The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


errno - error return value


#include <errno.h>


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.1-2017 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 undefined.

The symbolic values stored in errno are documented in the ERRORS sections 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.






Error Numbers

XBD <errno.h>


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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