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 filesystem type), and use of -t is therefore non-portable.
If an argument is the absolute file name of a special file containing a mounted filesystem, df shall show the space available on that filesystem rather than on the filesystem containing the special file (which is typically the root filesystem).
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 filesystem containing the special file. A defect report has been submitted for this case.