3 Reentrancy Properties of 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 Red Hat newlib 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.