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Update the documentation to describe the new mktemp() family behaviour.

Browse Source 
Also notes that mkstemp() first appeared in 4.4BSD (change obtained
from OpenBSD)

Reviewed by:	sheldonh
This commit is contained in:
Kris Kennaway 2000-11-19 10:21:32 +00:00
parent f743d11975
commit dc3f2db745
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=68892
1 changed files with 33 additions and 8 deletions
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41
lib/libc/stdio/mktemp.3
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@ -56,17 +56,18 @@ The
function
takes the given file name template and overwrites a portion of it
to create a file name.
This file name is unique and suitable for use
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
.Ql X Ns s
appended
to it, for example
.Pa /tmp/temp.XXXX .
.Pa /tmp/temp.XXXXXX .
The trailing
.Ql X Ns s
are replaced with the current process number and/or a
unique letter combination.
are replaced with a
unique alphanumeric combination.
The number of unique file names
.Fn mktemp
can return depends on the number of
@ -76,7 +77,7 @@ provided; six
will
result in
.Fn mktemp
testing roughly 26 ** 6 combinations.
selecting one of 56800235584 (62 ** 6) possible temporary file names.
.Pp
The
.Fn mkstemp
@ -188,14 +189,34 @@ See
.Xr gcc 1
for more information.
.Sh BUGS
This family of functions produces filenames which can be guessed.
This family of functions produces filenames which can be guessed,
though the risk is minimized when large numbers of
.Ql X Ns s
are used to
increase the number of possible temporary filenames.
This makes the race in
.Fn mktemp ,
between testing for a file's existence and opening it for use,
between testing for a file's existence (in the
.Fn mktemp
function call)
and opening it for use
(later in the user application)
particularly dangerous from a security perspective.
Whenever it is possible,
.Fn mkstemp
should be used instead.
should be used instead, since it does not have the race condition.
If
.Fn mkstemp
cannot be used, the filename created by
.Fn mktemp
should be created using the
.Dv O_EXCL
flag to
.Xr 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.
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr getpid 2 ,
@ -207,6 +228,10 @@ A
.Fn mktemp
function appeared in
.At v7 .
The
.Fn mkstemp
function appeared in
.Bx 4.4 .
The
.Fn mkdtemp
function first appeared in