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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)mktemp.3 8.1 (Berkeley) 6/4/93 .\" $FreeBSD: src/lib/libc/stdio/mktemp.3,v 1.22 2007/01/09 00:28:07 imp Exp $ .\" .Dd February 11, 1998 .Dt MKTEMP 3 .Os .Sh NAME .Nm mkdtemp , .Nm mkstemp , .Nm mkstemps , .Nm mktemp .Nd make temporary file name (unique) .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In unistd.h .Ft char * .Fo mkdtemp .Fa "char *template" .Fc .Ft int .Fo mkstemps .Fa "char *template" .Fa "int suffixlen" .Fc .In stdlib.h .Ft int .Fo mkstemp .Fa "char *template" .Fc .Ft char * .Fo mktemp .Fa "char *template" .Fc .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 LEGACY SYNOPSIS .Fd #include <unistd.h> .Pp The include file .In unistd.h is necessary and sufficient for all functions. .Sh SEE ALSO .Xr chmod 2 , .Xr getpid 2 , .Xr mkdir 2 , .Xr open 2 , .Xr stat 2 , .Xr compat 5 .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.