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A copy of the license is included in the section entitled "GNU 12Free Documentation License". --> 13<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 14<head> 15<title>STABS: Register Parameters</title> 16 17<meta name="description" content="STABS: Register Parameters"> 18<meta name="keywords" content="STABS: Register Parameters"> 19<meta name="resource-type" content="document"> 20<meta name="distribution" content="global"> 21<meta name="Generator" content="makeinfo"> 22<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 23<link href="index.html#Top" rel="start" title="Top"> 24<link href="Symbol-Types-Index.html#Symbol-Types-Index" rel="index" title="Symbol Types Index"> 25<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 26<link href="Parameters.html#Parameters" rel="up" title="Parameters"> 27<link href="Local-Variable-Parameters.html#Local-Variable-Parameters" rel="next" title="Local Variable Parameters"> 28<link href="Parameters.html#Parameters" rel="previous" title="Parameters"> 29<style type="text/css"> 30<!-- 31a.summary-letter {text-decoration: none} 32blockquote.smallquotation {font-size: smaller} 33div.display {margin-left: 3.2em} 34div.example {margin-left: 3.2em} 35div.indentedblock {margin-left: 3.2em} 36div.lisp {margin-left: 3.2em} 37div.smalldisplay {margin-left: 3.2em} 38div.smallexample {margin-left: 3.2em} 39div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 40div.smalllisp {margin-left: 3.2em} 41kbd {font-style:oblique} 42pre.display {font-family: inherit} 43pre.format {font-family: inherit} 44pre.menu-comment {font-family: serif} 45pre.menu-preformatted {font-family: serif} 46pre.smalldisplay {font-family: inherit; font-size: smaller} 47pre.smallexample {font-size: smaller} 48pre.smallformat {font-family: inherit; font-size: smaller} 49pre.smalllisp {font-size: smaller} 50span.nocodebreak {white-space:nowrap} 51span.nolinebreak {white-space:nowrap} 52span.roman {font-family:serif; font-weight:normal} 53span.sansserif {font-family:sans-serif; font-weight:normal} 54ul.no-bullet {list-style: none} 55--> 56</style> 57 58 59</head> 60 61<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 62<a name="Register-Parameters"></a> 63<div class="header"> 64<p> 65Next: <a href="Local-Variable-Parameters.html#Local-Variable-Parameters" accesskey="n" rel="next">Local Variable Parameters</a>, Up: <a href="Parameters.html#Parameters" accesskey="u" rel="up">Parameters</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> 66</div> 67<hr> 68<a name="Passing-Parameters-in-Registers"></a> 69<h4 class="subsection">4.7.1 Passing Parameters in Registers</h4> 70 71<p>If the parameter is passed in a register, then traditionally there are 72two symbols for each argument: 73</p> 74<div class="example"> 75<pre class="example">.stabs "arg:p1" . . . ; N_PSYM 76.stabs "arg:r1" . . . ; N_RSYM 77</pre></div> 78 79<p>Debuggers use the second one to find the value, and the first one to 80know that it is an argument. 81</p> 82<a name="index-C_005fRPSYM"></a> 83<a name="index-N_005fRSYM_002c-for-parameters"></a> 84<p>Because that approach is kind of ugly, some compilers use symbol 85descriptor ‘<samp>P</samp>’ or ‘<samp>R</samp>’ to indicate an argument which is in a 86register. Symbol type <code>C_RPSYM</code> is used in XCOFF and <code>N_RSYM</code> 87is used otherwise. The symbol’s value is the register number. ‘<samp>P</samp>’ 88and ‘<samp>R</samp>’ mean the same thing; the difference is that ‘<samp>P</samp>’ is a 89GNU invention and ‘<samp>R</samp>’ is an IBM (XCOFF) invention. As of version 904.9, GDB should handle either one. 91</p> 92<p>There is at least one case where GCC uses a ‘<samp>p</samp>’ and ‘<samp>r</samp>’ pair 93rather than ‘<samp>P</samp>’; this is where the argument is passed in the 94argument list and then loaded into a register. 95</p> 96<p>According to the AIX documentation, symbol descriptor ‘<samp>D</samp>’ is for a 97parameter passed in a floating point register. This seems 98unnecessary—why not just use ‘<samp>R</samp>’ with a register number which 99indicates that it’s a floating point register? I haven’t verified 100whether the system actually does what the documentation indicates. 101</p> 102<p>On the sparc and hppa, for a ‘<samp>P</samp>’ symbol whose type is a structure 103or union, the register contains the address of the structure. On the 104sparc, this is also true of a ‘<samp>p</samp>’ and ‘<samp>r</samp>’ pair (using Sun 105<code>cc</code>) or a ‘<samp>p</samp>’ symbol. However, if a (small) structure is 106really in a register, ‘<samp>r</samp>’ is used. And, to top it all off, on the 107hppa it might be a structure which was passed on the stack and loaded 108into a register and for which there is a ‘<samp>p</samp>’ and ‘<samp>r</samp>’ pair! I 109believe that symbol descriptor ‘<samp>i</samp>’ is supposed to deal with this 110case (it is said to mean "value parameter by reference, indirect 111access"; I don’t know the source for this information), but I don’t know 112details or what compilers or debuggers use it, if any (not GDB or GCC). 113It is not clear to me whether this case needs to be dealt with 114differently than parameters passed by reference (see <a href="Reference-Parameters.html#Reference-Parameters">Reference Parameters</a>). 115</p> 116<hr> 117<div class="header"> 118<p> 119Next: <a href="Local-Variable-Parameters.html#Local-Variable-Parameters" accesskey="n" rel="next">Local Variable Parameters</a>, Up: <a href="Parameters.html#Parameters" accesskey="u" rel="up">Parameters</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> 120</div> 121 122 123 124</body> 125</html> 126