The df command shall behave as specified in ISO POSIX (2003), but with differences as listed below.
If the -k option is not specified, disk space is shown in unspecified units. If the -P option is specified, the size of the unit shall be printed on the header line in the format "%4s-blocks". Applications should specify -k.
The XSI option -t has unspecified behavior. Applications should not specify -t.
Rationale: The most common implementation of df uses the -t option for a different purpose (restricting output to a particular file system type), and use of -t is therefore non-portable.
If an argument is the absolute file name of a special file containing a mounted file system, df shall show the space available on that file system rather than on the file system containing the special file (which is typically the root file system).
Note: In ISO POSIX (2003) the XSI optional behavior permits an operand to name a special file, but appears to require the operation be performed on the file system containing the special file. A defect report has been submitted for this case.