[DowsingSpoon]

Write once run anywhere? But C already is a "write once run anywhere" language! Though, you usually have to recompile first :)

The criticisms related to UB are not about understanding the target platform and the target compiler's behavior. Undefined Behavior is not the same thing as Implementation-defined Behavior, and lots of folks (including me) would be satisfied with reclassifying chunks of UB as the latter.

The behavior of the target platform isn't really the issue. C23 mandates two's complement for signed integers. Most hardware wraps on overflow, but that literally doesn't matter. The standard says a program exhibiting signed overflow is undefined, period.

In practice, UB rules mean the compiler is free to remove checks for signed overflow/underflow, checks for null pointers, etc. This can and does happen. Man, just a few weeks ago, I just had to deal with a crash in a C program that turned out to be due to the compiler removing a null check. That was a painful one.

[prerok]

> crash in a C program that turned out to be due to the compiler removing a null check.

The what now? Though not lately, I did program in C for 15 years and never seen something like this. I did see some compiler bugs on obscure platforms (SINIX, IRIX, HPUX on Itanium64, etc.) with proprietary compilers, this kind of thing would make really get me shouting.

Were you able to determine why the compiler did this? Is it a bug in the compiler?

[jsmith45]

If the compiler can find any operation prior to the null check that would be UB if the value is null (even if it is something that in assembly would be harmless, like performing pointer arithmetic on it), the compiler is allowed to assume the pointer is not null, and thus omit the null check. This could then lead to something that will in practice cause problems like dereferencing the pointer.

Compilers keep taking more and more advantage of inferring that a values in variables cannot be `x`, because if it were than some previous usage would have been UB. When people file bugs to complain, the compiler authors point at the spec which allows them to assume that UB behavior never happens, so the compiler behavior is legal. The only counterargument is if the compiler has chosen to document some specific behavior for this UB (possibly only with specific flags enabled) in which case the compiler testing that scenario as proof of impossibility is indeed a bug (when the required flags are set).