为什么会出现不兼容的指针类型警告？

Question

我正在使用内核 3.13.0 编写 Linux 设备驱动程序，但我很困惑为什么会收到此警告。

warning: initialization from incompatible pointer type [enabled by default]
     .read = read_proc,
     ^
warning: (near initialization for ‘proc_fops.read’) [enabled by default]

据我所知，proc 函数的 file_operations 设置与设备函数相同。我可以毫无问题地读取/写入 /dev/MyDevice 并且没有警告。 proc write 函数不会发出警告，只会发出警告。我做错了什么？

/*****************************************************************************/
//DEVICE OPERATIONS
/*****************************************************************************/ 
static ssize_t dev_read(struct file *pfil, char __user *pBuf, size_t
  len, loff_t *p_off)
{
    //Not relevant to this question
}

static ssize_t dev_write(struct file *pfil, const char __user *pBuf,
                         size_t len, loff_t *p_off)
{
    //Not relevant to this question
}

static struct file_operations dev_fops =
{ //None of these cause a warning but the code is identical the proc code below
    .owner = THIS_MODULE,
    .read = dev_read,
    .write = dev_write
};

/*****************************************************************************/
//PROCESS OPERATIONS
/*****************************************************************************/
static int read_proc(struct file *pfil, char __user *pBuf, size_t
              len, loff_t *p_off)
{
    //Not relevant to this question
}

static ssize_t write_proc(struct file *pfil, const char __user *pBuf,
                         size_t len, loff_t *p_off)
{
    //Not relevant to this question
}

struct file_operations proc_fops = 
{
    .owner = THIS_MODULE,
    .write = write_proc,
    .read = read_proc, //This line causes the warning.
};

编辑：所以答案是我是个白痴，因为我没有看到“int”与“ssize_t”。谢谢大家！ Codenheim 和 Andrew Medico 几乎同时给出了正确答案，但我选择了 Medico's，因为它对未来的访客来说更加迂腐和明显。

Answer 1

read_proc 函数的返回类型（引发警告）与干净编译的函数不匹配。

static ssize_t dev_read(struct file *pfil, char __user *pBuf, size_t len, loff_t *p_off)

对比

static int read_proc(struct file *pfil, char __user *pBuf, size_t len, loff_t *p_off)

ssize_t 和 int 的大小可能不同。你的函数的返回类型应该是ssize_t。

Answer 2

在处理文件操作时，只需根据此结构遵循规则

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);
    ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
    ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
    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 *);
    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 *, size_t, unsigned int);
    ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
    int (*setlease)(struct file *, long arg, struct file_lock **);
    long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
    int (*show_fdinfo)(struct seq_file *m, struct file *f); };

您可以在 Kernel Documentation Documentation/filesystems/vfs.txt 中找到这个结构，或者您可以使用来自 Kernel Source 的标签 vim -t file_operations 找到它，或者您可以查看头文件 /include/linux/fs.h。

只是你的错误是你使用 static int 而不是使用 ssize_t 的返回类型。

Answer 3

函数指针需要一个返回类型为 ssize_t 的函数，但您已将其指定为 int 之一

你需要一个ssize_t，而不是int。