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 .

IEEE/The Open Group                                   2013                                         ERRNO(3POSIX)