'parameter'에 해당되는 글 2건

  1. 2011.06.08 [Kernel] module & parameter initialization.
  2. 2008.05.02 [C/C++] Using Reference as function parameter...

[Kernel] module & parameter initialization.

Domain/Kernel 2011.06.08 11:20

Linux kernel uses lot's of sections to modularize it's code structure easily.
In case of kernel module and parameter, following section is used.

[ Module ]
macro   : module_init()
section : device_initcall (section name : .initcall6.init) <see init.h>
[ Parameter ]
macro   : module_param_named(), module_param(), core_param() etc.
section : __param <see moduleparam.h>

core_param() macro works like other module parameter. But this is NOT for module BUT for kernel booting parameter.
Kernel parameter doesn't have any prefix unlike other modules.
Module parameter's name has <module name> as it's prefix.
For example, parameter <param> of module <module> has name <module>.<param>
See source code for details.

KBUILD_MODNAME is preprocessor value for module name.
This is defined through complex script processing. See Makefile.libMakefile.mod* in scripts/.
But in most case, you can know it by intuition. That is,  name that seems like module name, is set as KBUILD_MODNAME.
(ex. <mod name>-y<mod-name>-m in Makefile)
So, usually, deep analysis about above scripts are not required.

Note:
It's possible that one object gets potentially linked into more than one module.
In that case KBUILD_MODNAME will be set to  foo_bar, where foo and bar are the name of the modules.

Module and parameter initialization.

[ Built-in module ]
module initialization : do_initcalls() <see 'main.c'> => initcall section is used
parameter sysfs node  : param_sysfs_init() -> param_sysfs_builtin() => __param section is used
[ Dynamic module ]
module & parameter initialization is done at system call
SYSCALL_DEFINE3(init_module, ...) => load_module()

Each parameter has permission mask. So, parameter value can be read or written at runtime through sysfs node.

/sys/module/<module name>/parameter/<parameter name>

But module parameter whose  permission is 0, is ignored at sysfs (Not shown at sysfs).
( See module_param_sysfs_setup(...) function. )

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[C/C++] Using Reference as function parameter...

Language/C&C++ 2008.05.02 23:39
[[ blog 이사 과정에서 정확한 posting날짜가 분실됨. 년도와 분기 정도는 맞지 않을까? ]]

Reference is enhanced concept from Pointer in C++.
So, many C++ books says "Reference is better than Pointer. So try to use Reference if possible."
Yes, reference has several advantages. It guarantees that there is enough memory space for the variable. So, we don't need to do so-called "NULL Check" in most cases (In some evil cases, address can be NULL even if it is reference).
But, in terms of readability, sometimes, using Pointer is better than Reference. Since C, programmers think that parameter is passed by value. Let's see below.

func(a); // --- (*1)
func(&a); // --- (*2)

At first look - before looking at declaration of "func()", programmer tends to think that "Value of 'a' is passed. So, even after (*1), value of 'a' will be preserved." But in case of (*2), they might think that "Address of 'a' is passed. So, after (*2), value of 'a' may be changed."
But, Reference stands against of this 'Popular' concept - reference parameter of 'func()' may change value of 'a'.
But, in case that 'func()' doesn't change 'a', using reference - should be declared as 'const' - as parameter can be good choice.
In other words, if value of parameter is changed, using pointer is better then using reference in terms of readability, in my point of view.

In summary, my recommendation is,

"If possible, use Reference as parameter only when value of parameter is not changed with 'const' keyword."


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