ATTRIBUTE(3) BSD Library Functions Manual ATTRIBUTE(3)
attribute -- non-standard GCC attribute extensions
The GNU Compiler Collection (GCC) provides many extensions to the standard C language.
Among these are the so-called attributes. In NetBSD all attributes are provided in a
restricted namespace. The described macros should be preferred instead of using the GCC's
__attribute__ extension directly.
The gcc(1) compiler knows that certain functions such as abort(3) and exit(3) can never
return any value. When such a function is equipped with __dead, certain optimizations
are possible. Obviously a __dead function can never have return type other than void.
A __pure function is defined to be one that has no effects except the return value,
which is assumed to depend only on the function parameters and/or global variables.
Any access to parameters and/or global variables must also be read-only. A function
that depends on volatile memory, or other comparable system resource that can change
between two consecutive calls, can never be __pure. Many math(3) functions satisfy the
definition of a __pure function, at least in theory. Other examples include strlen(3)
A ``const function'' is a stricter variant of ``pure functions''. In addition to the
restrictions of pure functions, a function declared with __constfunc can never access
global variables nor take pointers as parameters. The return value of these functions
must depend only on the passed-by-value parameters. Note also that a function that
calls non-const functions can not be __constfunc. The canonical example of a const
function would be abs(3). As with pure functions, certain micro-optimizations are pos-
sible for functions declared with __constfunc.
GCC is known for aggressive function inlining. Sometimes it is known that inlining is
undesirable or that a function will perform incorrectly when inlined. The __noinline
macro expands to a function attribute that prevents GCC for inlining the function,
irrespective whether the function was declared with the inline keyword. The attribute
takes precedence over all other compiler options related to inlining.
In most GCC versions the common -Wall flag enables warnings produced by functions that
are defined but unused. Marking an unused function with the __unused macro inhibits
The __used macro expands to an attribute that informs GCC that a static variable or
function is to be always retained in the object file even if it is unreferenced.
The __packed macro expands to an attribute that forces a variable or structure field to
have the smallest possible alignment, potentially disregarding architecture specific
alignment requirements. The smallest possible alignment is effectively one byte for
variables and one bit for fields. If specified on a struct or union, all variables
therein are also packed. The __packed macro is often useful when dealing with data
that is in a particular static format on the disk, wire, or memory.
The __aligned() macro expands to an attribute that specifies the minimum alignment in
bytes for a variable, structure field, or function. In other words, the specified
object should have an alignment of at least x bytes, as opposed to the minimum align-
ment requirements dictated by the architecture and the ABI. Possible use cases
o Mixing assembly and C code.
o Dealing with hardware that may impose alignment requirements greater than the
o Using instructions that may impose special alignment requirements. Typical
example would be alignment of frequently used objects along processor cache
Note that when used with functions, structures, or structure members, __aligned() can
only be used to increase the alignment. If the macro is however used as part of a
typedef, the alignment can both increase and decrease. Otherwise it is only possible
to decrease the alignment for variables and fields by using the __packed macro. The
effectiveness of __aligned() is largely dependent on the linker. The __alignof__(3)
operator can be used to examine the alignment.
The __section() macro expands to an attribute that specifies a particular section to
which a variable or function should be placed. Normally the compiler places the gener-
ated objects to sections such as ``data'' or ``text''. By using __section(), it is
possible to override this behavior, perhaps in order to place some variables into par-
ticular sections specific to unique hardware.
The __read_mostly macro uses __section() to place a variable or function into the
``.data.read_mostly'' section of the (kernel) elf(5). The use of __read_mostly allows
infrequently modified data to be grouped together; it is expected that the cachelines
of rarely and frequently modified data structures are this way separated. Candidates
for __read_mostly include variables that are initialized once, read very often, and
seldom written to.
The __cacheline_aligned macro behaves like __read_mostly, but the used section is
``.data.cacheline_aligned'' instead. It also uses __aligned() to set the minimum
alignment into a predefined coherency unit. This should ensure that frequently used
data structures are aligned on cacheline boundaries. Both __cacheline_aligned and
__read_mostly are only available for the kernel.
A branch is generally defined to be a conditional execution of a program depending on
whether a certain flow control mechanism is altered. Typical example would be a
``if-then-else'' sequence used in high-level languages or a jump instruction used in
machine-level code. A branch prediction would then be defined as an attempt to guess
whether a conditional branch will be taken.
The macros __predict_true() and __predict_false() annotate the likelihood of whether a
branch will evaluate to true or false. The rationale is to improve instruction pipe-
lining. Semantically __predict_true expects that the integral expression exp equals 1.
The __predict_false expands to an attribute that instructs the compiler to predict that
a given branch will be likely false. As programmers are notoriously bad at predicting
the likely behavior of their code, profiling and empirical evidence should precede the
use of __predict_false and __predict_true.
gcc(1), __builtin_object_size(3), cdefs(3), c(7)
It goes without saying that portable applications should steer clear from non-standard
extensions specific to any given compiler. Even when portability is not a concern, use
these macros sparsely and wisely.
BSD December 19, 2010 BSD