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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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How do you set only certain bits of a byte in C without affecting the rest?

Say I have a byte like this 1010XXXX where the X values could be anything. I want to set the lower four bits to a specific pattern, say 1100, while leaving the upper four bits unaffected. How would I do this the fastest in C?
PICyourBrain
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Is there way to match IP with IP+CIDR straight from SELECT query?

Something like SELECT COUNT(*) AS c FROM BANS WHERE typeid=6 AND (SELECT ipaddr,cidr FROM BANS) MATCH AGAINST 'this_ip'; So you don't first fetch all records from DB and then match them one-by one. If c > 0 then were matched. BANS table: id int…
raspi
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bitmask question?

I have the follow: public static final int LIMIT_ONE = 1; public static final int TRADEABLE = (1 << 1); public static final int SELLABLE = (1 << 2); public static final int STORABLE = (1 << 3); public static final int STORABLE_IN_WH = (1 <<…
Prix
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SELECT users from MySQL database by privileges bitmask?

I have users table and want to SELECT some rows by bitmask criteria. I'll try to explain my problem with small example. Structure of table users user_id int [primary key, auto_increment] user_email varchar(200) …
Wh1T3h4Ck5
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Is there any difference between integer and bit(n) data types for a bitmask?

I am working with a table in a PostgreSQL database that has several boolean columns that determine some state (e.g. published, visible, etc.). I want to make a single status column that will store all these values as well as possible new ones in a…
Igor Zinov'yev
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Why should I use bitwise/bitmask in PHP?

I am working on a user-role / permission system in PHP for a script. Below is a code using a bitmask method for permissions that I found on phpbuilder.com. Below that part is a much simpler version w3hich could do basicly the same thing without the…
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PHP debug_backtrace bitmask usage

Trying to understand this entry in the php manual on debug_backtrace. I don't understand what they mean by "this parameter is a bitmask for ...." I have done web searches on bitmasks and my head is spinning round so I have decided I don't really…
Dayo
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Getting upper and lower byte of an integer in C# and putting it as a char array to send to a com port, how?

In C I would do this int number = 3510; char upper = number >> 8; char lower = number && 8; SendByte(upper); SendByte(lower); Where upper and lower would both = 54 In C# I am doing this: int number = Convert.ToInt16("3510"); …
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create a permission bit mask in java

I want to do something like this: public enum Permissions { CanBlah1, CanBlah2, CanBlah3 } byte[] userPerm = Permissions.CanBlah1 | Permissions.CanBlah2; // check permssions // if(userPerm && Permissions.CanBlah1 ==…
mrblah
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How to create a mask with the least significant bits set to 1 in C

How does this function work? A mask with the least significant n bits set to 1. Example: n = 6 --> 0x2F, n = 17 --> 0x1FFFF // I don't get these at all, especially how n = 6 --> 0x2F Also, what is a mask?
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efficiently find the first element matching a bit mask

I have a list of N 64-bit integers whose bits represent small sets. Each integer has at most k bits set to 1. Given a bit mask, I would like to find the first element in the list that matches the mask, i.e. element & mask == element. Example: If my…
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Bitmask in PHP for settings?

Bits and bitmask are something I have been struggling to understand for a while, but I would like to learn how to use them for settings and things like that in PHP. I have finally found a class that claims to do exactly that, and as I can tell, it…
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How and Where to use Bit Mask in java

Please explain to me how and where to use Bit Mask in java: I don't understand the code below: int bitmask=1; if ((bitmask & 1) == 1) // what it does The other questions dint exactly answer why?
Spartan
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Shift masked bits to the lsb

When you and some data with a mask you get some result which is of the same size as the data/mask. What I want to do, is to take the masked bits in the result (where there was 1 in the mask) and shift them to the right so they are next to each other…
cen
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Bitmask: how to determine if only one bit is set

If I have a basic bitmask... cat = 0x1; dog = 0x2; chicken = 0x4; cow = 0x8; // OMD has a chicken and a cow onTheFarm = 0x12; ...how can I check if only one animal (i.e. one bit) is set? The value of onTheFarm must be 2n, but how can I check that…
Tim
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