https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114877
--- Comment #3 from Lénárd Szolnoki <leni536 at gmail dot com> --- (In reply to Richard Biener from comment #2) > The standard says the results are unspecified, it doesn't say *p is written > to, > so I'm not sure there's a bug here. C17 (N2176) says in the library section: "The frexp functions break a floating-point number into a normalized fraction and an integral power of 2. They store the integer in the int object pointed to by exp." "If value is not a floating-point number or if the integral power of 2 is outside the range of int, the results are unspecified. Otherwise, the frexp functions return the value x, such that x has a magnitude in the interval [1/2, 1) or zero, and value equals x × 2*exp . If value is zero, both parts of the result are zero." This might be a little bit ambiguous, but my reading is that an int value is always stored, but sometimes the "results are unspecified", where I assume "results" refer to the fractional part and the integer exponent. An unspecified result then is stored to the object pointed to by exp. However further down in Annex F (relevant for targets with IEC 60559, such as x86_64): "frexp(±∞, exp) returns ±∞, and stores an unspecified value in the object pointed to by exp." "frexp(NaN, exp) stores an unspecified value in the object pointed to by exp (and returns a NaN)." This is a lot less ambiguous.
