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Chapter 6. Functions

You write functions to specify all the actions that a program performs when it

executes. The type of a function tells you the type of result it returns (if any). It can

also tell you the types of any arguments that the function expects when you call it

from within an expression.

This document describes briefly just those aspect of functions most relevant to the

use of the Standard C library:

Argument promotion occurs when the type of the function fails to provide any

information about an argument. Promotion occurs if the function declaration is not

a function prototype or if the argument is one of the unnamed arguments in a

varying number of arguments. In this instance, the argument must be an rvalue

expression (page 15). Hence:

v An integer argument type is promoted.

v An lvalue of type array of T becomes an rvalue of type pointer to T.

v A function designator of type function returning T becomes an rvalue of type

pointer to function returning T.

v An argument of type float is converted to double.

A do statement executes a statement one or more times, while its test-context

expression (page 24) has a nonzero value:

statement

while (test);

An expression statement evaluates an expression in a side-effects context (page 24):

printf("hello\n");

call a function

y = m * x + b;

store a value

++count;

alter a stored value

A for statement executes a statement zero or more times, while the optional

test-context expression (page 24) test has a nonzero value. You can also write two

expressions, se-1 and se-2, in a for statement that are each in a side-effects context

(page 24):

for (se-1; test; se-2)

statement

An if statement executes a statement only if the test-context expression (page 24)

has a nonzero value:

if (test)

statement

An if-else statement executes one of two statements, depending on whether the

test-context expression (page 24) has a nonzero value:

if (test)

statement-1

else

statement-2

A return statement terminates execution of the function and transfers control to the

expression that called the function. If you write the optional rvalue expression

(page 15) within the return statement, the result must be assignment-compatible

with the type returned by the function. The program converts the value of the

expression to the type returned and returns it as the value of the function call:

return expression;

An expression that occurs in a side-effects context specifies no value and

designates no object or function. Hence, it can have type void. You typically

evaluate such an expression for its side effects -- any change in the state of the

program that occurs when evaluating an expression. Side effects occur when the

program stores a value in an object, accesses a value from an object of volatile

qualified type, or alters the state of a file.

A switch statement jumps to a place within a controlled statement, depending on

the value of an integer expression:

switch (expr)

{

case val-1:

stat-1;

break;

case val-2:

stat-2;

falls through to next

default:

stat-n

}

In a test-context expression the value of an expression causes control to flow one

way within the statement if the computed value is nonzero or another way if the

computed value is zero. You can write only an expression that has a scalar rvalue

result, because only scalars can be compared with zero.

A while statement executes a statement zero or more times, while the test-context

expression has a nonzero value:

while (test)

statement

Standard C++ Library

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