log1p, log1pf, log1pl
Computes the natural logarithm of 1 plus the specified value.
Syntax
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
#define log1p(X) // Requires C11 or higher
float log1p(float x); //C++ only
long double log1p(long double x); //C++ only
Parameters
x
The floating-point argument.
Return value
If successful, returns the natural (base-e) log of (x + 1).
Otherwise, may return one of the following values:
| Input | Result | SEH exception | errno |
|---|---|---|---|
| +INF | +INF | ||
| Denormals | Same as input | UNDERFLOW |
|
| ±0 | Same as input | ||
| -1 | -INF | DIVBYZERO |
ERANGE |
| < -1 | NaN | INVALID |
EDOM |
| -INF | NaN | INVALID |
EDOM |
| ±SNaN | Same as input | INVALID |
|
| ±QNaN, indefinite | Same as input |
The errno value is set to ERANGE if x = -1. The errno value is set to EDOM if x < -1.
Remarks
The log1p functions may be more accurate than using log(x + 1) when x is near 0.
Because C++ allows overloading, you can call overloads of log1p that take and return float and long double types. In a C program, unless you're using the <tgmath.h> macro to call this function, log1p always takes and returns a double.
If you use the <tgmath.h> log1p() macro, the type of the argument determines which version of the function is selected. See Type-generic math for details.
Where x is a natural number, this function returns the base e logarithm of x + 1.
By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.
Requirements
| Function | C header | C++ header |
|---|---|---|
log1p, log1pf, log1pl |
<math.h> |
<cmath> |
log1p macro |
<tgmath.h> |
For more compatibility information, see Compatibility.
See also
Alphabetical function reference
log2, log2f, log2l
log, logf, log10, log10f