or array |
| elements are placed more closely in memory, to save memory at the |
| expense of speed. |
| </p> |
| </dd> |
| <dt><code>s<var>size</var></code></dt> |
| <dd><p>Size in bits of a variable of this type. This is fully supported by GDB |
| 4.11 and later. |
| </p> |
| </dd> |
| <dt><code>S</code></dt> |
| <dd><p>Indicate that this type is a string instead of an array of characters, |
| or a bitstring instead of a set. It doesn’t change the layout of the |
| data being represented, but does enable the debugger to know which type |
| it is. |
| </p> |
| </dd> |
| <dt><code>V</code></dt> |
| <dd><p>Indicate that this type is a vector instead of an array. The only |
| major difference between vectors and arrays is that vectors are |
| passed by value instead of by reference (vector coprocessor extension). |
| </p> |
| </dd> |
| </dl> |
| |
| <p>All of this can make the string field quite long. All versions of GDB, |
| and some versions of dbx, can handle arbitrarily long strings. But many |
| versions of dbx (or assemblers or linkers, I’m not sure which) |
| cretinously limit the strings to about 80 characters, so compilers which |
| must work with such systems need to split the <code>.stabs</code> directive |
| into several <code>.stabs</code> directives. Each stab duplicates every field |
| except the string field. The string field of every stab except the last |
| is marked as continued with a backslash at the end (in the assembly code |
| this may be written as a double backslash, depending on the assembler). |
| Removing the backslashes and concatenating the string fields of each |
| stab produces the original, long string. Just to be incompatible (or so |
| they don’t have to worry about what the assembler does with |
| backslashes), AIX can use ‘<samp>?</samp>’ instead of backslash. |
| </p> |
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