Skip to content

handy.h: Avoid UB in nBIT_MASK() #23976

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
khwilliamson merged 1 commit into from
Dec 4, 2025
+9 −2
Merged

handy.h: Avoid UB in nBIT_MASK() #23976

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
11 changes: 9 additions & 2 deletions handy.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -342,8 +342,15 @@ don't, so that you can portably take advantage of this C99 feature.
# define isPOWER_OF_2(n) ((n) && ((n) & ((n)-1)) == 0)
#endif

/* Returns a mask with the lowest n bits set */
#define nBIT_MASK(n) ((UINTMAX_C(1) << (n)) - 1)
/* Returns a mask with the lowest n bits set. Avoids undefined behavior if n
* is, say, 64 on a 64-bit system, by:
* 1) shifting only 63 yielding 1000000...000
* 2) subtracting 1, yielding 0111111...111
* 3) shifting by 1, yielding 1111111...110
* 4) adding the final bit yielding all 1's */
#define nBIT_MASK(n) \
(((n) == 0) ? 0 : (((UINTMAX_C(1) << ((n) - 1)) - 1) << 1) | 1)
// |_________________________|

/* The largest unsigned number that will fit into n bits */
#define nBIT_UMAX(n) nBIT_MASK(n)
Expand Down
Loading