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| <a name="Procedures"></a> |
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| Next: <a href="Nested-Procedures.html#Nested-Procedures" accesskey="n" rel="next">Nested Procedures</a>, Previous: <a href="Line-Numbers.html#Line-Numbers" accesskey="p" rel="prev">Line Numbers</a>, Up: <a href="Program-Structure.html#Program-Structure" accesskey="u" rel="up">Program Structure</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="Procedures-1"></a> |
| <h3 class="section">2.5 Procedures</h3> |
| |
| <a name="index-N_005fFUN_002c-for-functions"></a> |
| <a name="index-N_005fFNAME"></a> |
| <a name="index-N_005fSTSYM_002c-for-functions-_0028Sun-acc_0029"></a> |
| <a name="index-N_005fGSYM_002c-for-functions-_0028Sun-acc_0029"></a> |
| <p>All of the following stabs normally use the <code>N_FUN</code> symbol type. |
| However, Sun’s <code>acc</code> compiler on SunOS4 uses <code>N_GSYM</code> and |
| <code>N_STSYM</code>, which means that the value of the stab for the function |
| is useless and the debugger must get the address of the function from |
| the non-stab symbols instead. On systems where non-stab symbols have |
| leading underscores, the stabs will lack underscores and the debugger |
| needs to know about the leading underscore to match up the stab and the |
| non-stab symbol. BSD Fortran is said to use <code>N_FNAME</code> with the |
| same restriction; the value of the symbol is not useful (I’m not sure it |
| really does use this, because GDB doesn’t handle this and no one has |
| complained). |
| </p> |
| <a name="index-C_005fFUN"></a> |
| <p>A function is represented by an ‘<samp>F</samp>’ symbol descriptor for a global |
| (extern) function, and ‘<samp>f</samp>’ for a static (local) function. For |
| a.out, the value of the symbol is the address of the start of the |
| function; it is already relocated. For stabs in ELF, the SunPRO |
| compiler version 2.0.1 and GCC put out an address which gets relocated |
| by the linker. In a future release SunPRO is planning to put out zero, |
| in which case the address can be found from the ELF (non-stab) symbol. |
| Because looking things up in the ELF symbols would probably be slow, I’m |
| not sure how to find which symbol of that name is the right one, and |
| this doesn’t provide any way to deal with nested functions, it would |
| probably be better to make the value of the stab an address relative to |
| the start of the file, or just absolute. See <a href="ELF-Linker-Relocation.html#ELF-Linker-Relocation">ELF Linker Relocation</a> for more information on linker relocation of stabs in ELF |
| files. For XCOFF, the stab uses the <code>C_FUN</code> storage class and the |
| value of the stab is meaningless; the address of the function can be |
| found from the csect symbol (XTY_LD/XMC_PR). |
| </p> |
| <p>The type information of the stab represents the return type of the |
| function; thus ‘<samp>foo:f5</samp>’ means that foo is a function returning type |
| 5. There is no need to try to get the line number of the start of the |
| function from the stab for the function; it is in the next |
| <code>N_SLINE</code> symbol. |
| </p> |
| <p>Some compilers (such as Sun’s Solaris compiler) support an extension for |
| specifying the types of the arguments. I suspect this extension is not |
| used for old (non-prototyped) function definitions in C. If the |
| extension is in use, the type information of the stab for the function |
| is followed by type information for each argument, with each argument |
| preceded by ‘<samp>;</samp>’. An argument type of 0 means that additional |
| arguments are being passed, whose types and number may vary (‘<samp>...</samp>’ |
| in ANSI C). GDB has tolerated this extension (parsed the syntax, if not |
| necessarily used the information) since at least version 4.8; I don’t |
| know whether all versions of dbx tolerate it. The argument types given |
| here are not redundant with the symbols for the formal parameters |
| (see <a href="Parameters.html#Parameters">Parameters</a>); they are the types of the arguments as they are |
| passed, before any conversions might take place. For example, if a C |
| function which is declared without a prototype takes a <code>float</code> |
| argument, the value is passed as a <code>double</code> but then converted to a |
| <code>float</code>. Debuggers need to use the types given in the arguments |
| when printing values, but when calling the function they need to use the |
| types given in the symbol defining the function. |
| </p> |
| <p>If the return type and types of arguments of a function which is defined |
| in another source file are specified (i.e., a function prototype in ANSI |
| C), traditionally compilers emit no stab; the only way for the debugger |
| to find the information is if the source file where the function is |
| defined was also compiled with debugging symbols. As an extension the |
| Solaris compiler uses symbol descriptor ‘<samp>P</samp>’ followed by the return |
| type of the function, followed by the arguments, each preceded by |
| ‘<samp>;</samp>’, as in a stab with symbol descriptor ‘<samp>f</samp>’ or ‘<samp>F</samp>’. |
| This use of symbol descriptor ‘<samp>P</samp>’ can be distinguished from its use |
| for register parameters (see <a href="Register-Parameters.html#Register-Parameters">Register Parameters</a>) by the fact that it has |
| symbol type <code>N_FUN</code>. |
| </p> |
| <p>The AIX documentation also defines symbol descriptor ‘<samp>J</samp>’ as an |
| internal function. I assume this means a function nested within another |
| function. It also says symbol descriptor ‘<samp>m</samp>’ is a module in |
| Modula-2 or extended Pascal. |
| </p> |
| <p>Procedures (functions which do not return values) are represented as |
| functions returning the <code>void</code> type in C. I don’t see why this couldn’t |
| be used for all languages (inventing a <code>void</code> type for this purpose if |
| necessary), but the AIX documentation defines ‘<samp>I</samp>’, ‘<samp>P</samp>’, and |
| ‘<samp>Q</samp>’ for internal, global, and static procedures, respectively. |
| These symbol descriptors are unusual in that they are not followed by |
| type information. |
| </p> |
| <p>The following example shows a stab for a function <code>main</code> which |
| returns type number <code>1</code>. The <code>_main</code> specified for the value |
| is a reference to an assembler label which is used to fill in the start |
| address of the function. |
| </p> |
| <div class="example"> |
| <pre class="example">.stabs "main:F1",36,0,0,_main # <span class="roman">36 is N_FUN</span> |
| </pre></div> |
| |
| <p>The stab representing a procedure is located immediately following the |
| code of the procedure. This stab is in turn directly followed by a |
| group of other stabs describing elements of the procedure. These other |
| stabs describe the procedure’s parameters, its block local variables, and |
| its block structure. |
| </p> |
| <p>If functions can appear in different sections, then the debugger may not |
| be able to find the end of a function. Recent versions of GCC will mark |
| the end of a function with an <code>N_FUN</code> symbol with an empty string |
| for the name. The value is the address of the end of the current |
| function. Without such a symbol, there is no indication of the address |
| of the end of a function, and you must assume that it ended at the |
| starting address of the next function or at the end of the text section |
| for the program. |
| </p> |
| <hr> |
| <div class="header"> |
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| Next: <a href="Nested-Procedures.html#Nested-Procedures" accesskey="n" rel="next">Nested Procedures</a>, Previous: <a href="Line-Numbers.html#Line-Numbers" accesskey="p" rel="prev">Line Numbers</a>, Up: <a href="Program-Structure.html#Program-Structure" accesskey="u" rel="up">Program Structure</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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