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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. friend void‏ @volatile_void Jun 23

      Out of Context Undefined Behavior Clang cannot optimize f's return value to 1.0f because caller1 is a valid calling context and has to produce 2.0f, but on the other hand, caller2 invokes undefined behavior by violating so-called “strict aliasing” rules. https://godbolt.org/g/aU5uZo pic.twitter.com/nKRjbtYoJn

      2 replies 0 retweets 8 likes
      Show this thread
    2. Rich Felker‏ @RichFelker Jun 23
      Replying to @volatile_void

      Where is the UB? Looks ok to me.

      1 reply 0 retweets 0 likes
    3. friend void‏ @volatile_void Jun 23
      Replying to @RichFelker

      With dynamically allocated memory, the effective type is reset by new writes. When called from caller2, after line 8, that part of memory is an array of unsigned char, it's wrong to read it as a float.

      2 replies 0 retweets 1 like
    4. Rich Felker‏ @RichFelker Jun 23
      Replying to @volatile_void

      I'd need to look up the specifics but the intent is clearly that memcpy-type operations work, and that it's valid to treat the effective type as any (one) type you produced a valid representation for.

      2 replies 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker Jun 23
      Replying to @RichFelker @volatile_void

      WG14 is of course notoriously bad at expressing intent and notoriously bad at admitting they expressed it wrong or imprecisely and fixing it...

      1 reply 0 retweets 0 likes
    6. Rich Felker‏ @RichFelker Jun 23
      Replying to @RichFelker @volatile_void

      By your argument, memcpy and memset wouldn't work at all on allocated storage.

      1 reply 0 retweets 0 likes
    7. friend void‏ @volatile_void Jun 23
      Replying to @RichFelker

      memcpy copies the effective type. memset falls in “For all other accesses to an object having no declared type, the effective type of the object is simply the type of the lvalue used for the access.”

      2 replies 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker Jun 23
      Replying to @volatile_void

      The memcpy function is not special. Doing the equivalent with your own char writes is equally valid.

      1 reply 0 retweets 0 likes
    9. friend void‏ @volatile_void Jun 23
      Replying to @RichFelker

      I think this is what C11 means with “using memcpy or memmove, or is copied as an array of character type”, but then am I allowed to compute the values of the chars off-line as I did in the first function f? How about the third example below? https://godbolt.org/g/5pMB7c pic.twitter.com/ckdBQ4V69E

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Jun 23
      Replying to @volatile_void

      There is no consistent interpretation possible without acknowledging/allowing that allocated storage can have *ambiguous* effective type.

      7:35 PM - 23 Jun 2018
      2 replies 0 retweets 0 likes
        1. friend void‏ @volatile_void Jun 23
          Replying to @RichFelker

          Yes, this is the situation where upon access, the effective type is determined by the lvalue of the lvalue used for reading. This applies to calloc, memset, but I think if the standard wanted it to apply to writing char by char they would have said so more clearly.

          0 replies 0 retweets 0 likes
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        2. Rich Felker‏ @RichFelker Jun 23
          Replying to @RichFelker @volatile_void

          I'm not 100% sure but my suspicion is that you can't make it consistent without leaving the effective type to be determined by the lvalue used for next access after any stores via char types have occurred.

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Jun 23
          Replying to @RichFelker @volatile_void

          IOW same as the memset case.

          0 replies 0 retweets 0 likes
        4. End of conversation

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