log, logf, logl - cppreference.com (original) (raw)

1-3) Computes the natural (base e) logarithm of arg.

1. Type-generic macro: If arg has type long double, logl is called. Otherwise, if arg has integer type or the type double, log is called. Otherwise, logf is called. If arg is complex or imaginary, then the macro invokes the corresponding complex function (clogf, clog, clogl).

[edit] Parameters

[edit] Return value

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

[edit] 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.

[edit] Example

#include <errno.h> #include <fenv.h> #include <float.h> #include <math.h> #include <stdio.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"); }

Output:

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

[edit] References

• C23 standard (ISO/IEC 9899:2024):

• 7.12.6.7 The log functions (p: TBD)

• 7.25 Type-generic math <tgmath.h> (p: TBD)

• F.10.3.7 The log functions (p: TBD)

• 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

[edit] See also