errno(3p) — Linux manual page


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

PROLOG         top

       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         top

       errno — error return value

SYNOPSIS         top

       #include <errno.h>

DESCRIPTION         top

       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         top


ERRORS         top


       The following sections are informative.

EXAMPLES         top



       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         top




SEE ALSO         top

       Section 2.3, Error Numbers

       The Base Definitions volume of POSIX.1‐2008, errno.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group                 2013                           ERRNO(3P)