函數(shù)指針定義的一個(gè)錯(cuò)誤
1. 問題
粉絲提問：某個(gè)函數(shù)指針的使用：編譯時(shí)出錯(cuò)了。
type defaults to 'int' in declaration of 'on_touch_messgae_handle'[-Wimplicit-int] typedef(*on_touch_messgae_handle)(touch_message_t);
粉絲源碼如下：
2. 分析
1) 結(jié)構(gòu)解析
- 1 struct touch_message
- 2 {
- 3 rt_uint16_t x;
- 4 rt_uint16_t y;
- 5 rt_uint8_t event;
- 6 };
- 7 typedef struct touch_message * touch_message_t;
- 8 typedef (*on_touch_messgae_handle)(touch_message_t);
首先看下7行這個(gè)類型定義：
- typedef struct touch_message * touch_message_t;
定義后
- touch_message_t
等價(jià)于
- struct touch_message *
就是說我們?nèi)绻胻ouch_message_t 定義的變量是一個(gè)struct touch_message類型的一個(gè)指針。
再來分析下8行這個(gè)定義：
- typedef (*on_touch_messgae_handle)(touch_message_t);
可以替換成下面這個(gè)定義
- typedef (*on_touch_messgae_handle)(struct touch_message *);
2) 分步解析
有的C語言基礎(chǔ)不是很好的朋友，可能無法一眼看出來這個(gè)定義， 為了讓新手更容易看懂，我們來看一下下面一個(gè)遞進(jìn)式的定義：
- int fun;
這是一個(gè)整型變量fun;
- int fun();
這是一個(gè)函數(shù)fun， 參數(shù) ：空 返回值：int型
- int fun(struct touch_message *);
這是一個(gè)函數(shù)fun， 參數(shù) ：struct touch_message *的一個(gè)指針 返回值：int型
上述的變化都好理解，下面我們將fun做如下修改：
- int (*fun)(struct touch_message *);
括號(hào)的優(yōu)先級(jí)最高，(fun)一旦如此定義，那么fun就要先和結(jié)合， 所以fun變成了一個(gè)指針，
那么該指針指向什么呢?就需要看外面是如何定義的， 右邊是(struct touch_message * )，左邊是int， 所以說明指針指向的是一個(gè)函數(shù)，
參數(shù) ：struct touch_message *的一個(gè)指針 返回值：int型
舉例：將函數(shù)my_fun賦值給函數(shù)指針fun。int my_fun(struct touch_message *) { } int (*fun)(struct touch_message *); fun = my_fun;
這里有一個(gè)隱藏的知識(shí)點(diǎn)，函數(shù)名其實(shí)也是一個(gè)地址，而且賦值的時(shí)候函數(shù)類型必須和函數(shù)指針類型一致。
- typedef int (*fun)(struct touch_message *);
如果左邊再加上typedef呢?相當(dāng)于是設(shè)置fun為新的類型，我們可以用fun來定義一個(gè)函數(shù)指針， 該函數(shù)類型同上。
舉例 用新的類型定義一個(gè)函數(shù)指針變量，并給他賦值。typedef int (*fun)(struct touch_message *); int my_fun(struct touch_message *) { } fun fun_ptr; fun_ptr = my_fun;
然后將參數(shù)修改為，touch_message_t，就得到了粉絲的源碼中的樣子，
- typedef int (*fun)(touch_message_t);
但是粉絲的源碼中定義的函數(shù)類型缺少了對(duì)函數(shù)返回值的描述，所以左側(cè)增加一個(gè)int或者其他類型即可即可。
3. 函數(shù)指針
函數(shù)指針在linux內(nèi)核中使用非常頻繁，
比如字符設(shè)備，內(nèi)核給多有的字符設(shè)備提供了一個(gè)統(tǒng)一的接口，我們對(duì)設(shè)備的所有操作被抽象成read、write、open、close等，并封裝到結(jié)構(gòu)體struct file_operations 中：
- struct file_operations {
- struct module *owner;
- loff_t (*llseek) (struct file *, loff_t, int);
- ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
- ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
- ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
- ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
- int (*iterate) (struct file *, struct dir_context *);
- unsigned int (*poll) (struct file *, struct poll_table_struct *);
- long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
- long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
- int (*mmap) (struct file *, struct vm_area_struct *);
- int (*open) (struct inode *, struct file *);
- int (*flush) (struct file *, fl_owner_t id);
- int (*release) (struct inode *, struct file *);
- int (*fsync) (struct file *, loff_t, loff_t, int datasync);
- int (*aio_fsync) (struct kiocb *, int datasync);
- int (*fasync) (int, struct file *, int);
- int (*lock) (struct file *, int, struct file_lock *);
- ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
- unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
- int (*check_flags)(int);
- int (*flock) (struct file *, int, struct file_lock *);
- ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
- ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
- int (*setlease)(struct file *, long, struct file_lock **);
- long (*fallocate)(struct file *file, int mode, loff_t offset,
- loff_t len);
- int (*show_fdinfo)(struct seq_file *m, struct file *f);
- };
那么我們應(yīng)該如何定義該結(jié)構(gòu)體變量并初始化呢?
- static struct file_operations hello_ops =
- {
- .open = hello_open,
- .release = hello_release,
- .read = hello_read,
- .write = hello_write,
- };
函數(shù)定義如下：
- static int hello_open (struct inode *inode, struct file *filep)
- {
- return 0;
- }
- static int hello_release (struct inode *inode, struct file *filep)
- {
- return 0;
- }
- static ssize_t hello_read (struct file *filep, char __user *buf, size_t size, loff_t *pos)
- {
- return size;
- }
- static ssize_t hello_write (struct file *filep, const char __user *buf, size_t size, loff_t *pos)
- {
- return size;
- }
注意，函數(shù)的參數(shù)和返回值，必須嚴(yán)格按照結(jié)構(gòu)體struct file_operations中的類型定義。【編輯推薦】
