The lvalue errno is used by many functions to return error values.
Many functions provide an error number in errno, which has type
int and is defined in <errno.h>. The value of errno
shall be defined only after a call to a function for which it is explicitly
stated to be set and until it is changed by the next function call or if the
application assigns it a value. The value of errno should only be
examined when it is indicated to be valid by a function's return value.
Applications shall obtain the definition of errno by the inclusion of
<errno.h>. No function in this volume of
IEEE Std 1003.1-2001 shall set errno to 0.
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 symbolic values stored in errno are documented in the ERRORS sections
on all relevant pages.
Previously both POSIX and X/Open documents were more restrictive than the
ISO C standard in that they required errno to be defined as an
external variable, whereas the ISO C standard required only that
errno be defined as a modifiable lvalue with type int.
An application that needs to examine the value of errno to determine the
error should set it to 0 before a function call, then inspect it before a
subsequent function call.
Portions 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