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PaperclipPerfector  3mo ago (text post)   27784 thread views

Culture War Roundup for the week of December 15, 2025

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You are correct.

I would still say that in C, under a few circumstances you can depend on a null pointer access crashing, e.g. if all of the following apply:
(1) You are in standard userland where nothing is normally mapped to 0.
(2) You know that your code will not be run in other settings (for example, you are not writing a library).
(3) You are not handling untrusted data.
(4) You are not in a privilege elevated mode (like in the kernel)

Then you can usually count on getting a segmentation violation for null pointer access, so that failing to do

if (!ptr)
    abort();

will be of limited badness. (Given all these caveats, it is probably less bad

Compare and contrast with another source of undefined behavior: out-of-bounds array accesses. These will typically not cause segfaults, but will instead silently corrupt program data and flow, often leading to arbitrary code execution if exploited. It is the difference between getting killed from smoking and getting killed from smoking while filling up your gas tank.

Now, you could make the point that anyone treating a segfault as a safety net (instead of ruining one's day as much as one's car airbag firing) should not be programming with raw pointers, and I might even agree. In my defense, my baseline is physicists who self-taught C while programming with ROOT, and most of whom have no business coding in any language unsafer than Python, who still happily use C arrays and for whom a segfault on every third compile is just normal.

I would still say that in C, under a few circumstances you can depend on a null pointer access crashing, e.g. if all of the following apply:

(5) Your compiler hasn't (ab)used the fact null pointer accesses are undefined behavior to "simplify" code in unexpected ways.

Eg, consider

int foo( int *p )
{
  if ( p ) function_with_side_effects();
  return *p < INT_MIN;
}

Since p is unconditionally dereferenced in the return statement which results in undefined behavior if p is null, the compiler is free to assume that it is never null and generate code that unconditionally calls function_with_side_effects() rather than inserting a branch as if ( p ) is only ever not true in the presence of undefined behavior. That is, it can treat the code as if it were instead written as

int foo( int *p )
{
  function_with_side_effects();
  return *p < INT_MIN;
}

Depending on exactly what function_with_side_effects() does, that could result in a lot of unsafe outcomes even in "unprivileged user code that never handles untrusted data". Even more fun, since *p < INT_MIN evaluates to 0 for all possible values of *p, the compiler can completely remove the dereference that you'd expect to cause a segfault, thereby treating the code as if it were written as

int foo( int *p )
{
  function_with_side_effects();
  return 0;
}

Thanks, that is a good point.

(Also, I am pretty sure that compiler developers who use undefined behavior that way to 'optimize' code will go to hell eventually, but that will not help me if I am stuck debugging such a thing.)

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