ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.18 Recommendation of postfix syntax when qualifying functions with ARM function
modifiers
Many ARM keyword extensions modify the behavior or calling sequence of a function. For
example, __pure, __irq, __swi, __swi_indirect , __softfp, and __value_in_regs all behave in this
way.
These function modifiers all have a common syntax. A function modifier such as __pure can
qualify a function declaration either:
• Before the function declaration. For example:
__pure int foo(int);
• After the closing parenthesis on the parameter list. For example:
int foo(int) __pure;
For simple function declarations, each syntax is unambiguous. However, for a function whose
return type or arguments are function pointers, the prefix syntax is imprecise. For example, the
following function returns a function pointer, but it is not clear whether __pure modifies the
function itself or its returned pointer type:
__pure int (*foo(int)) (int); /* declares 'foo' as a (pure?) function that
returns a pointer to a (pure?) function.
It is ambiguous which of the two function
types is pure. */
In fact, the single __pure keyword at the front of the declaration of foo modifies both foo itself
and the function pointer type returned by foo.
In contrast, the postfix syntax enables clear distinction between whether __pure applies to the
argument, the return type, or the base function, when declaring a function whose argument and
return types are function pointers. For example:
int (*foo1(int) __pure) (int); /* foo1 is a pure function returning
a pointer to a normal function */
int (*foo2(int)) (int) __pure; /* foo2 is a function returning
a pointer to a pure function */
int (*foo3(int) __pure) (int) __pure; /* foo3 is a pure function returning
a pointer to a pure function */
In this example:
• foo1 and foo3 are modified themselves
• foo2 and foo3 return a pointer to a modified function
• the functions foo3 and foo are identical.
Because the postfix syntax is more precise than the prefix syntax, it is recommended that, where
possible, you make use of the postfix syntax when qualifying functions with ARM function
modifiers.
5.18.1 See also
Reference
Compiler Reference:
• __irq on page 5-8
• __pure on page 5-11
• __softfp on page 5-12
• __svc on page 5-13
• __svc_indirect on page 5-14
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