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SF bug 705836: struct.pack of floats in non-native endian order

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pack_float, pack_double, save_float:  All the routines for creating
IEEE-format packed representations of floats and doubles simply ignored
that rounding can (in rare cases) propagate out of a long string of
1 bits.  At worst, the end-off carry can (by mistake) interfere with
the exponent value, and then unpacking yields a result wrong by a factor
of 2.  In less severe cases, it can end up losing more low-order bits
than intended, or fail to catch overflow *caused* by rounding.

Bugfix candidate, but I already backported this to 2.2.

In 2.3, this code remains in severe need of refactoring.
This commit is contained in:
Tim Peters 2003-03-20 18:32:13 +00:00
parent 62364ffb80
commit d50ade68ec
4 changed files with 115 additions and 18 deletions
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49
Modules/structmodule.c
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@ -224,12 +224,8 @@ pack_float(double x, /* The number to pack */
return -1;
}
if (e >= 128) {
/* XXX 128 itself is reserved for Inf/NaN */
PyErr_SetString(PyExc_OverflowError,
"float too large to pack with f format");
return -1;
}
if (e >= 128)
goto Overflow;
else if (e < -126) {
/* Gradual underflow */
f = ldexp(f, 126 + e);
@ -242,6 +238,14 @@ pack_float(double x, /* The number to pack */
f *= 8388608.0; /* 2**23 */
fbits = (long) floor(f + 0.5); /* Round */
assert(fbits <= 8388608);
if (fbits >> 23) {
/* The carry propagated out of a string of 23 1 bits. */
fbits = 0;
++e;
if (e >= 255)
goto Overflow;
}
/* First byte */
*p = (s<<7) | (e>>1);
@ -260,6 +264,11 @@ pack_float(double x, /* The number to pack */
/* Done */
return 0;
Overflow:
PyErr_SetString(PyExc_OverflowError,
"float too large to pack with f format");
return -1;
}
static int
@ -295,12 +304,8 @@ pack_double(double x, /* The number to pack */
return -1;
}
if (e >= 1024) {
/* XXX 1024 itself is reserved for Inf/NaN */
PyErr_SetString(PyExc_OverflowError,
"float too large to pack with d format");
return -1;
}
if (e >= 1024)
goto Overflow;
else if (e < -1022) {
/* Gradual underflow */
f = ldexp(f, 1022 + e);
@ -314,9 +319,24 @@ pack_double(double x, /* The number to pack */
/* fhi receives the high 28 bits; flo the low 24 bits (== 52 bits) */
f *= 268435456.0; /* 2**28 */
fhi = (long) floor(f); /* Truncate */
assert(fhi < 268435456);
f -= (double)fhi;
f *= 16777216.0; /* 2**24 */
flo = (long) floor(f + 0.5); /* Round */
assert(flo <= 16777216);
if (flo >> 24) {
/* The carry propagated out of a string of 24 1 bits. */
flo = 0;
++fhi;
if (fhi >> 28) {
/* And it also progagated out of the next 28 bits. */
fhi = 0;
++e;
if (e >= 2047)
goto Overflow;
}
}
/* First byte */
*p = (s<<7) | (e>>4);
@ -352,6 +372,11 @@ pack_double(double x, /* The number to pack */
/* Done */
return 0;
Overflow:
PyErr_SetString(PyExc_OverflowError,
"float too large to pack with d format");
return -1;
}
static PyObject *