.\" This file is part of the Public Domain C Library (PDCLib).
.\" Permission is granted to use, modify, and / or redistribute at will.
.\"
.Dd
.Dt ERRNO 3
.Os
.\"
.Sh NAME
.Nm errno
.Nd library error result
.\"
.Sh SYNOPSIS
.In errno.h
.Pp
.Sy #define errno <object of thread local storage duration>
.\"
.Sh DESCRIPTION
The header
.In errno.h
defines the name
.Va errno
to refer to an object of thread local storage duration. It is undefined whether
or not 
.Va errno
refers to a preprocessor symbol or not.
.\"
.Pp
At program startup, the runtime shall initialize
.Va errno
to contain the value zero (the initial value in other threads is undefined). It 
is guaranteed that no function provided by the C standard library shall set the 
value of 
.Va errno
to zero; therefore, for functions which do not have a unique return value for
indicating error, it is possible to determine whether an error occured by 
setting
.Va errno
to zero before the call, then after the call seeing if an error value has been
stored into 
.Va errno.
.Pp
.Bf Sy
Note that it is valid for any function which is not described as explicitly 
setting 
.Va errno 
to set it to a non-zero value. Checking for a non-zero value is only valid with
functions which are defined to modify 
.Va errno .
.Ef
.\"
.Pp
In some historical versions of the POSIX standard, the prescribed method of 
using errno was to define
.Va extern int errno .
This is no longer supported by the POSIX or C standards, and will not work on 
the majority of implementations, including this one.
.\"
.Sh SEE ALSO
.Xr errno.h 3
.Xr abort 3
.Xr assert 3
\"
.Sh STANDARDS
Conforming to
.St -isoC-90 ,
.St -isoC-99 .