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### 1.1 Error Handling

There are four different versions of the math library routines: IEEE, POSIX, X/Open, or SVID. The version may be selected at runtime by setting the global variable `_LIB_VERSION`, defined in `math.h`. It may be set to one of the following constants defined in `math.h`: `_IEEE_`, `_POSIX_`, `_XOPEN_`, or `_SVID_`. The `_LIB_VERSION` variable is not specific to any thread, and changing it will affect all threads.

The versions of the library differ only in how errors are handled.

In IEEE mode, the `matherr` function is never called, no warning messages are printed, and `errno` is never set.

In POSIX mode, `errno` is set correctly, but the `matherr` function is never called and no warning messages are printed.

In X/Open mode, `errno` is set correctly, and `matherr` is called, but warning message are not printed.

In SVID mode, functions which overflow return 3.40282346638528860e+38, the maximum single-precision floating-point value, rather than infinity. Also, `errno` is set correctly, `matherr` is called, and, if `matherr` returns 0, warning messages are printed for some errors. For example, by default ‘`log(-1.0)`’ writes this message on standard error output:

    log: DOMAIN error

The library is set to X/Open mode by default.

The aforementioned error reporting is the supported Newlib libm error handling method. However, the majority of the functions are written so as to produce the floating-point exceptions (e.g. "invalid", "divide-by-zero") as required by the C and POSIX standards, for floating-point implementations that support them. Newlib does not provide the floating-point exception access routines defined in the standards for fenv.h, though, which is why they are considered unsupported. It is mentioned in case you have separately-provided access routines so that you are aware that they can be caused.

### 1.2 Standards Compliance And Portability

Most of the individual function descriptions describe the standards to which each function complies. However, these descriptions are mostly out of date, having been written before C99 was released. One of these days we’ll get around to updating the rest of them. (If you’d like to help, please let us know.)

“C99” refers to ISO/IEC 9899:1999, “Programming languages–C”. “POSIX” refers to IEEE Standard 1003.1. POSIX® is a registered trademark of The IEEE.

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