errno

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 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.1‐2017 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(3P)