Why is it impractical to check -ENODEV on debugfs api
debugfs api like debugfs_create_dir said
If debugfs is not included in the kernel, -ENODEV will be returned. It is impractical to check for this value, but rather check for NULL or! NULL instead, to eliminate the need for #ifdef in the calling code.
But why is it not wise? Could you give some examples about eliminate the need for #ifdef in the calling code
?
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It is possible to interpret -ENODEV as a success, as long as dentry does not return something other than delete (debugfs_remove) and uses it as a directory for another file (debugfs_create_-family). All of these functions handle the case where CONFIG_DEBUG_FS is disabled correctly.
This is why it is only enough to check for NULL or! NULL.
In the rare case where the dentry returned by debugfs_create _ * () is passed explicitly (for example, to access private inode data), it is sufficient to check that only the first such call does not return -ENODEV.
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Yes. This advice doesn't make sense. If CONFIG_DEBUG_FS is not activated, the return value is -ENODEV (cast to pointer). It is clearly not NULL, but it is also not used as a valid pointer.
It would be better to use the return value instead of IS_ERR_OR_NULL (ptr). If CONFIG_DEBUG_FS is not defined, it will return -ENODEV. If CONFIG_DEBUG_FS is defined but the call fails, NULL is returned. This method handles every opportunity.
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You asked, "Could you provide any examples of eliminating the need for #ifdef in the calling code?"
Well one way to write your code:
#ifdef CONFIG_DEBUG_FS
< ... do your debugfs stuff ... >
#endif
I would guess that this meant documentation.
Many kernel developers think that too many pairs #ifdef - #endif
in the code should be ugly ("hairy"). See this kernel document .
Relevant core code snippet Document code: "Prefer to compile entire functions rather than parts of functions or parts of expressions. Instead of putting an ifdef in an expression, output part or all of the expressions into a separate helper function and apply conditionally to that function."
Considering that following his advice, if you'd rather not use #ifdef, then pl goes with the already posted answers.
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