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



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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 indi-
       cated 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 IEEE Std 1003.1-2001 shall set errno to 0.

       It is unspecified whether errno is a macro or an identifier declared with external linkage. If a macro definition is sup-
       pressed 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
       Error Numbers, the Base Definitions volume of IEEE Std 1003.1-2001, <errno.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     ERRNO(3P)