log2 (3p) - Linux Man Pages
log2: compute base 2 logarithm functions
PROLOGThis manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
log2, log2f, log2l - compute base 2 logarithm functions
These functions shall compute the base 2 logarithm of their argument x, log_2(x).
An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.
Upon successful completion, these functions shall return the base 2 logarithm of x.
If x is ±0, a pole error shall occur and log2(), log2f(), and log2l() shall 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 shall occur, and either a NaN (if supported), or an implementation-defined value shall be returned.
If x is NaN, a NaN shall be returned.
If x is 1, +0 shall be returned.
These functions shall fail if:
- Domain Error
- The finite value of x is less than zero, or x is -Inf.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.
- Pole Error
- The value of x is zero.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.
COPYRIGHTPortions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .