A flag is a value used to make a decision between two or more options in the execution of a program. For instance, the /w flag on the MS-DOS dir command causes the command to display filenames in several columns across the screen instead of displaying them one per line. In which a flag is used to indicate which of two possible types is held in a union. Because a flag has a small number of values (often only two), it is tempting to save memory space by not storing each flag in its own int or char.
Efficiency in this case is a tradeoff between size and speed. The most memory-space efficient way to store a flag value is as single bits or groups of bits just large enough to hold all the possible values. This is because most computers cannot address individual bits in memory, so the bit or bits of interest must be extracted from the bytes that contain it.
The most time-efficient way to store flag values is to keep each in its own integer variable. Unfortunately, this method can waste up to 31 bits of a 32-bit variable, which can lead to very inefficient use of memory. If there are only a few flags, it doesn’t matter how they are stored. If there are many flags, it might be advantageous to store them packed in an array of characters or integers. They must then be extracted by a process called bit masking, in which unwanted bits are removed from the ones of interest.
2. What is meant by “bit masking”?
Bit masking means selecting only certain bits from byte(s) that might have many bits set. To examine some bits of a byte, the byte is bitwise “ANDed” with a mask that is a number consisting of only those bits of interest. For instance, to look at the one’s digit (rightmost digit) of the variable flags, you bitwise AND it with a mask of one (the bitwise AND operator in C is &):
flags & 1;
To set the bits of interest, the number is bitwise “ORed” with the bit mask (the bitwise OR operator in C is |). For instance, you could set the one’s digit of flags like so:
flags = flags | 1;
Or, equivalently, you could set it like this:
flags |= 1;
To clear the bits of interest, the number is bitwise ANDed with the one’s complement of the bit mask. The “one’s complement” of a number is the number with all its one bits changed to zeros and all its zero bits changed to ones. The one’s complement operator in C is ~. For instance, you could clear the one’s digit of flags like so:
flags = flags & ~1;
Or, equivalently, you could clear it like this:
flags &= ~1;
Sometimes it is easier to use macros to manipulate flag values.
Example Program : Macros that make manipulating flags easier.
/* Bit Masking */
/* Bit masking can be used to switch a character
between lowercase and uppercase */
#define BIT_POS(N) ( 1U << (N) )
#define SET_FLAG(N, F) ( (N) |= (F) )
#define CLR_FLAG(N, F) ( (N) &= -(F) )
#define TST_FLAG(N, F) ( (N) & (F) )
#define BIT_RANGE(N, M) ( BIT_POS((M)+1 – (N))-1 << (N) )
#define BIT_SHIFTL(B, N) ( (unsigned)(B) << (N) )
#define BIT_SHIFTR(B, N) ( (unsigned)(B) >> (N) )
#define SET_MFLAG(N, F, V) ( CLR_FLAG(N, F), SET_FLAG(N, V) )
#define CLR_MFLAG(N, F) ( (N) &= ~(F) )
#define GET_MFLAG(N, F) ( (N) & (F) )
unsigned char ascii_char = ‘A’; /* char = 8 bits only */
int test_nbr = 10;
printf(“Starting character = %c\n”, ascii_char);
/* The 5th bit position determines if the character is
uppercase or lowercase.
5th bit = 0 – Uppercase
5th bit = 1 – Lowercase */
printf(“\nTurn 5th bit on = %c\n”, SET_FLAG(ascii_char, BIT_POS(5)) );
printf(“Turn 5th bit off = %c\n\n”, CLR_FLAG(ascii_char, BIT_POS(5)) );
printf(“Look at shifting bits\n”);
printf(“Current value = %d\n”, test_nbr);
printf(“Shifting one position left = %d\n”,
test_nbr = BIT_SHIFTL(test_nbr, 1) );
printf(“Shifting two positions right = %d\n”,
BIT_SHIFTR(test_nbr, 2) );
BIT_POS(N) takes an integer N and returns a bit mask corresponding to that single bit position (BIT_POS(0)returns a bit mask for the one’s digit, BIT_POS(1) returns a bit mask for the two’s digit, and so on). So instead of writing
#define A_FLAG 4096
#define B_FLAG 8192
you can write
#define A_FLAG BIT_POS(12)
#define B_FLAG BIT_POS(13)
which is less prone to errors.
The SET_FLAG(N, F) macro sets the bit at position F of variable N. Its opposite is CLR_FLAG(N, F), which clears the bit at position F of variable N. Finally, TST_FLAG(N, F) can be used to test the value of the bit at position F of variable N, as in
if (TST_FLAG(flags, A_FLAG))
/* do something */;
The macro BIT_RANGE(N, M) produces a bit mask corresponding to bit positions N through M, inclusive. With this macro, instead of writing
#define FIRST_OCTAL_DIGIT 7 /* 111 */
#define SECOND_OCTAL_DIGIT 56 /* 111000 */
you can write
#define FIRST_OCTAL_DIGIT BIT_RANGE(0, 2) /* 111 */
#define SECOND_OCTAL_DIGIT BIT_RANGE(3, 5) /* 111000 */
which more clearly indicates which bits are meant.
The macro BIT_SHIFT(B, N) can be used to shift value B into the proper bit range (starting with bit N). For instance, if you had a flag called C that could take on one of five possible colors, the colors might be defined like this:
#define C_FLAG BIT_RANGE(8, 10) /* 11100000000 */
/* here are all the values the C flag can take on */
#define C_BLACK BIT_SHIFTL(0, 8) /* 00000000000 */
#define C_RED BIT_SHIFTL(1, 8) /* 00100000000 */
#define C_GREEN BIT_SHIFTL(2, 8) /* 01000000000 */
#define C_BLUE BIT_SHIFTL(3, 8) /* 01100000000 */
#define C_WHITE BIT_SHIFTL(4, 8) /* 10000000000 */
#define C_ZERO C_BLACK
#define C_LARGEST C_WHITE
/* A truly paranoid programmer might do this */
#if C_LARGEST > C_FLAG
Cause an error message. The flag C_FLAG is not
big enough to hold all its possible values.
#endif /* C_LARGEST > C_FLAG */
3. Are bit fields portable?
Bit fields are not portable. Because bit fields cannot span machine words, and because the number of bits in a machine word is different on different machines, a particular program using bit fields might not even compile on a particular machine.
Assuming that your program does compile, the order in which bits are assigned to bit fields is not defined. Therefore, different compilers, or even different versions of the same compiler, could produce code that would not work properly on data generated by compiled older code. Stay away from using bit fields, except in cases in which the machine can directly address bits in memory and the compiler can generate code to take advantage of it and the increase in speed to be gained would be essential to the operation of the program.
4. Is it better to bitshift a value than to multiply by 2?
Any decent optimizing compiler will generate the same code no matter which way you write it. Use whichever form is more readable in the context in which it appears. The following program’s assembler code can be viewed with a tool such as CODEVIEW on DOS/Windows or the disassembler (usually called “dis”) on UNIX machines:
Example: Multiplying by 2 and shifting left by 1 are often the same.
unsigned int test_nbr = 300;
test_nbr *= 2;
test_nbr = 300;
test_nbr <<= 1;