libc/stdlib/mktemp.3

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.Dd $Mdocdate: January 19 2024 $
.Dt MKTEMP 3
.Os
.Sh NAME
.Nm mktemp ,
.Nm mkstemp ,
.Nm mkostemp ,
.Nm mkstemps ,
.Nm mkostemps ,
.Nm mkdtemp
.Nd make temporary file name (unique)
.Sh SYNOPSIS
.In stdlib.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"
.In stdlib.h
.In fcntl.h
.Ft int
.Fn mkostemp "char *template" "int flags"
.Ft int
.Fn mkostemps "char *template" "int suffixlen" "int flags"
.Sh DESCRIPTION
The
.Fn mktemp
family of functions take the given file name template and overwrite
a portion of it to create a new file name.
This file name is unique and suitable for use by the application.
The template may be any file name with at least six trailing
.Em X Ns s ,
for example
.Pa /tmp/temp.XXXXXXXX .
The trailing
.Em X Ns s
are replaced with a unique digit and letter combination.
The number of unique file names that can be returned
depends on the number of
.Em X Ns s
provided;
.Fn mktemp
will try at least 2 ** 31 combinations before giving up.
At least six
.Em X Ns s
must be used, though 10 is much better.
.Pp
The
.Fn mktemp
function generates a temporary file name based on a template as
described above.
Because
.Fn mktemp
does not actually create the temporary file, there is a window of
opportunity during which another process can open the file instead.
Because of this race condition,
.Fn mktemp
should not be used where
.Fn mkstemp
can be used instead.
.Fn mktemp
was marked as a legacy interface in
.St -p1003.1-2001 .
.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 mkostemp
function acts the same as
.Fn mkstemp ,
except that the
.Fa flags
argument may contain zero or more of the following flags for the underlying
.Xr open 2
system call:
.Pp
.Bl -tag -width "O_CLOEXECXX" -offset indent -compact
.It Dv O_APPEND
Append on each write.
.It Dv O_CLOEXEC
Set the close-on-exec flag on the new file descriptor.
.It Dv O_SYNC
Perform synchronous I/O operations.
.El
.Pp
The
.Fn mkstemps
and
.Fn mkostemps
functions act the same as
.Fn mkstemp
and
.Fn mkostemp ,
except they permit a suffix to exist in the template.
The template should be of the form
.Pa /tmp/tmpXXXXXXXXXXsuffix .
.Fn mkstemps
and
.Fn mkostemps
are told the length of the suffix string, i.e.,
.Li strlen("suffix") .
.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 ,
.Fn mkostemp ,
.Fn mkstemps ,
and
.Fn mkostemps
functions return \-1 if no suitable file could be created.
If any call fails, an error code is placed in the global variable
.Va errno .
.Sh EXAMPLES
Quite often a programmer will want to replace a use of
.Fn mktemp
with
.Fn mkstemp ,
usually to avoid the problems described above.
Doing this correctly requires a good understanding of the code in question.
.Pp
For instance, code of this form:
.Bd -literal -offset indent
char sfn[19];
FILE *sfp;

strlcpy(sfn, "/tmp/ed.XXXXXXXXXX", sizeof(sfn));
if (mktemp(sfn) == NULL || (sfp = fopen(sfn, "w+")) == NULL) {
	warn("%s", sfn);
	return (NULL);
}
return (sfp);
.Ed
.Pp
should be rewritten like this:
.Bd -literal -offset indent
char sfn[19];
FILE *sfp;
int fd;

strlcpy(sfn, "/tmp/ed.XXXXXXXXXX", sizeof(sfn));
if ((fd = mkstemp(sfn)) == -1 ||
    (sfp = fdopen(fd, "w+")) == NULL) {
	if (fd != -1) {
		unlink(sfn);
		close(fd);
	}
	warn("%s", sfn);
	return (NULL);
}
return (sfp);
.Ed
.Pp
Often one will find code which uses
.Fn mktemp
very early on, perhaps to globally initialize the template nicely, but the
code which calls
.Xr open 2
or
.Xr fopen 3
on that file name will occur much later.
(In almost all cases, the use of
.Xr fopen 3
will mean that the flags
.Dv O_CREAT
|
.Dv O_EXCL
are not given to
.Xr open 2 ,
and thus a symbolic link race becomes possible, hence making
necessary the use of
.Xr fdopen 3
as seen above.)
Furthermore, one must be careful about code which opens, closes, and then
re-opens the file in question.
Finally, one must ensure that upon error the temporary file is
removed correctly.
.Pp
There are also cases where modifying the code to use
.Fn mktemp ,
in concert with
.Xr open 2
using the flags
.Dv O_CREAT
|
.Dv O_EXCL ,
is better, as long as the code retries a new template if
.Xr open 2
fails with an
.Va errno
of
.Er EEXIST .
.Sh ERRORS
The
.Fn mktemp ,
.Fn mkstemp ,
.Fn mkostemp ,
and
.Fn mkdtemp
functions may set
.Va errno
to one of the following values:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Ar template
argument has fewer than six trailing
.Em X Ns s .
.It Bq Er EEXIST
All file names tried are already in use.
Consider appending more
.Em X Ns s to the
.Ar template .
.El
.Pp
The
.Fn mkstemps
and
.Fn mkostemps
functions may set
.Va errno
to
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Ar template
argument length is less than
.Ar suffixlen
or it has fewer than six
.Em X Ns s
before the suffix.
.It Bq Er EEXIST
All file names tried are already in use.
Consider appending more
.Em X Ns s to the
.Ar template .
.El
.Pp
In addition, the
.Fn mkostemp
and
.Fn mkostemps
functions may also set
.Va errno
to
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa flags
is invalid.
.El
.Pp
The
.Fn mktemp
function may also set
.Va errno
to any value specified by the
.Xr lstat 2
function.
.Pp
The
.Fn mkstemp ,
.Fn mkostemp ,
.Fn mkstemps ,
and
.Fn mkostemps
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 SEE ALSO
.Xr chmod 2 ,
.Xr lstat 2 ,
.Xr mkdir 2 ,
.Xr open 2 ,
.Xr tempnam 3 ,
.Xr tmpfile 3 ,
.Xr tmpnam 3
.Sh STANDARDS
The
.Fn mkdtemp
and
.Fn mkstemp
functions conform to the
.St -p1003.1-2008
specification.
The ability to specify more than six
.Em X Ns s
is an extension to that standard.
The
.Fn mkostemp
function is expected to conform to a future revision of that standard.
.Pp
The
.Fn mktemp
function conforms to
.St -p1003.1-2001 ;
as of
.St -p1003.1-2008
it is no longer a part of the standard.
.Pp
The
.Fn mkstemps
and
.Fn mkostemps
functions are non-standard and should not be used if portability is required.
.Sh HISTORY
A
.Fn mktemp
function appeared in
.At v7 .
The
.Fn mkdtemp
function appeared in
.Ox 2.2 .
The
.Fn mkstemp
function appeared in
.Bx 4.3 .
The
.Fn mkstemps
function appeared in
.Ox 2.3 .
The
.Fn mkostemp
and
.Fn mkostemps
functions appeared in
.Ox 5.7 .
.Sh BUGS
For
.Fn mktemp
there is an obvious race between file name selection and file
creation and deletion: the program is typically written to call
.Xr tmpnam 3 ,
.Xr tempnam 3 ,
or
.Fn mktemp .
Subsequently, the program calls
.Xr open 2
or
.Xr fopen 3
and erroneously opens a file (or symbolic link, FIFO or other
device) that the attacker has created in the expected file location.
Hence
.Fn mkstemp
is recommended, since it atomically creates the file.
An attacker can guess the file names produced by
.Fn mktemp .
Whenever it is possible,
.Fn mkstemp
or
.Fn mkdtemp
should be used instead.
.Pp
For this reason,
.Xr ld 1
will output a warning message whenever it links code that uses
.Fn mktemp .