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

       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

       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.1‐2017, errno.h(0p)

COPYRIGHT         top

       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 .

       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               2017                         ERRNO(3P)