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<HTML><HEAD><TITLE>Functions</TITLE></HEAD><BODY><H1><A NAME="Functions">Functions</A></H1><HR><P>You write functions to specify all the actions that a programperforms when it executes. The type of a function tells you the typeof result it returns (if any). It can also tell you the types of anyarguments that the function expects when you call it from within anexpression.</P><P>This document describes briefly just those aspectof functions most relevant to the use of the Standard C library:</P><P><B><A NAME="argument promotion">Argument promotion</A></B>occurs when the type of the function fails to provide any informationabout an argument. Promotion occurs if the function declarationis not a function prototype or if the argument is one ofthe unnamed arguments in a<A HREF="stdarg.html#varying number of arguments">varying numberof arguments</A>. In this instance, the argument must be an<A HREF="express.html#rvalue expression">rvalue expression</A>. Hence:</P><UL><LI>An integer argument type is promoted.</LI><LI>An lvalue of type array of <CODE>Ty</CODE> becomes an rvalue of typepointer to <CODE>Ty</CODE>.</LI><LI>A function designator of type function returning <CODE>Ty</CODE> becomesan rvalue of type pointer to function returning <CODE>Ty</CODE>.<LI>An argument of type <I>float</I> is converted to <I>double.</I></UL><P>A <B><A NAME="Do Statement"><I>do</I> statement</A></B>executes a statement one or more times, while its<A HREF="#test-context expression">test-context expression</A>has a nonzero value:</P><PRE> do <I>statement</I> while (<I>test</I>);</PRE><P>An <B><A NAME="Expression Statement"><I>expression</I> statement</A></B>evaluates an expression in a<A HREF="#side-effects context">side-effects context</A>:</P><PRE> printf("hello\n"); <B>call a function</B> y = m * x + b; <B>store a value</B> ++count; <B>alter a stored value</B></PRE><P>A <B><A NAME="For Statement"><I>for</I> statement</A></B>executes a statement zero or more times, while the optional<A HREF="#test-context expression">test-context expression</A><CODE><I>test</I></CODE> has a nonzero value.You can also write two expressions, <CODE><I>se-1</I></CODE>and <CODE><I>se-2</I></CODE>, in a <I>for</I> statement that are each in a<A HREF="#side-effects context">side-effects context</A>:</P><PRE> for (<I>se-1</I>; <I>test</I>; <I>se-2</I>) <I>statement</I></PRE><P>An <B><A NAME="If Statement"><I>if</I> statement</A></B>executes a statement only if the<A HREF="#test-context expression">test-context expression</A>has a nonzero value:</P><PRE> if (<I>test</I>) <I>statement</I></PRE><P>An <B><A NAME="If-Else Statement"><I>if-else</I> statement</A></B>executes one of two statements, depending on whether the<A HREF="#test-context expression">test-context expression</A>has a nonzero value:<PRE> if (<I>test</I>) <I>statement-1</I> else <I>statement-2</I></PRE><P>A <B><A NAME="Return Statement"><I>return</I> statement</A></B>terminates execution of the function and transfers controlto the expression that called the function. If you write the optional<A HREF="express.html#rvalue expression">rvalue expression</A>within the <I>return</I> statement,the result must be assignment-compatible with the type returnedby the function. The program converts the value of the expressionto the type returned and returns it as the value of the function call:</P><PRE> return <I>expression</I>;</PRE><P>An expression that occurs in a<B><A NAME="side-effects context">side-effects context</A></B>specifies no value and designates no object or function.Hence, it can have type <I>void.</I>You typically evaluate such an expression for its<B><A NAME="side effects">side effects</A></B>-- any change in the state of the program that occurs when evaluatingan expression. Side effects occur when the programstores a value in an object, accesses a value from an objectof <I>volatile</I> qualified type, or alters the state of a file.</P><P>A <B><A NAME="Switch Statement"><I>switch</I> statement</A></B>jumps to a place within a controlled statement, dependingon the value of an integer expression:</P><PRE> switch (<I>expr</I>) { case <I>val-1</I>: <I>stat-1</I>; break; case <I>val-2</I>: <I>stat-2</I>; <B>falls through to next</B> default: <I>stat-n</I> }</PRE><P>In a<B><A NAME="test-context expression">test-context expression</A></B>the value of an expression causes controlto flow one way within the statement if the computed value is nonzeroor another way if the computed value is zero. You can write only anexpression that has a scalar rvalue result, because only scalars canbe compared with zero.</P><P>A <B><A NAME="While Statement"><I>while</I> statement</A></B>executes a statement zero or more times, while the test-contextexpression has a nonzero value:</P><PRE> while (<I>test</I>) <I>statement</I></PRE><HR><P>See also the<B><A HREF="index.html#Table of Contents">Table of Contents</A></B> and the<B><A HREF="_index.html">Index</A></B>.</P><P><I><A HREF="crit_pb.html">Copyright</A> © 1989-2002by P.J. Plauger and Jim Brodie. All rights reserved.</I></P><!--V4.01:1125--></BODY></HTML>⌨️ 快捷键说明
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