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cbit-manipulationbit

A function to find whether a bit is on, with type char*

I'm trying to write a function that receives a char* pointer and an index and checks whether the bit in the index is on.

i got somthing like that:

int bitIsOn(char *ptr, int index){
    return (ptr[index/8] & (1<< (index%8)));
}

note: I need to use c90. I am getting the following error message: error: invalid operands to binary & (have 'char *' and 'int')

How can I avoid type collision here

Solution

  • Use unsigned data for bitwise operations. Use standard definition CHAR_BIT instead of magic number 8 (it is defined in limits.h header file).

    int checkbit(const void *data, const size_t index)
{
    const unsigned char *udata = data;
    return udata[index / CHAR_BIT] & (1 << (index % CHAR_BIT));
}

    If you want to return 0 or 1 you can use double logical NOT operator:

    return !!(udata[index / CHAR_BIT] & (1 << (index % CHAR_BIT)));