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76 lines
2.5 KiB
76 lines
2.5 KiB
4 years ago
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---
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title: Booleans are wasted memory
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date: "2020-11-06"
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---
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A boolean is either `true` or `false`. That translates to 1 or 0. If you think that one bit is enough to store this information, you'd be wrong.
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In order to keep the binary layout of a program simple and convenient, most languages store information in either 4 bit or 8 bit blocks. If you allocate a `bool` in C, you will get [an integer constant with a value of either 1 or 0](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdbool.h.html), taking up 4 bits.
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If you allocate a bool in Rust or (most) other languages that are based on LLVM, [it will take up 1 `i1`, or 8 bit of memory](https://llvm.org/docs/LangRef.html#simple-constants).
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## There's a better way
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If you find yourself having to store multiple boolean states somewhere, you might simply declare those booleans and call it a day:
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```c
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#include <stdbool.h>
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#include <stdio.h>
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int main()
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{
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bool can_read = true;
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bool can_write = false;
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bool can_execute = true;
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if (can_read)
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printf("read bit set\n");
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if (can_write)
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printf("write bit set\n");
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if (can_execute)
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printf("execute bit set\n");
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// Output:
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// read bit set
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// write bit set
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}
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```
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An alternative approach to store boolean values is to share a "chunk" of bits with other values. This is usually done using bitwise operations:
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```c
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#include <stdbool.h>
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#include <stdio.h>
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// Define permissions
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#define PERM_NONE 0b000
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#define PERM_READ 0b001
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#define PERM_WRITE 0b010
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#define PERM_EXECUTE 0b100
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#define PERM_ALL PERM_READ | PERM_WRITE | PERM_EXECUTE
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int main()
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{
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// Set permissions
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int permissions = PERM_READ | PERM_WRITE;
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if (permissions & PERM_READ)
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printf("write bit set\n");
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if (permissions & PERM_WRITE)
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printf("read bit set\n");
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if (permissions & PERM_EXECUTE)
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printf("execute bit set\n");
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// Output:
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// read bit set
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// write bit set
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}
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```
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This example still wastes 1 bit since we only use 3 out of 4 possible bits of the integer type, but I'm sure you get the point. Whenever you find yourself needing multiple boolean values, think twice if you can use this pattern.
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Microcontrollers have a very constrainted environment, therefore bitwise operations are essential in those scenarios. 7 wasted bytes are a lot if there are only 4 kb of total memory available. For larger systems we often forget about these constraints, until they add up.
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## My Plea
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* Be mindful about the software you create.
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* Appreciate the resources at your disposal.
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