The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> errno - error return value

#### SYNOPSIS

> ``#include <errno.h>``

#### DESCRIPTION

> 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

> None.

#### ERRORS

> None.

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> 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

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> Error Numbers

> XBD <errno.h>

#### CHANGE HISTORY

> First released in Issue 1. Derived from Issue 1 of the SVID.

#### Issue 5

> The following sentence is deleted from the DESCRIPTION: "The value of errno is 0 at program start-up, but is never set to 0 by any XSI function".

> The DESCRIPTION also no longer states that conforming implementations may support the declaration:

>     extern int errno;

#### Issue 6

> Obsolescent text regarding defining errno as:

>     extern int errno

> is removed.

> Text regarding no function setting errno to zero to indicate an error is changed to no function shall set errno to zero. This is for alignment with the ISO/IEC 9899:1999 standard.

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/23 is applied, adding text to the DESCRIPTION stating that the setting of errno after a successful call to a function is unspecified unless the description of the function requires that it will not be modified.

End of informative text.

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