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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. Tor Myklebust‏ @t2mykleb Mar 5
      Replying to @RichFelker

      It's under the user's control. In particular, it's not under the compiler's control at the time it's making decisions about how to mess with the programmer's __attr__((const)) function. I think the only avenue of argument here is that malloc/free have a forbidden "effect."

      1 reply 0 retweets 0 likes
    2. Rich Felker‏ @RichFelker Mar 5
      Replying to @t2mykleb

      If it's under the user's control, user input changes the result of the function, => nonpure.

      1 reply 0 retweets 0 likes
    3. Tor Myklebust‏ @t2mykleb Mar 5
      Replying to @RichFelker

      That ship already sailed. nexttoward() is __attr__((const)) in my headers yet it's exposed as a weak symbol => can be overriden by user via LD_PRELOAD and similar => nonpure, according to you. :)

      1 reply 0 retweets 0 likes
    4. Rich Felker‏ @RichFelker Mar 5
      Replying to @t2mykleb

      No, that's not my claim. Redefining the std functions is UB.

      1 reply 0 retweets 0 likes
    5. Tor Myklebust‏ @t2mykleb Mar 5
      Replying to @RichFelker

      I'm not certain whether LD_PRELOAD (or dynamic linking in general) constitute "redefining" standard library functions according to the C standard, but that seems like another ship that has sailed in practice.

      1 reply 0 retweets 0 likes
    6. Tor Myklebust‏ @t2mykleb Mar 5
      Replying to @t2mykleb @RichFelker

      Ultimately, the compiler needs to make conservative assumptions about malloc() and nexttoward(). Given that C compilers today are decoupled from standard libraries, the only properties the compiler can make use of are those guaranteed by the standard.

      1 reply 0 retweets 0 likes
    7. Tor Myklebust‏ @t2mykleb Mar 5
      Replying to @t2mykleb @RichFelker

      In particular, the existence of a malloc impl where malloc never returns 0 when running a given program implies that the compiler cannot assume that a given "malloc fails" branch is reachable.

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker Mar 5
      Replying to @t2mykleb

      I think we just have different views here thst can't be reconciled. To me a program that depends on behavior of a particular malloc impl to satisfy invariants is simply not valid.

      2 replies 0 retweets 0 likes
    9. Tor Myklebust‏ @t2mykleb Mar 5
      Replying to @RichFelker

      You can reasonably argue that such a program is not portable or that it represents very bad engineering. But the programmer gets to use the freedom created by the compiler being conservative.

      1 reply 0 retweets 0 likes
    10. Rich Felker‏ @RichFelker Mar 5
      Replying to @t2mykleb

      My view is that the implementation has freedom to change internals at any time as long as it did not _document_ their consequences as contracts.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Mar 5
      Replying to @RichFelker @t2mykleb

      "malloc always succeeds here because a previously freed chunk gets reused" is an implementation detail that need not be stable and that's not knowable from a black box perspective.

      9:44 PM - 5 Mar 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Tor Myklebust‏ @t2mykleb Mar 5
          Replying to @RichFelker

          But in practice we've broken "the implementation" into a compiler and a standard library, and we permit the user to swap out standard library implementations. Impl details are knowable to the user, but not to the compiler.

          1 reply 0 retweets 0 likes
        3. Tor Myklebust‏ @t2mykleb Mar 5
          Replying to @t2mykleb @RichFelker

          If the compiler uses assumptions about the stdlib that go beyond what's guaranteed by the standard, then by definition it's generating code that's wrong for some stdlib impl.

          2 replies 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker Mar 5
          Replying to @t2mykleb

          Again I understand what you're saying and I disagree. The application cannot depend on stdlib properties that are neither contractual or testable in order to satisfy its promises to the compiler.

          1 reply 0 retweets 0 likes
        5. Tor Myklebust‏ @t2mykleb Mar 5
          Replying to @RichFelker

          The specific __attr__((const)) discussion we had is clouded by the attribute's ambiguous natural-language specification. If you're granting that, then we have a genuine disagreement. Sadly, it is late and I need sleep too much to engage further.

          0 replies 0 retweets 2 likes
        6. End of conversation

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