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Questions tagged [bitmask]

Bitmask is a technique used to isolate specific bits in a byte in order to work only on the desired ones. They are used in IP addresses to separate the network prefix and the host number for example.

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Regarding bit masking in C. Why (~(~0 << N)) is preferred than ((1 << N) -1)?

I do know that ~0 will evaluate the maximum word sized bit 1s (and thus takes caring of portability), but I am still not getting why ((1 << N) - 1) is discouraged? Please share if you used the second form and got into any trouble.
MS.
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Given a list and a bitmask, how do I return the values at the indices that are True?

I start with the following list s and bitmask b: s = ['baa', 'baa', 'black', 'sheep', 'have', 'you', 'any', 'wool'] b = [1, 0, 0, 0, 1, 1, 1, 0] # or any iterable with boolean values How do I write some function apply_bitmask(s, b) so that it…
Kit
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How to efficiently check against bitmask?

I am using inotify and want to efficiently check against the reported bitmask event (see inotify man page). Now I could brutely check against every bit on every event, but that would be extremely crude, if not stupid, as I would have N conditionals…
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Combining Enum Value using Bitmask

I understand it's possible to use bitmasks in enum values, but I don't know how to create it. I have a simple enum : enum State { minimizing = 0, maximizing, minimized, maximized }; A state is always State.minimized or…
Thomas Joulin
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Fastest way to produce a mask with n ones starting at position i

What is the fastest way (in terms of cpu cycles on common modern architecture), to produce a mask with len bits set to 1 starting at position pos: template constexpr T make_mask(std::size_t pos, std::size_t len) { // Body of the…
Vincent
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NumPy boolean array warning?

I have a few numpy arrays, lets say a, b, and c, and have created a mask to apply to all of them. I am trying to mask them as such: a = a[mask] where mask is a bool array. It is worth noting that I have verified that len(a) = len(b) = len(c) =…
pretzlstyle
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Most efficient way to extract bit flags

I have these possible bit flags. 1, 2, 4, 8, 16, 64, 128, 256, 512, 2048, 4096, 16384, 32768, 65536 So each number is like a true/false statement on the server side. So if the first 3 items, and only the first 3 items are marked "true" on the…
Nathan
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Bitmask switch statement

I have this code in a section of my project: enum myEnum { invalid = -1, val1 = 1, val2 = 2, val3 = 4 }; int bitmask = val1 | val3; if(bitmask & val1) ... if(bitmask & val2) ... if(bitmask & val3) ... This is fine, and…
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Creating a mask with N least significant bits set

I would like to create a macro or function1 mask(n) which given a number n returns an unsigned integer with its n least significant bits set. Although this seems like it should be a basic primitive with heavily discussed implementations which…
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Bit masking (javascript): how to check ALL flags

If I have the number '00001000' and the mask '00101000', how can i check, with a binary operation, if both the bits in the number are set? number & mask return true if at least one bit match, but I need to check for all of them to match. How to?
pistacchio
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Does this type of function or technique have a name?

HI there, I'm slightly new to programming, more of a hobby. I am wondering if a the following logic or technique has a specific name, or term. My current project has 7 check boxes, one for each day of the week. I needed an easy to save which boxes…
user297459
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Matching binary patterns in C

I'm currently developing a C program that needs to parse some bespoke data structures, fortunately I know how they are structured, however I'm not sure of how to implement my parser in C. Each of the structures are 32-bits in length, and each…
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Correct usage of bitmask?

heyhey, just have a question about bitmasks. I think I know now what they are and where they can be used. I want to store specific permissions like BUILD, BREAK and INTERACT and maybe more for specific groups. The code below should do this but Im…
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Using sigaction(), c

I was doing a little reading about sigaction() (sources are from my course notes) and I'm not sure I understand this text: The signal mask is calculated and installed only for the duration of the signal handler. By default, the signal “sig” is…
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C++11 and [17.5.2.1.3] Bitmask Types

The Standard allows one to choose between an integer type, an enum, and a std::bitset. Why would a library implementor use one over the other given these choices? Case in point, llvm's libcxx appears to use a combination of (at least) two of these…
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