POW

Section: POSIX Programmer's Manual (3P)
Updated: 2003
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PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.  

NAME

pow, powf, powl - power function  

SYNOPSIS

#include <math.h>

double pow(double x, double y);
float powf(float x, float y);
long double powl(long double x, long double y);
 

DESCRIPTION

These functions shall compute the value of x raised to the power y, x**y. If x is negative, the application shall ensure that y is an integer value.

An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.  

RETURN VALUE

Upon successful completion, these functions shall return the value of x raised to the power y.

For finite values of x < 0, and finite non-integer values of y, a domain error shall occur and either a NaN (if representable), or an implementation-defined value shall be returned.

If the correct value would cause overflow, a range error shall occur and pow(), powf(), and powl() shall return ±HUGE_VAL, ±HUGE_VALF, and ±HUGE_VALL, respectively, with the same sign as the correct value of the function.

If the correct value would cause underflow, and is not representable, a range error may occur, and either 0.0 (if supported), or an implementation-defined value shall be returned.

If x or y is a NaN, a NaN shall be returned (unless specified elsewhere in this description).

For any value of y (including NaN), if x is +1, 1.0 shall be returned.

For any value of x (including NaN), if y is ±0, 1.0 shall be returned.

For any odd integer value of y > 0, if x is ±0, ±0 shall be returned.

For y > 0 and not an odd integer, if x is ±0, +0 shall be returned.

If x is -1, and y is ±Inf, 1.0 shall be returned.

For |x| < 1, if y is -Inf, +Inf shall be returned.

For |x| > 1, if y is -Inf, +0 shall be returned.

For |x| < 1, if y is +Inf, +0 shall be returned.

For |x| > 1, if y is +Inf, +Inf shall be returned.

For y an odd integer < 0, if x is -Inf, -0 shall be returned.

For y < 0 and not an odd integer, if x is -Inf, +0 shall be returned.

For y an odd integer > 0, if x is -Inf, -Inf shall be returned.

For y > 0 and not an odd integer, if x is -Inf, +Inf shall be returned.

For y < 0, if x is +Inf, +0 shall be returned.

For y > 0, if x is +Inf, +Inf shall be returned.

For y an odd integer < 0, if x is ±0, a pole error shall occur and ±HUGE_VAL, ±HUGE_VALF, and ±HUGE_VALL shall be returned for pow(), powf(), and powl(), respectively.

For y < 0 and not an odd integer, if x is ±0, a pole error shall occur and HUGE_VAL, HUGE_VALF, and HUGE_VALL shall be returned for pow(), powf(), and powl(), respectively.

If the correct value would cause underflow, and is representable, a range error may occur and the correct value shall be returned.  

ERRORS

These functions shall fail if:

Domain Error
The value of x is negative and y is a finite non-integer.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.

Pole Error
The value of x is zero and y is negative.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.

Range Error
The result overflows.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the overflow floating-point exception shall be raised.

These functions may fail if:

Range Error
The result underflows.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the underflow floating-point exception shall be raised.

The following sections are informative.  

EXAMPLES

None.  

APPLICATION USAGE

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.  

RATIONALE

None.  

FUTURE DIRECTIONS

None.  

SEE ALSO

exp(), feclearexcept(), fetestexcept(), isnan(), the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <math.h>  

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .