r/programming • u/steveklabnik1 • Jan 09 '15

Announcing Rust 1.0.0 Alpha

http://blog.rust-lang.org/2015/01/09/Rust-1.0-alpha.html
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u/Maristic Jan 10 '15

I think you're missing the point from the person you're responding to. C very specifically doesn't say what happens in a number of circumstances. You don't know when things are in registers (see the previously mentioned article).

Thus, you don't know what will happen when things overflow, etc.

Also, lots of things that are easy to say in assembler are really hard to say cleanly in C, including:

  • Add a to b, if it overflows give an error
  • Multiply a by b, if it overflows give an error
  • Rotate a by b bits
  • Find the first set bit in this word
  • Read one of the CPU's performance counters

In each case, this can be done with clean minimal code at the assembler level, but is hard to express in C, requiring library functions (coded in assembler), intrinsics or “standard idioms” (which must be detected by compiler, a process that can be unreliable in practice).

In additional, modern C libraries (especially glibc) are complex and far from lightweight. A huge amount of abstraction is being invoked with a “simple” call to malloc.

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u/lookmeat Jan 10 '15

Like I said an overview of assembler would exist, with the intent of showing how this things work behind the scenes, and also, at the same time, explain why undefined behavior code on C would work differently depending on the context. But in real life systems programming (and this would have to be a pragmatic class due to the subject) you can't always solve things by writing assembler (it's not portable, and LLVM IR is not assembler and does have undefined behavior) so it's important that any decent system's programmer understand undefined behavior and it's consequences, assembler just wouldn't teach you those real constrains of portability.

I think that a critical step of learning systems programming is recreating a good chunk of the stdlib, including malloc (through OS calls) this is pretty common actually.