This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                         ERRNO(3POSIX)

       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  POSIX.1‐2017 shall set errno to 0. The setting of errno after a successful call to a function
       is unspecified unless the description of that function specifies that errno shall not be modified.

       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.

       None.

       Thefollowingsectionsareinformative.

       None.

       None.

       errno — error return value

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       None.

Section2.3, ErrorNumbers

       The Base Definitions volume of POSIX.1‐2017, <errno.h>

       #include <errno.h>