5 ms·

Old school comment... back in the day, there were computers on which dereferencing the null pointer would in fact return zero. And given that a typical linked

Old school comment... back in the day, there were computers on which dereferencing the null pointer would in fact return zero. And given that a typical linked list is terminated by a null, mistakes could creep in where "points to a 0" was checked when "pointer is 0" should have been. Needless to say, code like this would crash in mysterious ways when compiled on a machine where location 0, while readable, had nonzero contents. The alternative, getting a segfault because 0 wasn't mapped, is preferable because then you see the problem right away. Anyhow it's just barely possible that this assignment was a hack to make this sort of code work on a machine where address 0 is (uninitialized) nonzero and can be harmlessly made zero.

AlotOfReading 3y ago
I've actually seen code equivalent to this used for control flow. It was a very old bit of safety-critical code(!) that initialized a bunch of hardware from predefined tables containing pointers to overrides. For table values that weren't overridden (very few by the time I came along), it would access the null pointer to intentionally trap so an interrupt could insert the correct default and restart the instruction. Certain higher ups determined that behavior was a business requirement when I refused to reimplement it on a new platform.
pavlov 3y ago
IBM AIX is a Unix-like system on which dereferencing a null pointer for a read is guaranteed to return zero. But writes will crash. It’s pretty weird.
- twoodfin 3y ago
  HP-UX binaries also had an attribute that would control this behavior. https://docstore.mik.ua/manuals/hp-ux/en/B2355-60130/chatr_pa.1.html https://docstore.mik.ua/manuals/hp-ux/en/B2355-60130/chatr_p... Wonderful fun when they shipped a Kerberos library that unconditionally dereferenced a potentially null optional pointer-to-struct, relying on the executable to be set in the “null tolerant” mode!
rsaxvc 3y ago
Many(most?) Cortex-M4 systems boot from the reset vector and stack pointer stored in the first 8B of memory, so it's a valid address on a ton of small systems today. You can relocate the vector table and to block it off with the MPU if you have one and memory to spare.
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AlotOfReading 3y ago

I've actually seen code equivalent to this used for control flow. It was a very old bit of safety-critical code(!) that initialized a bunch of hardware from predefined tables containing pointers to overrides. For table values that weren't overridden (very few by the time I came along), it would access the null pointer to intentionally trap so an interrupt could insert the correct default and restart the instruction. Certain higher ups determined that behavior was a business requirement when I refused to reimplement it on a new platform.

pavlov 3y ago

IBM AIX is a Unix-like system on which dereferencing a null pointer for a read is guaranteed to return zero. But writes will crash. It’s pretty weird.

twoodfin 3y ago
HP-UX binaries also had an attribute that would control this behavior. https://docstore.mik.ua/manuals/hp-ux/en/B2355-60130/chatr_pa.1.html https://docstore.mik.ua/manuals/hp-ux/en/B2355-60130/chatr_p... Wonderful fun when they shipped a Kerberos library that unconditionally dereferenced a potentially null optional pointer-to-struct, relying on the executable to be set in the “null tolerant” mode!

rsaxvc 3y ago

Many(most?) Cortex-M4 systems boot from the reset vector and stack pointer stored in the first 8B of memory, so it's a valid address on a ton of small systems today. You can relocate the vector table and to block it off with the MPU if you have one and memory to spare.

deleted 3y ago

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