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bug#6131: [PATCH]: fiemap support for efficient sparse file copy
From: |
Jim Meyering |
Subject: |
bug#6131: [PATCH]: fiemap support for efficient sparse file copy |
Date: |
Sun, 26 Sep 2010 10:46:56 +0200 |
Hi Jeff,
This function has problems:
- the inner "zeros" declaration shadows the outer one
and ends up being useless.
- the "sizeof zeros" resolves to 4 or 8. obviously not what you intended.
...
> static bool
> +write_zeros (int fd, uint64_t n_bytes)
> {
> - bool last = false;
> - union { struct fiemap f; char c[4096]; } fiemap_buf;
> - struct fiemap *fiemap = &fiemap_buf.f;
> - struct fiemap_extent *fm_ext = &fiemap->fm_extents[0];
> - enum { count = (sizeof fiemap_buf - sizeof *fiemap) / sizeof *fm_ext };
> - verify (count != 0);
> + char *zeros = calloc (IO_BUFSIZE, sizeof (char));
> + if (! zeros)
> + {
> + /* Try a small buffer. */
> + static char zeros[1024];
> + }
>
> + while (n_bytes)
> + {
> + uint64_t n = MIN (sizeof zeros, n_bytes);
> + if ((full_write (fd, zeros, n)) != n)
> + return false;
> + n_bytes -= n;
> + }
> +
> + return true;
> +}
Please use the following instead.
I'll review the rest tomorrow or Tuesday.
static bool
write_zeros (int fd, uint64_t n_bytes)
{
static char *zeros;
static size_t nz = IO_BUFSIZE;
if (zeros == NULL)
{
static char fallback[1024];
zeros = calloc (nz, 1);
if (zeros == NULL)
{
zeros = fallback;
nz = sizeof fallback;
}
}
while (n_bytes)
{
uint64_t n = MIN (nz, n_bytes);
if ((full_write (fd, zeros, n)) != n)
return false;
n_bytes -= n;
}
return true;
}