errno man page on Archlinux

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ERRNO(3P)		   POSIX Programmer's Manual		     ERRNO(3P)

       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.

       errno — error return value

       #include <errno.h>

       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.1‐2008 shall set errno to 0. The set‐
       ting 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 sec‐
       tions on all relevant pages.



       The following sections are informative.


       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.



       Section 2.3, Error Numbers

       The Base Definitions volume of POSIX.1‐2008, <errno.h>

       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal  and	 Electronics  Engineers,  Inc  and  The	 Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum	 1  applied.)  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 .

       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.ker‐ .

IEEE/The Open Group		     2013			     ERRNO(3P)

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