How can I read and write binary floats with Chicken Scheme?

Question

I am reading a binary data format using Chicken, and so far I've gotten ints working by doing stuff like (fx+ (fxshl (read-byte) 8) (read-byte)) (Big Endian).

How can I read and write floats in? I have to be able to read and write IEEE 754-2008 32-bit and 64-bit binary floats.

Answer 1

I haven't found any good libraries to do this so far, but I've hacked together something that works. Note that I only have read-byte and read-string available to me as input operations.

  ;;
  ;; These are some unfun C routines to convert 4 int-promoted bytes to a float
  ;; by manually assembling the float using bitwise operators
  ;;
  ;; Caveat! These will only work on platforms in which floats are 32-bit Big
  ;; Endian IEEE754-2008 numbers and doubles are 64-bit Big Endian IEEE754-2008
  ;; numbers! Also, stdint.h.
  ;;
  (define (readFloat)
    (let ([c-read-float
            (foreign-lambda* float
              ((int i1)
               (int i2)
               (int i3)
               (int i4))
               "uint8_t b1 = (uint8_t) i1;
                uint8_t b2 = (uint8_t) i2;
                uint8_t b3 = (uint8_t) i3;
                uint8_t b4 = (uint8_t) i4;

                uint32_t i = 0;

                i = b1;
                i = (i << 8) | b2;
                i = (i << 8) | b3;
                i = (i << 8) | b4;

                float f = *(float*)&i;

                C_return(f);")])
        (let* ([i1 (read-byte)]
               [i2 (read-byte)]
               [i3 (read-byte)]
               [i4 (read-byte)])
          (c-read-float i1 i2 i3 i4))))