/C

# log, logf, logl

Defined in header `<math.h>`
`float       logf( float arg );`
(1) (since C99)
`double      log( double arg );`
(2)
`long double logl( long double arg );`
(3) (since C99)
Defined in header `<tgmath.h>`
`#define log( arg )`
(4) (since C99)
1-3) Computes the natural (base e) logarithm of `arg`.
4) 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`).

### Parameters

 arg - floating point value

### 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");
}```

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```

### References

• 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