C Pocket Reference

Error Handling for Mathematical Functions

Error conditions are customarily detected by examining the return value of a function and/or the global error variable errno . The variable errno is declared with type int in the header file errno.h.

If a function is passed an argument that is outside the domain for which the function is defined, a “domain error” occurs, and errno is assigned the value of the macro EDOM . Similarly, if the result of a function cannot be represented by the type of the function’s return value, then a “range error” occurs, and errno is assigned the value ERANGE . In the case of an overflow—that is, if the magnitude of the result is too great for the specified type—the function returns the value of the macro HUGE_VAL , with the appropriate sign. In case of an underflow—i. e., the magnitude of the result is too small—the function returns 0.

In addition to HUGE_VAL (with type double), ANSI C99 also provides the macros HUGE_VALF (type float) and HUGE_VALL (type long double), which are returned by functions of the corresponding types.

Furthermore, ANSI C99 introduces the macros FP_ILOGB0 and FP_ILOGBNAN . The function ilogb( x ) returns FP_ILOGB0 if x is equal to 0. If x is “not a number” (NaN), ilogb( x ) returns the value of FP_ILOGBNAN.