5 ms·

You haven't read the code properly. The memcpy() copies one character. After shgetc() reads that and possibly a few more characters, the entire FILE object go

by JdeBP 6y ago

You haven't read the code properly. The memcpy() copies one character. After shgetc() reads that and possibly a few more characters, the entire FILE object goes away and the next call to sscanf() sets up an entirely fresh nonce FILE object and does the initial fill all over again, including scanning the stuff that it has already scanned for NUL, a few characters further on. As I just explained.

Jasper_ 6y ago
Yes, but it's not scanning the entire string, it's scanning at most 256 bytes. In MSVC, the number of bytes scanned is `M*N` where M is the number of calls to scanf and N is the size of the file. In musl, N is now 257, no matter the size of the file. Compared to ~10MB of data, that's basically free. Sure, it's scanning a bit more than maybe it needs to, but it's much closer to a non-scanning libc in the amount of data it's consuming.
- JdeBP 6y ago
  And this will be the third time that I've written the phrase "not entirely free of this behaviour", in comparison with the implementations that are entirely free of this behaviour.
  - wnoise 6y ago
    It's not entirely free of overhead. The overhead is constant per invocation though, so it's entirely free of _quadratic_ behavior. Which is what's being discussed.
    - JdeBP 6y ago
      No, that was not what was being discussed, neither here nor in that discussion. See https://news.ycombinator.com/item?id=26297612 https://news.ycombinator.com/item?id=26297612 . None of us mentioned quadratic behaviour. We are talking about sscanf() calling strlen(), and in the original discussion why.
      - wnoise 6y ago
        Sure, the quadratic behavior is attached to the "calling sscanf in a loop". The point is linear behavior for a call to sscanf. Which isn't the case for musl. Nor does musl call strlen, or even effectively call it. It will bizarrely scan for a NUL in up to 256 bytes ahead of where in the string it is currently. But that's not actually the same behavior as calling strlen or having other linear behavior.

by JdeBP 6y ago

Jasper_ 6y ago

Yes, but it's not scanning the entire string, it's scanning at most 256 bytes. In MSVC, the number of bytes scanned is `M*N` where M is the number of calls to scanf and N is the size of the file. In musl, N is now 257, no matter the size of the file. Compared to ~10MB of data, that's basically free. Sure, it's scanning a bit more than maybe it needs to, but it's much closer to a non-scanning libc in the amount of data it's consuming.

JdeBP 6y ago
And this will be the third time that I've written the phrase "not entirely free of this behaviour", in comparison with the implementations that are entirely free of this behaviour.
- wnoise 6y ago
  It's not entirely free of overhead. The overhead is constant per invocation though, so it's entirely free of _quadratic_ behavior. Which is what's being discussed.
  - JdeBP 6y ago
    No, that was not what was being discussed, neither here nor in that discussion. See https://news.ycombinator.com/item?id=26297612 https://news.ycombinator.com/item?id=26297612 . None of us mentioned quadratic behaviour. We are talking about sscanf() calling strlen(), and in the original discussion why.
    - wnoise 6y ago
      Sure, the quadratic behavior is attached to the "calling sscanf in a loop". The point is linear behavior for a call to sscanf. Which isn't the case for musl. Nor does musl call strlen, or even effectively call it. It will bizarrely scan for a NUL in up to 256 bytes ahead of where in the string it is currently. But that's not actually the same behavior as calling strlen or having other linear behavior.