POSIX defines certain system-specific options in the system calls for operating on files. Some systems support these options and others do not. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using. They can also vary between file systems on a single machine.
This section describes the macros you can test to determine whether a
particular option is supported on your machine. If a given macro is
defined in `unistd.h', then its value says whether the
corresponding feature is supported. (A value of -1 indicates no;
any other value indicates yes.) If the macro is undefined, it means
particular files may or may not support the feature.
Since all the machines that support the GNU C library also support NFS,
one can never make a general statement about whether all file systems
support the _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC
features. So these names are never defined as macros in the GNU C
library.
chown function is restricted so
that the only changes permitted to nonprivileged processes is to change
the group owner of a file to either be the effective group ID of the
process, or one of its supplementary group IDs. See section File Owner.
NAME_MAX generate an ENAMETOOLONG error. Otherwise, file
name components that are too long are silently truncated.
If one of these macros is undefined, that means that the option might be
in effect for some files and not for others. To inquire about a
particular file, call pathconf or fpathconf.
See section Using pathconf.
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