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osdep: Document differences in rounding macros 

Make it obvious which macros are safe in which situations.

Useful since QEMU_ALIGN_UP and ROUND_UP both purport to do
the same thing, but differ on whether the alignment must be
a power of 2.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1469129688-22848-4-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Eric Blake 2016-07-21 13:34:47 -06:00 committed by Paolo Bonzini
parent 7423f41782
commit e9fd416e66
1 changed files with 5 additions and 1 deletions
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6
include/qemu/osdep.h
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@ -158,7 +158,8 @@ extern int daemon(int, int);
/* Round number down to multiple */
#define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))
/* Round number up to multiple */
/* Round number up to multiple. Safe when m is not a power of 2 (see
* ROUND_UP for a faster version when a power of 2 is guaranteed) */
#define QEMU_ALIGN_UP(n, m) QEMU_ALIGN_DOWN((n) + (m) - 1, (m))
/* Check if n is a multiple of m */
@ -175,6 +176,9 @@ extern int daemon(int, int);
/* Check if pointer p is n-bytes aligned */
#define QEMU_PTR_IS_ALIGNED(p, n) QEMU_IS_ALIGNED((uintptr_t)(p), (n))
/* Round number up to multiple. Requires that d be a power of 2 (see
* QEMU_ALIGN_UP for a safer but slower version on arbitrary
* numbers) */
#ifndef ROUND_UP
#define ROUND_UP(n,d) (((n) + (d) - 1) & -(d))
#endif