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 IEEE Std 1003.1-2001 shall set errno to 0.
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 , the Base Definitions volume of IEEE Std 1003.1-2001, <errno.h>