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    SubjectRe: Style question: Should one check for NULL pointers?
    On Fri, 11 Jul 2003, Richard B. Johnson wrote:

    > On Fri, 11 Jul 2003, David D. Hagood wrote:
    >
    > > But consider the following code:
    > >
    > > sscanf(0,0);
    > >
    > > That IS an error condition - both the string to scan and the format
    > > string are NULL. In this case sscanf should return EITHER 0 (no items
    > > matched) or better still -1 (error).
    > >
    >
    > But it does neither. Instead, it seg-faults your code!
    >
    > The problem lies with the original question. The question
    > referred to "Style" (look at the subject-line). It is
    > not a question about style, but a question about utility
    > and design. Style has nothing to do with it.

    This isn't really an example of the sort of thing I was asking about. I
    was referring specifically to kernel code, not general-purpose userspace C
    library routines like sscanf.

    But maybe you mean the kernel's internal implementation of sscanf. If
    someone in the kernel did write "sscanf(0,0);" then that would definitely
    be an error in the source. It should be brought to the author's attention
    as soon as possible, and a segfault (or BUG_ON) is a much more effective
    way of doing so than simply returning -1.

    > If you are writing code for an embedded system, the code
    > must always run even if RAM got trashed from alpha particles
    > or EMP. In that case, you trap every possible condition and
    > force a restart off a hardware timer, refreshing everything in
    > RAM from PROM or NVRAM.

    Okay, but there's no way at the source level to protect against all kinds
    of events like alpha particles changing a stored value. And what's the
    point in checking just _some_ of them in the source if you're going to
    have to check _all_ of them at a lower (hardware) level anyway?

    > If you are writing code to examine the contents of sys_errlist[],
    > prior to adding another error-code, then you don't check anything
    > and it's file-name is probably a.out, compiled from xxx.c.

    I don't understand that comment.

    > So, the extent to which one checks for exceptions and provides
    > utility for handling exceptions depends upon the code design, not
    > it's style.

    But the kernel already provides a utility for handling exceptions and has
    a fairly fixed design. The extent to which one writes exception checks of
    dubious value is indeed a stylistic issue, at least in this one setting.

    Alan Stern

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