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man pages section 3: Extended Library Functions, Volume 2

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Updated: July 2017



log10, log10f, log10l - base 10 logarithm function


c99 [ flag... ] file... –lm [ library... ]
#include <math.h>

double log10(double x);
float log10f(float x);
long double log10l(long double x);


These functions compute the base 10 logarithm of x, log10(x).

Return Values

Upon successful completion, log10() returns the base 10 logarithm of x.

If x is ±0, a pole error occurs and log10(), log10f(), and log10l() return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively.

For finite values of x that are less than 0, or if x is −Inf, a domain error occurs and a NaN is returned.

If x is NaN, a NaN is returned.

If x is 1, +0 is returned.

If x is +Inf, x is returned.

For exceptional cases, matherr(3M) tabulates the values to be returned by log10() as specified by SVID3 and XPG3.


These functions will fail if:

Domain Error

The finite value of x is negative, or x is -Inf.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised.

The log10() function sets errno to EDOM if the value of x is negative.

Pole Error

The value of x is 0.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the divide-by-zero floating-point exception is raised.


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 non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.

An application can also set errno to 0 before calling log10(). On return, if errno is non-zero, an error has occurred. The log10f() and log10l() functions do not set errno.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

feclearexcept(3M), fetestexcept(3M), isnan(3M), log(3M), math.h(3HEAD), matherr(3M), pow(3M), attributes(5), standards(5)