log, logf, logl

float       logf( float arg );

double      log( double arg );

long double logl( long double arg );

#define log( arg )

Parameters

Return value

If no errors occur, the natural (base-e) logarithm of arg (ln(arg) or log_e(arg)) is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.

Error handling

Errors are reported as specified in math_errhandling.

Domain error occurs if arg is less than zero.

Pole error may occur if arg is zero.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

• If the argument is ±0, -∞ is returned and FE_DIVBYZERO is raised.
• If the argument is 1, +0 is returned
• If the argument is negative, NaN is returned and FE_INVALID is raised.
• If the argument is +∞, +∞ is returned
• If the argument is NaN, NaN is returned

Example

#include <stdio.h>
#include <math.h>
#include <float.h>
#include <errno.h>
#include <fenv.h>
// #pragma STDC FENV_ACCESS ON
int main(void)
{
    printf("log(1) = %f\n", log(1));
    printf("base-5 logarithm of 125 = %f\n", log(125)/log(5));
    // special values
    printf("log(1) = %f\n", log(1));
    printf("log(+Inf) = %f\n", log(INFINITY));
    //error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("log(0) = %f\n", log(0));
    if(errno == ERANGE) perror("    errno == ERANGE");
    if(fetestexcept(FE_DIVBYZERO)) puts("    FE_DIVBYZERO raised");
}

log(1) = 0.000000
base-5 logarithm of 125 = 3.000000
log(1) = 0.000000
log(+Inf) = inf
log(0) = -inf
    errno == ERANGE: Numerical result out of range
    FE_DIVBYZERO raised

References

• C17 standard (ISO/IEC 9899:2018):
• 7.12.6.7 The log functions (p: 178-179)
• 7.25 Type-generic math <tgmath.h> (p: 272-273)
• F.10.3.7 The log functions (p: 380)
• C11 standard (ISO/IEC 9899:2011):
• 7.12.6.7 The log functions (p: 244-245)
• 7.25 Type-generic math <tgmath.h> (p: 373-375)
• F.10.3.7 The log functions (p: 522)
• C99 standard (ISO/IEC 9899:1999):
• 7.12.6.7 The log functions (p: 225)
• 7.22 Type-generic math <tgmath.h> (p: 335-337)
• F.9.3.7 The log functions (p: 459)
• C89/C90 standard (ISO/IEC 9899:1990):
• 4.5.4.4 The log function

See also