2. 注释

单行注释应采用/*␣comment␣*/的形式,用空格把界定符和文字分开。多行注释最常见的是这种形式:

/*

␣*␣Multi-line

␣*␣comment

␣*/

也有更花哨的形式:

/*************\

* Multi-line  *

* comment     *

\*************/

使用注释的场合主要有以下几种。

1、整个源文件的顶部注释。说明此模块的相关信息,例如文件名、作者和版本历史等,顶头写不缩进。例如内核源代码目录下的kernel/sched.c文件的开头:

/*

 *  kernel/sched.c

 *

 *  Kernel scheduler and related syscalls

 *

 *  Copyright (C) 1991-2002  Linus Torvalds

 *

 *  1996-12-23  Modified by Dave Grothe to fix bugs in semaphores and

 *              make semaphores SMP safe

 *  1998-11-19  Implemented schedule_timeout() and related stuff

 *              by Andrea Arcangeli

 *  2002-01-04  New ultra-scalable O(1) scheduler by Ingo Molnar:

 *              hybrid priority-list and round-robin design with

 *              an array-switch method of distributing timeslices

 *              and per-CPU runqueues.  Cleanups and useful suggestions

 *              by Davide Libenzi, preemptible kernel bits by Robert Love.

 *  2003-09-03  Interactivity tuning by Con Kolivas.

 *  2004-04-02  Scheduler domains code by Nick Piggin

 */

2、函数注释。说明此函数的功能、参数、返回值、错误码等,写在函数定义上侧,和此函数定义之间不留空行,顶头写不缩进。

3、相对独立的语句组注释。对这一组语句做特别说明,写在语句组上侧,和此语句组之间不留空行,与当前语句组的缩进一致。

4、代码行右侧的简短注释。对当前代码行做特别说明,一般为单行注释,和代码之间至少用一个空格隔开,一个源文件中所有的右侧注释最好能上下对齐。尽管例 2.1 “带更多注释的Hello World”讲过注释可以穿插在一行代码中间,但不建议这么写。内核源代码目录下的lib/radix-tree.c文件中的一个函数包含了上述三种注释:

/**

 *      radix_tree_insert    -    insert into a radix tree

 *      @root:          radix tree root

 *      @index:         index key

 *      @item:          item to insert

 *

 *      Insert an item into the radix tree at position @index.

 */

int radix_tree_insert(struct radix_tree_root *root,

                        unsigned long index, void *item)

{

        struct radix_tree_node *node = NULL, *slot;

        unsigned int height, shift;

        int offset;

        int error;



        /* Make sure the tree is high enough.  */

        if ((!index && !root->rnode) ||

                        index > radix_tree_maxindex(root->height)) {

                error = radix_tree_extend(root, index);

                if (error)

                        return error;

        }



        slot = root->rnode;

        height = root->height;

        shift = (height-1) * RADIX_TREE_MAP_SHIFT;



        offset = 0;                     /* uninitialised var warning */

        do {

                if (slot == NULL) {

                        /* Have to add a child node.  */

                        if (!(slot = radix_tree_node_alloc(root)))

                                return -ENOMEM;

                        if (node) {

                                node->slots[offset] = slot;

                                node->count++;

                        } else

                                root->rnode = slot;

                }



                /* Go a level down */

                offset = (index >> shift) & RADIX_TREE_MAP_MASK;

                node = slot;

                slot = node->slots[offset];

                shift -= RADIX_TREE_MAP_SHIFT;

                height--;

        } while (height > 0);



        if (slot != NULL)

                return -EEXIST;



        BUG_ON(!node);

        node->count++;

        node->slots[offset] = item;

        BUG_ON(tag_get(node, 0, offset));

        BUG_ON(tag_get(node, 1, offset));



        return 0;

}

[CodingStyle]中特别指出,函数内的注释要尽可能少用。写注释主要是为了说明你的代码“能做什么”(比如函数接口定义),而不是为了说明“怎样做”,只要代码写得足够清晰,“怎样做”是一目了然的,如果你需要用注释才能解释清楚,那就表示你的代码可读性很差,除非是特别需要提醒注意的地方才使用函数内注释。

5、复杂的结构体定义比函数更需要注释。例如内核源代码目录下的kernel/sched.c文件中定义了这样一个结构体:

/*

 * This is the main, per-CPU runqueue data structure.

 *

 * Locking rule: those places that want to lock multiple runqueues

 * (such as the load balancing or the thread migration code), lock

 * acquire operations must be ordered by ascending &runqueue.

 */

struct runqueue {

        spinlock_t lock;



        /*

         * nr_running and cpu_load should be in the same cacheline because

         * remote CPUs use both these fields when doing load calculation.

         */

        unsigned long nr_running;

#ifdef CONFIG_SMP

        unsigned long cpu_load[3];

#endif

        unsigned long long nr_switches;



        /*

         * This is part of a global counter where only the total sum

         * over all CPUs matters. A task can increase this counter on

         * one CPU and if it got migrated afterwards it may decrease

         * it on another CPU. Always updated under the runqueue lock:

         */

        unsigned long nr_uninterruptible;



        unsigned long expired_timestamp;

        unsigned long long timestamp_last_tick;

        task_t *curr, *idle;

        struct mm_struct *prev_mm;

        prio_array_t *active, *expired, arrays[2];

        int best_expired_prio;

        atomic_t nr_iowait;



#ifdef CONFIG_SMP

        struct sched_domain *sd;



        /* For active balancing */

        int active_balance;

        int push_cpu;



        task_t *migration_thread;

        struct list_head migration_queue;

        int cpu;

#endif



#ifdef CONFIG_SCHEDSTATS

        /* latency stats */

        struct sched_info rq_sched_info;



        /* sys_sched_yield() stats */

        unsigned long yld_exp_empty;

        unsigned long yld_act_empty;

        unsigned long yld_both_empty;

        unsigned long yld_cnt;



        /* schedule() stats */

        unsigned long sched_switch;

        unsigned long sched_cnt;

        unsigned long sched_goidle;



        /* try_to_wake_up() stats */

        unsigned long ttwu_cnt;

        unsigned long ttwu_local;

#endif

};

6、复杂的宏定义和变量声明也需要注释。例如内核源代码目录下的include/linux/jiffies.h文件中的定义:

/* TICK_USEC_TO_NSEC is the time between ticks in nsec assuming real ACTHZ and  */

/* a value TUSEC for TICK_USEC (can be set bij adjtimex)                */

#define TICK_USEC_TO_NSEC(TUSEC) (SH_DIV (TUSEC * USER_HZ * 1000, ACTHZ, 8))



/* some arch's have a small-data section that can be accessed register-relative

 * but that can only take up to, say, 4-byte variables. jiffies being part of

 * an 8-byte variable may not be correctly accessed unless we force the issue

 */

#define __jiffy_data  __attribute__((section(".data")))



/*

 * The 64-bit value is not volatile - you MUST NOT read it

 * without sampling the sequence number in xtime_lock.

 * get_jiffies_64() will do this for you as appropriate.

 */

extern u64 __jiffy_data jiffies_64;

extern unsigned long volatile __jiffy_data jiffies;

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