According to this excerpt from The Open Group Base Specifications
Issue 6 (http://pubs.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html
),
'>0' indeed means that the functionality is implemented and can be
used.
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The following symbolic constants, if defined in <unistd.h>, shall have
a value of -1, 0, or greater, unless otherwise specified below. If
these are undefined, the fpathconf(), pathconf(), or sysconf()
functions can be used to determine whether the option is provided for
a particular invocation of the application.
If a symbolic constant is defined with the value -1, the option is not
supported. Headers, data types, and function interfaces required only
for the option need not be supplied. An application that attempts to
use anything associated only with the option is considered to be
requiring an extension.
If a symbolic constant is defined with a value greater than zero, the
option shall always be supported when the application is executed. All
headers, data types, and functions shall be present and shall operate
as specified.
If a symbolic constant is defined with the value zero, all headers,
data types, and functions shall be present. The application can check
at runtime to see whether the option is supported by calling
fpathconf(), pathconf(), or sysconf() with the indicated name
parameter.
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