PROLOG

       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.

NAME

       errno — error return value

SYNOPSIS

       #include <errno.h>

DESCRIPTION

       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 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.

RETURN VALUE

       None.

ERRORS

       None.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       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.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       Section 2.3, Error Numbers

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

COPYRIGHT

       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 Electrical 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 http://www.unix.org/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 .