cc [ flag... ] file... lmvec [ library... ] void vrsqrt_(int *n, double * restrict x, int *stridex, double * restrict y, int *stridey);
void vrsqrtf_(int *n, float * restrict x, int *stridex, float * restrict y, int *stridey);
These functions evaluate the function rsqrt(x), defined by rsqrt(x) = 1 / sqrt(x), for an entire vector of values at once. The first parameter specifies the number of values to compute. Subsequent parameters specify the argument and result vectors. Each vector is described by a pointer to the first element and a stride, which is the increment between successive elements.
Specifically, vrsqrt_(n, x, sx, y, sy) computes y[i * *sy] = rsqrt(x[i * *sx]) for each i = 0, 1, ..., *n  1. The vrsqrtf_() function performs the same computation for single precision data.
These functions are not guaranteed to deliver results that are identical to the results of evaluating 1.0 / sqrt(x) given the same arguments. Nonexceptional results, however, are accurate to within a unit in the last place.
The element count *n must be greater than zero. The strides for the argument and result arrays can be arbitrary integers, but the arrays themselves must not be the same or overlap. A zero stride effectively collapses an entire vector into a single element. A negative stride causes a vector to be accessed in descending memory order, but note that the corresponding pointer must still point to the first element of the vector to be used; if the stride is negative, this will be the highestaddressed element in memory. This convention differs from the Level 1 BLAS, in which array parameters always refer to the lowestaddressed element in memory even when negative increments are used.
These functions assume that the default roundtonearest rounding direction mode is in effect. On x86, these functions also assume that the default roundto64bit rounding precision mode is in effect. The result of calling a vector function with a nondefault rounding mode in effect is undefined.
These functions handle special cases and exceptions in the spirit of IEEE 754. In particular,
An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID  FE_DIVBYZERO  FE_OVERFLOW  FE_UNDERFLOW) is nonzero, an exception has been raised. The application can then examine the result or argument vectors for exceptional values. Some vector functions can raise the inexact exception even if all elements of the argument array are such that the numerical results are exact.
See attributes(5) for descriptions of the following attributes:

sqrt(3M), feclearexcept(3M), fetestexcept(3M), attributes(5)