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<code>errno</code> is defined by the [[ISO C]] standard to be a modifiable [[lvalue]] of type [[Integer (computer science)|int]], and must not be explicitly declared.
It was common in traditional C to declare errno manually (i.e., extern int errno) instead of including <errno.h>. It will not work with modern versions of the C library. However, on (very) old Unix systems, there may be no <errno.h> and the declaration is needed.
In such situations, the source code must be modified to replace all <code>extern int errno</code> lines with a single include <code>#include <errno.h></code>.
==See also==
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