Le 22/03/16 18:43, Garrett D'Amore a écrit :
> Running head on /dev/zero is … wrong. Arguably without -c it should loop
> forever until it crashes due to malloc() failure, or it should copy an
> infinite sequence of zero bytes because it never reaches a newline.
>
> The problem is that zero is a special character (end of string) — its unclear
> to me what the correct behavior here should be. What happens if you embed
> zero bytes in a file?
>
> Note that in POSIX, head lacks -c, so the behavior is not part of any
> standard.
>
> I *think* the “right” answer here is that NUL should be treated like any
> other character, which precludes the use of normal C string handling
> functions in head. Even fgets(3C) and getline(3C) cannot be used.
>
> Seriously, head and copy are for use in text files, not with arbitrary binary
> data files, and so their handling (or mishandling) of binary data should not
> come as a surprise.
>
> That said, on BSD (Darwin) head -c 1 copies a single zero byte. head without
> -c just returns an error. (Not sure what the error is, because I can’t
> dtruss it in new Darwin. Thanks Apple.)
>
> - Garrett
>
Well, 'man' does say that it copies 'n' bytes, not characters.
I can dd a byte from /dev/zero and head -c 1 now returns
but it seems still buggy...
Truss shows it reads twice:
> open64("/tmp/fooz", O_RDONLY) = 3
> fstat64(3, 0x08047570) = 0
> fstat64(3, 0x08047470) = 0
> ioctl(3, TCGETA, 0x0804752E) Err#25 ENOTTY
> read(3, "\0", 4096) = 1
> read(3, 0x0806346C, 4096) = 0
> ioctl(1, TCGETA, 0x080467BE) = 0
> fstat64(1, 0x08046700) = 0
> read(3, 0x0806346C, 4096) = 0
> llseek(3, 0, SEEK_CUR) = 1
> _exit(0)
and if I try to do a 'wc -c'
> $ head -c 1 /tmp/fooz |wc -c
> 0
> $ sh -c 'head -c 1 /tmp/fooz |wc -c'
> 1
> $ ghead -c 1 /tmp/fooz |wc -c
> 1
just to compare to gnu head and sh builtin
--
Richard PALO
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