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> This feels like one of those “you’re a fan of X? Name every Y” memes. How can you identify legal C without understanding undefined behavior? Or rather, what

> This feels like one of those “you’re a fan of X? Name every Y” memes. How can you identify legal C without understanding undefined behavior? Or rather, what is the list of UB that nay professional C developer should be expected to know? > If someone has some examples that a reasonable person would write, specifically examples that look like the simplest way to do something, then I’d love to hear them. Basically all OOP-style C. For instance, the Windows macro #define CONTAINING_RECORD(address, type, field) ((type *)( \ (PCHAR)(address) - \ (ULONG_PTR)(&((type *)0)->field))) The Linux kernel is also an excellent source of these. Also, serialization/deserialization libraries are an excellent source for UB. And huge amounts of code depends on UB by using unsigned char arrays for storage of other types. Furthermore, in practice, C code assumes most of the following: - null pointers are represented as 0, e.g. if(ptr). - pointer provenance does not exist - char and unsigned char are 8-bit bytes - two's-complement arithmetic - page-based memory protection - no types have trap representations - the values of base types, besides floating point, do not have multiple representations. - etc

gnubison 5y ago
> what is the list of UB that nay professional C developer should be expected to know? Off the top of my head, as a non professional C programmer: - can’t read from uninitialized memory - one malloc = one free - signed overflow is undefined - pointers to stack variables can’t outlive the stack variable itself - can’t dereference NULL pointers (Obviously take this with a massive grain of salt, there are almost certainly important ones that I’m forgetting.) > null pointers are represented as 0, e.g. if(ptr). If(ptr) is guaranteed by the standard to work. Memset(ptr, 0, len) is not (and is very broken in other ways as well). > char and unsigned char are 8-bit bytes They’re guaranteed to be one byte, where a byte is at least 8 bits. So unless you’re trying to simulate %256 with an implicit conversion, or relying on unsigned overflow to turn 0xFF into 0x00, then you’ll be fine.
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gnubison 5y ago

> what is the list of UB that nay professional C developer should be expected to know? Off the top of my head, as a non professional C programmer: - can’t read from uninitialized memory - one malloc = one free - signed overflow is undefined - pointers to stack variables can’t outlive the stack variable itself - can’t dereference NULL pointers (Obviously take this with a massive grain of salt, there are almost certainly important ones that I’m forgetting.) > null pointers are represented as 0, e.g. if(ptr). If(ptr) is guaranteed by the standard to work. Memset(ptr, 0, len) is not (and is very broken in other ways as well). > char and unsigned char are 8-bit bytes They’re guaranteed to be one byte, where a byte is at least 8 bits. So unless you’re trying to simulate %256 with an implicit conversion, or relying on unsigned overflow to turn 0xFF into 0x00, then you’ll be fine.

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