freebsd-dev/lib/libc/stdio/mktemp.3

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.\"     @(#)mktemp.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd February 11, 1998
.Dt MKTEMP 3
.Os
.Sh NAME
.Nm mktemp
.Nd make temporary file name (unique)
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft char *
.Fn mktemp "char *template"
.Ft int
.Fn mkstemp "char *template"
.Ft int
.Fn mkstemps "char *template" "int suffixlen"
.Ft char *
.Fn mkdtemp "char *template"
.Sh DESCRIPTION
The
.Fn 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
.Ql X Ns s
appended
to it, for example
.Pa /tmp/temp.XXXXXX .
The trailing
.Ql X Ns s
are replaced with a
unique alphanumeric combination.
The number of unique file names
.Fn mktemp
can return depends on the number of
.Ql X Ns s
provided; six
.Ql X Ns s
will
result in
.Fn mktemp
selecting one of 56800235584 (62 ** 6) possible temporary file names.
.Pp
The
.Fn 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.
.Pp
The
.Fn mkstemps
function acts the same as
.Fn mkstemp ,
except it permits a suffix to exist in the template.
The template should be of the form
.Pa /tmp/tmpXXXXXXsuffix .
The
.Fn mkstemps
function
is told the length of the suffix string.
.Pp
The
.Fn mkdtemp
function makes the same replacement to the template as in
.Fn mktemp
and creates the template directory, mode 0700.
.Sh RETURN VALUES
The
.Fn mktemp
and
.Fn mkdtemp
functions return a pointer to the template on success and
.Dv NULL
on failure.
The
.Fn mkstemp
and
.Fn mkstemps
functions
return \-1 if no suitable file could be created.
If either call fails an error code is placed in the global variable
.Va errno .
.Sh ERRORS
The
.Fn mkstemp ,
.Fn mkstemps
and
.Fn mkdtemp
functions
may set
.Va errno
to one of the following values:
.Bl -tag -width Er
.It Bq Er ENOTDIR
The pathname portion of the template is not an existing directory.
.El
.Pp
The
.Fn mkstemp ,
.Fn mkstemps
and
.Fn mkdtemp
functions
may also set
.Va errno
to any value specified by the
.Xr stat 2
function.
.Pp
The
.Fn mkstemp
and
.Fn mkstemps
functions
may also set
.Va errno
to any value specified by the
.Xr open 2
function.
.Pp
The
.Fn mkdtemp
function
may also set
.Va errno
to any value specified by the
.Xr mkdir 2
function.
.Sh NOTES
A common problem that results in a core dump is that the programmer
passes in a read-only string to
.Fn mktemp ,
.Fn mkstemp ,
.Fn mkstemps
or
.Fn mkdtemp .
This is common with programs that were developed before
.St -isoC
compilers were common.
For example, calling
.Fn mkstemp
with an argument of
.Qq /tmp/tempfile.XXXXXX
will result in a core dump due to
.Fn 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
.Xr gcc 1
for more information.
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr getpid 2 ,
.Xr mkdir 2 ,
.Xr open 2 ,
.Xr stat 2
.Sh HISTORY
A
.Fn mktemp
function appeared in
.At v7 .
The
.Fn mkstemp
function appeared in
.Bx 4.4 .
The
.Fn mkdtemp
function first appeared in
.Ox 2.2 ,
and later in
.Fx 3.2 .
The
.Fn mkstemps
function first appeared in
.Ox 2.4 ,
and later in
.Fx 3.4 .
.Sh BUGS
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 (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, 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.
.Pp
The implementation of these functions calls
.Xr arc4random 3 ,
which is not reentrant.
You must provide your own locking around this and other consumers of the
.Xr arc4random 3
API.