Reserve space C

Question

Can you tell me the best way to reserve for example 1000 bits from memory in C? I'm doing a program and i need to manage flags (values =0 or =1). What is the most efficient way to do that? I was thinking in reserve the space needed and control the flags (bits) using masks. Thank you

Which is more important: minimizing the amount of space this takes up, or minimizing the time it takes to access an individual value? — robert, Jun 02 '12 at 19:47
I would be surprised if toggling flags would be a bottle neck. Usually its the process that the flags correspond to? — Mikhail, Jun 02 '12 at 23:39

score 3 · Answer 1 · edited May 23 '17 at 11:56

3

The least amount of memory you can access/address in C is a byte which is of CHAR_BIT bits width (which is guranteed to be at least 8 bits long). So, if you are looking for a packed structure you could use ceil(1000/8) bytes.

You may want to take a look at this SO question for bit manipulation details.

edited May 23 '17 at 11:56

Community

1
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answered Jun 02 '12 at 19:47

dirkgently

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+1: This solution minimize memory usage. Since the OP didn't specify what "efficient" means, both this technique and Ouah's answer the question. – wallyk Jun 02 '12 at 19:59

score 2 · Answer 2 · answered Jun 02 '12 at 19:54

2

You can use the bool type:

#include <stdbool.h> // for bool
#include <stdlib.h>  // for malloc

#define N  1000

bool *b = malloc(N * sizeof *b);

// for example to set the 42th element to 1
b[41] = 1;

answered Jun 02 '12 at 19:54

ouah

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+1: This solution minimizes access time. No need to use `malloc()` though. It could be a non-heap variable, either local to a function or global. – wallyk Jun 02 '12 at 19:58

score 0 · Answer 3 · edited May 23 '17 at 11:56

something like?

struct bitf
  {

    unsigned char b:1;
  };
struct bitf[1000];

if you won't to use bitmask the max size is the max size of a variable on your computer. Error trying to define a 1,024-bit (128 Byte) Bit Field

or for min memory:

#include <stdio.h>

int main(void)
{

  struct bitf
  {

    unsigned char a:1;
    unsigned char b:1;
    unsigned char c:1;
    unsigned char d:1;
    unsigned char e:1;
    unsigned char f:1;
    unsigned char g:1;
    unsigned char h:1;

  };

  struct bitf number[125];

  number[100].a = 1;
  number[110].h =1;
  printf("size: %d  a:%d   h:%d\n",sizeof(number), number[100].a ==0, number[110].h ==1);


  return 0;
}

output:

size: 125  a:0   h:1

Reserve space C

3 Answers3