Nonstandard compiler attribute extension

As C Standard extension , Some compilers allow functions 、 Variables or types use some nonstandard properties . In brief, these macros are used for compiler optimization .GNU Compiler collection （GCC） and LLVM/Clang Use __attribute__ Syntax to specify these attributes . Different compilers may support different properties , It is also possible to use a completely different syntax .

NetBSD Code should generally avoid direct use __attribute__ Or some other similar syntax provided by a particular compiler .

however NetBSD The in <sys/cdefs.h> The following attribute macros are provided in the header file , These attribute macros can be automatically extended to the corresponding compiler syntax .

     
     __dead
     __pure
     __constfunc
     __noinline
     __unused
     __used
     __diagused
     __debugused
     __packed
     __aligned(x);
     __section(section);
     __read_mostly
     __cacheline_aligned
     __predict_true(exp);
     __predict_false(exp);
     __printflike(fmtarg, firstvararg);
     __sysloglike(fmtarg, firstvararg);

__dead

Certain functions, such as abort(3) and exit(3), can never return. 

Some functions such as abort(3),exit(3), There is no return value . When used _dead When you declare a function , The compiler can do some optimization , Related warning May also be removed . Obviously with _dead Decorated functions except void No other types will be returned ._dead This is to tell the compiler that it can let go and optimize . similar __attribute__((noreturn)).

 __pure

By __pure Modified function , Is defined as a function that has no effect other than the return value , The return value is assumed to depend only on function parameters and / Or global variables , For parameters and / Or global variables must also be read-only . If a function is to be correct volatile Variable （ Register variables, etc ） To read and write , Or access system resources that can be changed between successive calls , Can't use __pure Of . And __pure The equivalent function attribute is _attribute__((const))

 __constfunc

__constfunc It's better than __pure More stringent variants , except __pure Beyond the limits of , The function it modifies can never access global variables , Nor can I get a pointer as a parameter . __constfunc The return value of the decorated function must only depend on the parameters passed by the value .