libm
When a libm function detects an exceptional case, errno may be
set, the matherr function may be called, and a error message
may be written to the standard error stream. This behavior may not
be reentrant.
With reentrant C libraries like the Cygnus C library, errno is
a macro which expands to the per-thread error value. This makes it thread
safe.
When the user provides his own matherr function it must be
reentrant for the math library as a whole to be reentrant.
In normal debugged programs, there are usually no math subroutine
errors---and therefore no assignments to errno and no matherr
calls; in that situation, the math functions behave reentrantly.
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