'\" et

.TH ERRNO "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH 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.

.SH NAME

errno

\(em error return value

.SH SYNOPSIS

.LP

.nf

#include <errno.h>

.fi

.SH DESCRIPTION

The lvalue

.IR errno

is used by many functions to return error values.

.P

Many functions provide an error number in

.IR errno ,

which has type

.BR int

and is defined in

.IR <errno.h> .

The value of

.IR 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

.IR errno

should only be examined when it is indicated to be valid by a

function's return value. Applications shall obtain the definition of

.IR errno

by the inclusion of

.IR <errno.h> .

No function in this volume of POSIX.1\(hy2008 shall set

.IR errno

to 0. The setting of

.IR errno

after a successful call to a function is unspecified unless the

description of that function specifies that

.IR errno

shall not be modified.

.P

It is unspecified whether

.IR 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

.IR errno ,

the behavior is undefined.

.P

The symbolic values stored in

.IR errno

are documented in the ERRORS sections on all relevant pages.

.SH "RETURN VALUE"

None.

.SH ERRORS

None.

.LP

.IR "The following sections are informative."

.SH EXAMPLES

None.

.SH "APPLICATION USAGE"

Previously both POSIX and X/Open documents were more restrictive than

the ISO\ C standard in that they required

.IR errno

to be defined as an external variable, whereas the ISO\ C standard required only

that

.IR errno

be defined as a modifiable lvalue with type

.BR int .

.P

An application that needs to examine the value of

.IR errno

to determine the error should set it to 0 before a function call, then

inspect it before a subsequent function call.

.SH RATIONALE

None.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "Section 2.3" ", " "Error Numbers"

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "\fB<errno.h>\fP"

.SH COPYRIGHT

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

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.kernel.org/doc/man-pages/reporting_bugs.html .