linux-ext4 - Re: /fs/ext4/ext4.h add a comment to ext4_dir_entry

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Date:   Mon, 4 May 2020 16:26:35 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Jonny Grant <jg@...k.org>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: /fs/ext4/ext4.h add a comment to ext4_dir_entry_2


> On May 3, 2020, at 6:52 AM, Jonny Grant <jg@...k.org> wrote:
> 
> Hello
> 
> Could a comment be added to clarify 'file_type' ?
> 
> struct ext4_dir_entry_2 {
>    __le32    inode;            /* Inode number */
>    __le16    rec_len;        /* Directory entry length */
>    __u8    name_len;        /* Name length */
>    __u8    file_type;
>    char    name[EXT4_NAME_LEN];    /* File name */
> };
> 
> 
> 
> This what I am proposing to add:
> 
>    __u8    file_type;        /* See directory file type macros below */

For this kind of structure field, it makes sense to reference the macro
names directly, like:

	__u8	file_type;	/* See EXT4_FT_* type macros below */

since "macros below" may be ambiguous as the header changes over time.


Even better (IMHO) is to use a named enum for this, like:

        enum ext4_file_type file_type:8; /* See EXT4_FT_ types below */

/*
 * Ext4 directory file types.  Only the low 3 bits are used.  The
 * other bits are reserved for now.
 */
enum ext4_file_type {
	EXT4_FT_UNKNOWN		= 0,
	EXT4_FT_REG_FILE	= 1,
	EXT4_FT_DIR		= 2,
	EXT4_FT_CHRDEV		= 3,
	EXT4_FT_BLKDEV		= 4,
	EXT4_FT_FIFO		= 5,
	EXT4_FT_SOCK		= 6,
	EXT4_FT_SYMLINK		= 7,
	EXT4_FT_MAX,
	EXT4_FT_DIR_CSUM	= 0xDE
};

so that the allowed values for this field are clear from the definition.
However, the use of a fixed-with bitfield (enum :8) is a GCC-ism and Ted
may be against that for portability reasons, since the kernel and
userspace headers should be as similar as possible.

Cheers, Andreas






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