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.
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.
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.
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
IEEE/The Open Group 2017 ERRNO(3P)