ptg10805159910 Solutions to Selected Exercises Appendix C
tempfile.Wehave to use thedfcommand in this example to see the actual
amount of free space on the file system.
4.9 If the link being removed is not the last link to the file, the file is not removed. In
this case, the changed-status time of the file is updated. But if the link being
removed is the last link to the file, it makes no sense to update this time, because
all the information about the file (the i-node) is removed with the file.4.10 We recursively call our functiondopathafter opening a directory withopendir.
Assuming thatopendiruses a single file descriptor,this means that each time we
descend one level, we use another descriptor.(We assume that the descriptor isn’t
closed until we’refinished with a directory and callclosedir.) This limits the
depth of the file system tree that we can traverse to the maximum number of open
descriptors for the process. Note that thenftwfunction as specified in the XSI
option of the Single UNIX Specification allows the caller to specify the number of
descriptors to use, implying that it can close and reuse descriptors.
4.12 Thechrootfunction is used by the Internet File Transfer Protocol(FTP)program
to aid in security.Users without accounts on a system (termedanonymous FTP)
areplaced in a separate directory,and achrootis done to that directory.This
prevents the user from accessing any file outside this new root directory.
In addition,chrootcan be used to build a copy of a file system hierarchy at a
new location and then modify this new copy without changing the original file
system. This could be used, for example, to test the installation of new software
packages.
Only the superuser can executechroot,and once you change the root of a
process, it (and all its descendants) can never get back to the original root.4.13 First, callstatto fetch the three times for the file; then callutimeto set the
desired value. The value that we don’t want to change in the call toutime
should be the corresponding value fromstat.
4.14 Thefinger( 1 )command callsstaton the mailbox. The last-modification time
is the time that mail was last received, and the last-access time is when the mail
was last read.
4.15 Bothcpioandtarstoreonly the modification time (st_mtime) in the archive.
The access time isn’t stored, because its value corresponds to the time the archive
was created, since the file has to be read to be archived. The-aoption tocpio
has it reset the access time of each input file after the file has been read. This way,
the creation of the archive doesn’t change the access time. (Resetting the access
time, however,does modify the changed-status time.) The changed-status time
isn’t stored in the archive, because we can’t set this value on extraction even if it
was archived. (Theutimesfunction and its related functions,futimensand
utimensat,can change only the access time and the modification time.)
When the archive is read back (extracted), tar, by default, restores the
modification time to the value in the archive. Themoption totartells it to not