How can I check my byte flag, verifying that a specific bit is at 1 or 0?

Question

I use a byte to store some flag like 10101010, and I would like to know how to verify that a specific bit is at 1 or 0.

Answer 1

As an extension of @Daoks answer

When doing bit-manipulation it really helps to have a very solid knowledge of bitwise operators.

Also the bitwise "AND" operator in C is &, so what you are wanting to do is:

unsigned char a = 0xAA; // 10101010 in hex
unsigned char b = (1 << bitpos); //Where bitpos is the position you want to check

if(a & b) {
    //bit set
}

else {
    //not set
}

Above I used the bitwise "AND" (& in C) to check whether a particular bit was set or not. I also used two different ways of formulating binary numbers. I highly recommend you check out the Wikipedia link above.

Answer 2

You can do as Daok says and you make a bit to bit OR to the resulting of the previous AND operation. In this case you will have a final result of 1 or 0.

Answer 3

Use a bitwise (not logical!) and to compare the value against a bitmask.

if (var & 0x08) {
  /* the fourth bit is set */
}

Answer 4

byte THIRDBIT = 4; // 4 = 00000100 i.e third bit is set

int isThirdBitSet(byte in) {
 return in & THIRDBIT; // Returns 1 if the third bit is set, 0 otherwise
}

Answer 5

Traditionally, to check if the lowest bit is set, this will look something like:

int MY_FLAG = 0x0001;
if ((value & MY_FLAG) == MY_FLAG)
    doSomething();

Answer 6

Kristopher Johnson's answer is very good if you like working with individual fields like this. I prefer to make the code easier to read by using bit fields in C.

For example:

struct fieldsample
{
  unsigned short field1 : 1;
  unsigned short field2 : 1;
  unsigned short field3 : 1;
  unsigned short field4 : 1;
}

Here you have a simple struct with four fields, each 1 bit in size. Then you can write your code using simple structure access.

void codesample()
{
  //Declare the struct on the stack.
  fieldsample fields;
  //Initialize values.
  fields.f1 = 1;
  fields.f2 = 0;
  fields.f3 = 0;
  fields.f4 = 1;
  ...
  //Check the value of a field.
  if(fields.f1 == 1) {}
  ...
}

You get the same small size advantage, plus readable code because you can give your fields meaningful names inside the structure.