New macros found in C99 that are not found in C90 include:

INFINITY // positive infinity
NAN // IEEE not-a-number

New generic function macros found in C99 that are not found in C90 include:

#define isinf(x) // non-zero only if x is positive or negative infinity
#define isnan(x) // non-zero only if x is NaN
#define isless(x, y) // 1 only if x < y and x and y are not NaN, and 0 otherwise
#define isunordered(x, y) // 1 only if either x or y is NaN, and 0 otherwise

New mathematical functions found in C99 that are not found in C90 include:

double acosh(double x); // hyperbolic arccosine of x
double asinh(double x); // hyperbolic arcsine of x
double atanh(double x); // hyperbolic arctangent of x
double erf(double x); // returns the error function of x
double round(double x); // returns x rounded to the nearest integer
double tgamma(double x); // returns the gamma function of x

C99 supports the new mathematical functions for all real floating-point types.

Single precision versions of all existing `<math.h>`

functions are
also supported.