<cmath> (math.h)

type
<cmath> <ctgmath>

double_t

Floating-point type
Alias of one of the fundamental floating-point types at least as wide as double.

It is the type used by the implementation to evaluate values of type double, as determined by FLT_EVAL_METHOD:

FLT_EVAL_METHODfloat_tdouble_t
0floatdouble
1doubledouble
2long doublelong double
otherimplementation-definedimplementation-defined