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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gdb/Variables.html | |
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diff --git a/share/doc/gdb/Variables.html b/share/doc/gdb/Variables.html new file mode 100644 index 0000000..20a5ef5 --- /dev/null +++ b/share/doc/gdb/Variables.html @@ -0,0 +1,301 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Free Software" and "Free Software Needs +Free Documentation", with the Front-Cover Texts being "A GNU Manual," +and with the Back-Cover Texts as in (a) below. + +(a) The FSF's Back-Cover Text is: "You are free to copy and modify +this GNU Manual. Buying copies from GNU Press supports the FSF in +developing GNU and promoting software freedom." --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Debugging with GDB: Variables</title> + +<meta name="description" content="Debugging with GDB: Variables"> +<meta name="keywords" content="Debugging with GDB: Variables"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Data.html#Data" rel="up" title="Data"> +<link href="Arrays.html#Arrays" rel="next" title="Arrays"> +<link href="Ambiguous-Expressions.html#Ambiguous-Expressions" rel="previous" title="Ambiguous Expressions"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Variables"></a> +<div class="header"> +<p> +Next: <a href="Arrays.html#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="p" rel="previous">Ambiguous Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Program-Variables"></a> +<h3 class="section">10.3 Program Variables</h3> + +<p>The most common kind of expression to use is the name of a variable +in your program. +</p> +<p>Variables in expressions are understood in the selected stack frame +(see <a href="Selection.html#Selection">Selecting a Frame</a>); they must be either: +</p> +<ul> +<li> global (or file-static) +</li></ul> + +<p>or +</p> +<ul> +<li> visible according to the scope rules of the +programming language from the point of execution in that frame +</li></ul> + +<p>This means that in the function +</p> +<div class="smallexample"> +<pre class="smallexample">foo (a) + int a; +{ + bar (a); + { + int b = test (); + bar (b); + } +} +</pre></div> + +<p>you can examine and use the variable <code>a</code> whenever your program is +executing within the function <code>foo</code>, but you can only use or +examine the variable <code>b</code> while your program is executing inside +the block where <code>b</code> is declared. +</p> +<a name="index-variable-name-conflict"></a> +<p>There is an exception: you can refer to a variable or function whose +scope is a single source file even if the current execution point is not +in this file. But it is possible to have more than one such variable or +function with the same name (in different source files). If that +happens, referring to that name has unpredictable effects. If you wish, +you can specify a static variable in a particular function or file by +using the colon-colon (<code>::</code>) notation: +</p> +<a name="index-colon_002dcolon_002c-context-for-variables_002ffunctions"></a> +<a name="index-_003a_003a_002c-context-for-variables_002ffunctions"></a> +<div class="smallexample"> +<pre class="smallexample"><var>file</var>::<var>variable</var> +<var>function</var>::<var>variable</var> +</pre></div> + +<p>Here <var>file</var> or <var>function</var> is the name of the context for the +static <var>variable</var>. In the case of file names, you can use quotes to +make sure <small>GDB</small> parses the file name as a single word—for example, +to print a global value of <code>x</code> defined in <samp>f2.c</samp>: +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) p 'f2.c'::x +</pre></div> + +<p>The <code>::</code> notation is normally used for referring to +static variables, since you typically disambiguate uses of local variables +in functions by selecting the appropriate frame and using the +simple name of the variable. However, you may also use this notation +to refer to local variables in frames enclosing the selected frame: +</p> +<div class="smallexample"> +<pre class="smallexample">void +foo (int a) +{ + if (a < 10) + bar (a); + else + process (a); /* Stop here */ +} + +int +bar (int a) +{ + foo (a + 5); +} +</pre></div> + +<p>For example, if there is a breakpoint at the commented line, +here is what you might see +when the program stops after executing the call <code>bar(0)</code>: +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) p a +$1 = 10 +(gdb) p bar::a +$2 = 5 +(gdb) up 2 +#2 0x080483d0 in foo (a=5) at foobar.c:12 +(gdb) p a +$3 = 5 +(gdb) p bar::a +$4 = 0 +</pre></div> + +<a name="index-C_002b_002b-scope-resolution"></a> +<p>These uses of ‘<samp>::</samp>’ are very rarely in conflict with the very +similar use of the same notation in C<tt>++</tt>. When they are in +conflict, the C<tt>++</tt> meaning takes precedence; however, this can be +overridden by quoting the file or function name with single quotes. +</p> +<p>For example, suppose the program is stopped in a method of a class +that has a field named <code>includefile</code>, and there is also an +include file named <samp>includefile</samp> that defines a variable, +<code>some_global</code>. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) p includefile +$1 = 23 +(gdb) p includefile::some_global +A syntax error in expression, near `'. +(gdb) p 'includefile'::some_global +$2 = 27 +</pre></div> + +<a name="index-wrong-values"></a> +<a name="index-variable-values_002c-wrong"></a> +<a name="index-function-entry_002fexit_002c-wrong-values-of-variables"></a> +<a name="index-optimized-code_002c-wrong-values-of-variables"></a> +<blockquote> +<p><em>Warning:</em> Occasionally, a local variable may appear to have the +wrong value at certain points in a function—just after entry to a new +scope, and just before exit. +</p></blockquote> +<p>You may see this problem when you are stepping by machine instructions. +This is because, on most machines, it takes more than one instruction to +set up a stack frame (including local variable definitions); if you are +stepping by machine instructions, variables may appear to have the wrong +values until the stack frame is completely built. On exit, it usually +also takes more than one machine instruction to destroy a stack frame; +after you begin stepping through that group of instructions, local +variable definitions may be gone. +</p> +<p>This may also happen when the compiler does significant optimizations. +To be sure of always seeing accurate values, turn off all optimization +when compiling. +</p> +<a name="index-_0060_0060No-symbol-_0022foo_0022-in-current-context_0027_0027"></a> +<p>Another possible effect of compiler optimizations is to optimize +unused variables out of existence, or assign variables to registers (as +opposed to memory addresses). Depending on the support for such cases +offered by the debug info format used by the compiler, <small>GDB</small> +might not be able to display values for such local variables. If that +happens, <small>GDB</small> will print a message like this: +</p> +<div class="smallexample"> +<pre class="smallexample">No symbol "foo" in current context. +</pre></div> + +<p>To solve such problems, either recompile without optimizations, or use a +different debug info format, if the compiler supports several such +formats. See <a href="Compilation.html#Compilation">Compilation</a>, for more information on choosing compiler +options. See <a href="C.html#C">C and C<tt>++</tt></a>, for more information about debug +info formats that are best suited to C<tt>++</tt> programs. +</p> +<p>If you ask to print an object whose contents are unknown to +<small>GDB</small>, e.g., because its data type is not completely specified +by the debug information, <small>GDB</small> will say ‘<samp><incomplete +type></samp>’. See <a href="Symbols.html#Symbols">incomplete type</a>, for more about this. +</p> +<a name="index-no-debug-info-variables"></a> +<p>If you try to examine or use the value of a (global) variable for +which <small>GDB</small> has no type information, e.g., because the program +includes no debug information, <small>GDB</small> displays an error message. +See <a href="Symbols.html#Symbols">unknown type</a>, for more about unknown types. If you +cast the variable to its declared type, <small>GDB</small> gets the +variable’s value using the cast-to type as the variable’s type. For +example, in a C program: +</p> +<div class="smallexample"> +<pre class="smallexample"> (gdb) p var + 'var' has unknown type; cast it to its declared type + (gdb) p (float) var + $1 = 3.14 +</pre></div> + +<p>If you append <kbd>@entry</kbd> string to a function parameter name you get its +value at the time the function got called. If the value is not available an +error message is printed. Entry values are available only with some compilers. +Entry values are normally also printed at the function parameter list according +to <a href="Print-Settings.html#set-print-entry_002dvalues">set print entry-values</a>. +</p> +<div class="smallexample"> +<pre class="smallexample">Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29 +29 i++; +(gdb) next +30 e (i); +(gdb) print i +$1 = 31 +(gdb) print i@entry +$2 = 30 +</pre></div> + +<p>Strings are identified as arrays of <code>char</code> values without specified +signedness. Arrays of either <code>signed char</code> or <code>unsigned char</code> get +printed as arrays of 1 byte sized integers. <code>-fsigned-char</code> or +<code>-funsigned-char</code> <small>GCC</small> options have no effect as <small>GDB</small> +defines literal string type <code>"char"</code> as <code>char</code> without a sign. +For program code +</p> +<div class="smallexample"> +<pre class="smallexample">char var0[] = "A"; +signed char var1[] = "A"; +</pre></div> + +<p>You get during debugging +</p><div class="smallexample"> +<pre class="smallexample">(gdb) print var0 +$1 = "A" +(gdb) print var1 +$2 = {65 'A', 0 '\0'} +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Arrays.html#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="p" rel="previous">Ambiguous Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |