111 lines
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111 lines
6.4 KiB
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<title>C Constants - Debugging with GDB</title>
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<div class="node">
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<a name="C-Constants"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="C-Plus-Plus-Expressions.html#C-Plus-Plus-Expressions">C Plus Plus Expressions</a>,
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Previous: <a rel="previous" accesskey="p" href="C-Operators.html#C-Operators">C Operators</a>,
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Up: <a rel="up" accesskey="u" href="C.html#C">C</a>
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<hr>
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</div>
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<h5 class="subsubsection">15.4.1.2 C and C<tt>++</tt> Constants</h5>
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<p><a name="index-C-and-C_0040t_007b_002b_002b_007d-constants-987"></a>
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<span class="sc">gdb</span> allows you to express the constants of C and C<tt>++</tt> in the
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following ways:
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<ul>
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<li>Integer constants are a sequence of digits. Octal constants are
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specified by a leading ‘<samp><span class="samp">0</span></samp>’ (i.e. zero), and hexadecimal constants
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by a leading ‘<samp><span class="samp">0x</span></samp>’ or ‘<samp><span class="samp">0X</span></samp>’. Constants may also end with a letter
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‘<samp><span class="samp">l</span></samp>’, specifying that the constant should be treated as a
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<code>long</code> value.
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<li>Floating point constants are a sequence of digits, followed by a decimal
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point, followed by a sequence of digits, and optionally followed by an
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exponent. An exponent is of the form:
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‘<samp><span class="samp">e[[+]|-]</span><var>nnn</var><span class="samp"><!-- /@w --></span></samp>’, where <var>nnn</var> is another
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sequence of digits. The ‘<samp><span class="samp">+</span></samp>’ is optional for positive exponents.
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A floating-point constant may also end with a letter ‘<samp><span class="samp">f</span></samp>’ or
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‘<samp><span class="samp">F</span></samp>’, specifying that the constant should be treated as being of
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the <code>float</code> (as opposed to the default <code>double</code>) type; or with
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a letter ‘<samp><span class="samp">l</span></samp>’ or ‘<samp><span class="samp">L</span></samp>’, which specifies a <code>long double</code>
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constant.
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<li>Enumerated constants consist of enumerated identifiers, or their
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integral equivalents.
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<li>Character constants are a single character surrounded by single quotes
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(<code>'</code>), or a number—the ordinal value of the corresponding character
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(usually its <span class="sc">ascii</span> value). Within quotes, the single character may
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be represented by a letter or by <dfn>escape sequences</dfn>, which are of
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the form ‘<samp><span class="samp">\</span><var>nnn</var></samp>’, where <var>nnn</var> is the octal representation
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of the character's ordinal value; or of the form ‘<samp><span class="samp">\</span><var>x</var></samp>’, where
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‘<samp><var>x</var></samp>’ is a predefined special character—for example,
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‘<samp><span class="samp">\n</span></samp>’ for newline.
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<p>Wide character constants can be written by prefixing a character
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constant with ‘<samp><span class="samp">L</span></samp>’, as in C. For example, ‘<samp><span class="samp">L'x'</span></samp>’ is the wide
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form of ‘<samp><span class="samp">x</span></samp>’. The target wide character set is used when
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computing the value of this constant (see <a href="Character-Sets.html#Character-Sets">Character Sets</a>).
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<li>String constants are a sequence of character constants surrounded by
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double quotes (<code>"</code>). Any valid character constant (as described
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above) may appear. Double quotes within the string must be preceded by
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a backslash, so for instance ‘<samp><span class="samp">"a\"b'c"</span></samp>’ is a string of five
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characters.
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<p>Wide string constants can be written by prefixing a string constant
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with ‘<samp><span class="samp">L</span></samp>’, as in C. The target wide character set is used when
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computing the value of this constant (see <a href="Character-Sets.html#Character-Sets">Character Sets</a>).
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<li>Pointer constants are an integral value. You can also write pointers
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to constants using the C operator ‘<samp><span class="samp">&</span></samp>’.
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<li>Array constants are comma-separated lists surrounded by braces ‘<samp><span class="samp">{</span></samp>’
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and ‘<samp><span class="samp">}</span></samp>’; for example, ‘<samp><span class="samp">{1,2,3}</span></samp>’ is a three-element array of
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integers, ‘<samp><span class="samp">{{1,2}, {3,4}, {5,6}}</span></samp>’ is a three-by-two array,
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and ‘<samp><span class="samp">{&"hi", &"there", &"fred"}</span></samp>’ is a three-element array of pointers.
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</ul>
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</body></html>
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