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



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

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.1-2017 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.

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.1-2017, <errno.h>

COPYRIGHT
       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(3)
