Binary Literals in C

Binary Literals in C: "A couple of years ago, Netrino engineer Dan Smith was writing stepper motor control firmware that interfaced to lots of registers with binary fields and sub-fields. After struggling a bit with the usual error-prone 'off by 1 bit shift' masking and conversion from binary to hexadecimal literals in C, he happened across a useful post on a forum.

In a nutshell, the 'binary literal' technique involves the following set of C preprocessor macros:

// Internal Macros
#define HEX__(n) 0x##n##LU
#define B8__(x) ((x&0x0000000FLU)?1:0) \
+((x&0x000000F0LU)?2:0) \
+((x&0x00000F00LU)?4:0) \
+((x&0x0000F000LU)?8:0) \
+((x&0x000F0000LU)?16:0) \
+((x&0x00F00000LU)?32:0) \
+((x&0x0F000000LU)?64:0) \
+((x&0xF0000000LU)?128:0)

// User-visible Macros
#define B8(d) ((unsigned char)B8__(HEX__(d)))
#define B16(dmsb,dlsb) (((unsigned short)B8(dmsb)<<8) + B8(dlsb))
#define B32(dmsb,db2,db3,dlsb) \
(((unsigned long)B8(dmsb)<<24) \
+ ((unsigned long)B8(db2)<<16) \
+ ((unsigned long)B8(db3)<<8) \
+ B8(dlsb))

Here are some examples of the usage of these macros:
B8(01010101) // 85
B16(10101010,01010101) // 43,605
B32(10000000,11111111,10101010,01010101) // 2,164,238,933

So if you had a memory-mapped 8-bit control register of the format XXXYYZZZ (where XXX, YY, and ZZZ are subfields), you could initialize it like so:

*p_reg = ( (B8(010) << 5) | (B8(11) << 3) | (B8(101) << 0) )

which sets the XXX bits to 010, YY to 11, and ZZZ to 101. If I ever needed to change XXX to 011, just change a single 0 to a 1 in the source code, and everything magically changes. Best of all, it's all done at compile-time. No error-prone conversion to hexadecimal necessary, no figuring out which bits belong to which nibbles, etc.

What is that old saying? -- 'good programmers write good code; great programmers steal great code'

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