POW(3) Library Functions Manual POW(3)

pow powf powl — power function

#include <math.h>

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

[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

These functions shall compute the value of x raised to the power 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 (FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or (FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

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 [MX] 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, [MXX] and is not representable, a range error may occur, and pow (), powf (), and powl () shall return [MXX] 0.0, or (if IEC 60559 Floating-Point is not supported) an implementation-defined value no greater in magnitude than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.

For y < 0, if x is zero, a [CX] pole error may occur and pow (), powf (), and powl () shall return ±HUGE_VAL, ±HUGE_VALF, and ±HUGE_VALL, respectively. [MX] On systems that support the IEC 60559 Floating-Point option, if x is ±0:

  • If y is an odd integer, a pole error shall occur and pow (), powf (), and powl () shall return ±HUGE_VAL, ±HUGE_VALF, and ±HUGE_VALL, respectively.
  • If y is finite and is not an odd integer, a pole error shall occur and pow (), powf (), and powl () shall return HUGE_VAL, HUGE_VALF, and HUGE_VALL, respectively.
  • If y is -Inf, a pole error may occur and pow (), powf (), and powl () shall return HUGE_VAL, HUGE_VALF, and HUGE_VALL, respectively.

[MX] 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.

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

These functions shall fail if:

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.

[MX] 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.

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:

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.

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.

None.

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.

None.

None.

exp(3), feclearexcept(3), fetestexcept(3), isnan(3)

XBD V1_chap04(7), <math.h>

First released in Issue 1. Derived from Issue 1 of the SVID.

The DESCRIPTION is updated to indicate how an application should check for an error. This text was previously published in the APPLICATION USAGE section.

The normative text is updated to avoid use of the term "must" for application requirements.

The powf () and powl () functions are added for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard.

The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 (“ISO C99”) standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 (“ISO C99”) standard are marked.

IEEE Std 1003.1-2001 (“POSIX.1”)/Cor 1-2002, item XSH/TC1/D6/42 is applied, correcting the third paragraph in the RETURN VALUE section.

ISO/IEC 9899:1999 (“ISO C99”) standard, Technical Corrigendum 2 #51 (SD5-XSH-ERN-81) is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0443 [68], XSH/TC1-2008/0444 [148], and XSH/TC1-2008/0445 [68] are applied.

Austin Group Defect 1302 is applied, aligning these functions with the ISO/IEC 9899:2018 standard.

June 14, 2024 posix.fail