errno(3p) [posix man page]

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 (consult the correspond-
       ing 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 applica-
       tion 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. Applica-
       tions 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 Technol-
       ogy  --	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 Stan-
       dard 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(3P)