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.
NAMEerrno — error return value
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 set‐
ting of errno after a successful call to a function is unspecified
unless the description of that function specifies that errno shall not
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 sec‐
tions on all relevant pages.
The following sections are informative.
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.
Section 2.3, Error Numbers
The Base Definitions volume of POSIX.1‐2008, <errno.h>
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 Electri‐
cal 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 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.ker‐
IEEE/The Open Group 2013 ERRNO(3P)