Datasheet

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ARM Compiler Reference

3.1.2 Function keywords

Several keywords tell the compiler to give a function special treatment. These are all

ARM extensions to the ANSI C specification:

Declarations inside functions

Declarations inside a function indicate that the following statements are processed

differently. The

asm

keyword does not modify the surrounding function, but it does

indicate that the statements following the keyword are different.

__asm

This instructs the compiler that the following code is written in assembler

language (see Inline assembler on page 3-18).

Function qualifiers

Function qualifiers affect the type of a function. The qualifiers are placed after the

parameter list in the same position that

const

and

volatile

can appear for C++ member

function types.

__irq

This enables a C or C++ function to be used as an interrupt routine called

by the

IRQ

FIQ

vectors. All corrupted registers except floating-point

registers are preserved, not only those that are normally preserved under

the ATPCS. The default ATPCS mode must be used. The function exits

by setting the pc to lr–4 and the CPSR to the value in SPSR. It is not

available in tcc or tcpp. No arguments or return values can be used with

__irq

functions.

See the chapter on Handling Processor Exceptions in the ADS Developer

Guide for detailed information on using

__irq

__pure

This asserts that a function declaration is pure. Functions that are pure are

candidates for common subexpression elimination. By default, functions

are assumed to be impure (causing side-effects). A function is properly

defined as pure only if:

• its result depends exclusively on the values of its arguments

• it has no side effects, for example it cannot call impure functions.

So, a pure function cannot use global variables or dereference pointers,

because the compiler assumes that the function does not access memory

(except stack memory) at all. When called twice with the same

parameters, a pure function must return the same value each time.

The __pure declaration can also be used as a prefix or postfix declaration.

In some cases the prefix form can be ambiguous and readability is

improved by using the postfix form: