On Thu, Aug 20, 2020 at 01:02:55PM +0200, Peter Krempa wrote:
On Tue, Jul 07, 2020 at 21:46:26 +0200, Michal Privoznik wrote:
This function will be used to detect zero buffers (which are going to be interpreted as hole in virStream later).
I shamelessly took inspiration from coreutils.
Coreutils is proudly GPLv3 ...
Signed-off-by: Michal Privoznik <mprivozn@redhat.com> --- src/libvirt_private.syms | 1 + src/util/virstring.c | 38 ++++++++++++++++++++++++++++++++ src/util/virstring.h | 2 ++ tests/virstringtest.c | 47 ++++++++++++++++++++++++++++++++++++++++ 4 files changed, 88 insertions(+)
[...]
diff --git a/src/util/virstring.c b/src/util/virstring.c index e9e792f3bf..c26bc770d4 100644 --- a/src/util/virstring.c +++ b/src/util/virstring.c @@ -1404,3 +1404,41 @@ int virStringParseYesNo(const char *str, bool *result)
return 0; } + + +/** + * virStringIsNull:
IsNull might indicate that this does a check if the pointer is NULL. You are checking for NUL bytes.
+ * @buf: buffer to check + * @len: the length of the buffer + * + * For given buffer @buf and its size @len determine whether + * it contains only zero bytes (NUL) or not.
Given the semantics of C strings being terminated by the NUL byte I don't think this function qualifies as a string helper and thus should probably reside somewhere outside of virstring.h
+ * + * Returns: true if buffer is full of zero bytes, + * false otherwise. + */ +bool virStringIsNull(const char *buf, size_t len) +{ + const char *p = buf; + + if (!len) + return true; + + /* Check up to 16 first bytes. */ + for (;;) { + if (*p) + return false; + + p++; + len--; + + if (!len) + return true; + + if ((len & 0xf) == 0) + break; + }
Do we really need to do this optimization? We could arguably simplify this to:
if (*buf != '\0') return false;
return memcmp(buf, buf + 1, len - 1);
Depends whether we care about this sparsification having goood performance or not. As a point of reference, QEMU has invested tonnes of effort in its impl, using highly tuned impls for SSE, AVX, etc switched at runtime. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|