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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. Rich Felker‏ @RichFelker Mar 5
      Replying to @t2mykleb

      Indeed gcc's definition allows global reads so it's questionable. I was thinking something in between gcc's pure & const. Surely the optimization is valid for const though.

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

      If the function is attr-const it must return the same thing it would if called in a context where malloc would fail.

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

      What if no such context occurs in my program?

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

      It's not under the program's control. Aside from necessarily failing after somewhere on the order of 2^(CHAR_BIT*sizeof(void*)) objects are in existence, it's up to the implementation if/when malloc fails. There is no context where it "can't fail".

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

      The program is under no obligation to attempt to create so many objects simultaneously. Even if it did so, it's under no obligation to call that __attr__((const)) function in such a state.

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

      I think you misread. The point was that under extreme conditions the implementations is forced to make malloc fail, but it's never forced to make it succeed. Failure is always an option.

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

      In particular on any implementation where the invoking user can control resource limits, it's always possible for any particular call to malloc to fail.

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

      That isn't true and you know it isn't true. Classic linked list malloc that never returns memory to the OS will always succeed if you just freed a block of the size you're trying to allocate.

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

      The nature of the malloc implementation isn't under the application's control. On an implementation with shared libs it can change just by upgrade. Failure is always potentially-reachable.

      1 reply 0 retweets 0 likes
    10. 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
      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.

      9:08 PM - 5 Mar 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. 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
        3. 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
        4. 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
        5. 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
        6. 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
        7. 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
        8. Rich Felker‏ @RichFelker Mar 5
          Replying to @RichFelker @t2mykleb

          To be valid it would have to meet them with all possible mallocs.

          0 replies 0 retweets 0 likes
        9. End of conversation

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