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    SubjectRe: [PATCH 2/4] arch/arm/mach-at91/clock.c: Add missing IS_ERR test
    On Tue, 25 Jan 2011, Russell King - ARM Linux wrote:

    > On Tue, Jan 25, 2011 at 12:18:40PM +0100, Julia Lawall wrote:
    > > On Tue, 25 Jan 2011, walter harms wrote:
    > > > So these is a bug ? They should return -ENOENT ?
    > > >
    > > > The interessting question is: what to do with an error ?
    > > >
    > > > Obviously some architecture can live with NULL, so it is not an critical
    > > > error. An the patch shows a code that is simply a return, not even the
    > > > user is informed that something did not work as expected.
    > > >
    > > > From that point of view i would like question if it is useful to have
    > > > a "detailed" error instead of just returning NULL.
    > >
    > > Somewhat unrelatedly, I often run into code where error handling code is
    > > needed, but not present, and the function returns void, so nothing is
    > > provided for propagating the error further. I generally consider these
    > > cases to be beyond my expertise to fix...
    > That is a pain, but so is returning NULL in error conditions. If you've
    > got several layers of nesting, and every level returns NULL on error,
    > it's an awful lot of debugging to find out _why_ a failure happened.
    > With error codes, it narrows down the number of places which could have
    > returned that error code, and as error codes can be descriptive, it
    > turns it into an "oh, I forgot about doing X" or "it's failing *there*"
    > rather than a puzzle.
    > The only place where it really makes sense to return NULL is with memory
    > allocators. NULL is an accepted value for meaning "I couldn't allocate
    > memory" as its not a useful pointer value.
    > The alternative is to have an API like:
    > struct clk *clk_get(int *error, ...)
    > or
    > int clk_get(struct clk **, ...)
    > but that then leads to _additional_ errors made by driver authors and by
    > implementations - you can no longer guarantee that *error will always be
    > initialized, and this is why the whole ERR_PTR/PTR_ERR/IS_ERR stuff was
    > implemented. The kernel used to have such things in it and they were
    > buggy.

    I agree that error codes are very useful. The problem is rather how to
    propagate any sort of error indicator, whether ERR_PTR, NULL, an negative
    integer, etc.


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