.. index:: errno.h

Errors ``<errno.h>``
********************
The header ``<errno.h>`` defines several macros, all relating to the reporting
of error conditions.

.. index:: EDOM macro, EILSEQ macro, ERANGE macro, errno

The macros are:

  EDOM
  EILSEQ
  ERANGE

which expand to integer constant expressions with type ``int``, distinct
positive values, and which are suitable for use in ``#if`` preprocessing
directives; and ``errno`` which expands to a modifiable lvalue [#]_ that has
type ``int``, the value of which is set to a positive error number by several
library functions. It is unspecified whether ``errno`` is a macro or an
identifier declared with external linkage. If a macro definition is
suppressed in order to access an actual object, or a program defines an
identifier with the name ``errno``, the behavior is undefined.

The value of ``errno`` is zero at program startup, but is never set to zero by
any library function. [#]_ The value of ``errno`` may be set to nonzero by a
library function call whether or not there is an error, provided the use of
``errno`` is not documented in the description of the function in this
International Standard.

Additional macro definitions, beginning with ``E`` and a digit or ``E`` and an
uppercase letter, may also be specified by the implementation.

.. [#] The macro errno need not be the identifier of an object. It might expand
  to a modifiable lvalue resulting from a function call (for example,
  ``*errno()``).
.. [#] Thus, a program that uses ``errno`` for error checking should set it to
  zero before a library function call, then inspect it before a subsequent
  library function call. Of course, a library function can save the value of
  ``errno`` on entry and then set it to zero, as long as the original value is
  restored if ``errno``'s value is still zero just before the return.