#include <unistd.h> char *
mktemp (char *template); int
mkstemp (char *template); int
mkstemps (char *template int suffixlen); char *
mkdtemp (char *template);
DESCRIPTION
The
mktemp ();
function
takes the given file name template and overwrites a portion of it
to create a file name.
This file name is guaranteed not to exist at the time of function invocation
and is suitable for use
by the application.
The template may be any file name with some number of
`X'
Ns s
appended
to it, for example
/tmp/temp.XXXXXX
The trailing
`X'
Ns s
are replaced with a
unique alphanumeric combination.
The number of unique file names
mktemp ();
can return depends on the number of
`X'
Ns s
provided; six
`X'
Ns s
will
result in
mktemp ();
selecting one of 56800235584 (62 ** 6) possible temporary file names.
The
mkstemp ();
function
makes the same replacement to the template and creates the template file,
mode 0600, returning a file descriptor opened for reading and writing.
This avoids the race between testing for a file's existence and opening it
for use.
The
mkstemps ();
function acts the same as
mkstemp (,);
except it permits a suffix to exist in the template.
The template should be of the form
/tmp/tmpXXXXXXsuffix
The
mkstemps ();
function
is told the length of the suffix string.
The
mkdtemp ();
function makes the same replacement to the template as in
mktemp ();
and creates the template directory, mode 0700.
RETURN VALUES
The
mktemp ();
and
mkdtemp ();
functions return a pointer to the template on success and
NULL
on failure.
The
mkstemp ();
and
mkstemps ();
functions
return -1 if no suitable file could be created.
If either call fails an error code is placed in the global variable
errno
ERRORS
The
mkstemp (,);
mkstemps ();
and
mkdtemp ();
functions
may set
errno
to one of the following values:
Bq Er ENOTDIR
The pathname portion of the template is not an existing directory.
The
mkstemp (,);
mkstemps ();
and
mkdtemp ();
functions
may also set
errno
to any value specified by the
stat(2)
function.
The
mkstemp ();
and
mkstemps ();
functions
may also set
errno
to any value specified by the
open(2)
function.
The
mkdtemp ();
function
may also set
errno
to any value specified by the
mkdir(2)
function.
NOTES
A common problem that results in a core dump is that the programmer
passes in a read-only string to
mktemp (,);
mkstemp (,);
mkstemps ();
or
mkdtemp (.);
This is common with programs that were developed before
St -isoC
compilers were common.
For example, calling
mkstemp ();
with an argument of
Qq /tmp/tempfile.XXXXXX
will result in a core dump due to
mkstemp ();
attempting to modify the string constant that was given.
If the program in question makes heavy use of that type
of function call, you do have the option of compiling the program
so that it will store string constants in a writable segment of memory.
See
gcc(1)
for more information.
A
mktemp ();
function appeared in
AT&T System
v7 .
The
mkstemp ();
function appeared in
BSD 4.4
The
mkdtemp ();
function first appeared in
Ox 2.2 ,
and later in
Fx 3.2 .
The
mkstemps ();
function first appeared in
Ox 2.4 ,
and later in
Fx 3.4 .
BUGS
This family of functions produces filenames which can be guessed,
though the risk is minimized when large numbers of
`X'
Ns s
are used to
increase the number of possible temporary filenames.
This makes the race in
mktemp (,);
between testing for a file's existence (in the
mktemp ();
function call)
and opening it for use
(later in the user application)
particularly dangerous from a security perspective.
Whenever it is possible,
mkstemp ();
should be used instead, since it does not have the race condition.
If
mkstemp ();
cannot be used, the filename created by
mktemp ();
should be created using the
O_EXCL
flag to
open(2)
and the return status of the call should be tested for failure.
This will ensure that the program does not continue blindly
in the event that an attacker has already created the file
with the intention of manipulating or reading its contents.
The implementation of these functions calls
arc4random(3),
which is not reentrant.
You must provide your own locking around this and other consumers of the
arc4random(3)
API.
