_PyFloat_Pack8

_PyFloat_{Pack,Unpack}{4,8}

The struct and pickle (at least) modules need an efficient platform- independent way to store floating-point values as byte strings. The Pack routines produce a string from a C double, and the Unpack routines produce a C double from such a string. The suffix (4 or 8) specifies the number of bytes in the string.

On platforms that appear to use (see _PyFloat_Init()) IEEE-754 formats these functions work by copying bits. On other platforms, the formats the 4- byte format is identical to the IEEE-754 single precision format, and the 8-byte format to the IEEE-754 double precision format, although the packing of INFs and NaNs (if such things exist on the platform) isn't handled correctly, and attempting to unpack a string containing an IEEE INF or NaN will raise an exception.

On non-IEEE platforms with more precision, or larger dynamic range, than 754 supports, not all values can be packed; on non-IEEE platforms with less precision, or smaller dynamic range, not all values can be unpacked. What happens in such cases is partly accidental (alas).

The pack routines write 4 or 8 bytes, starting at p. le is a bool argument, true if you want the string in little-endian format (exponent last, at p+3 or p+7), false if you want big-endian format (exponent first, at p). Return value: 0 if all is OK, -1 if error (and an exception is set, most likely OverflowError). There are two problems on non-IEEE platforms: 1): What this does is undefined if x is a NaN or infinity. 2): -0.0 and +0.0 produce the same string.

  1. int _PyFloat_Pack4(double x, ubyte* p, int le)
  2. int _PyFloat_Pack8(double x, ubyte* p, int le)
    extern (C)
    int
    _PyFloat_Pack8
    (
    double x
    ,
    ubyte* p
    ,
    int le
    )

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