libc/stdio/mktemp.3

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.Dd July 07, 2025
.Dt MKTEMP 3
.Os
.Sh NAME
.Nm mktemp
.Nd make temporary file name (unique)
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft char *
.Fn mktemp "char *template"
.Ft int
.Fn mkstemp "char *template"
.Ft int
.Fn mkostemp "char *template" "int oflags"
.Ft int
.Fn mkostemps "char *template" "int suffixlen" "int oflags"
.Ft int
.Fn mkostempsat "int dfd" "char *template" "int suffixlen" "int oflags"
.Ft char *
.Fn mkdtemp "char *template"
.In unistd.h
.Ft int
.Fn mkstemps "char *template" "int suffixlen"
.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.XXXXXXXXXX .
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 mkostemp
function
is like
.Fn mkstemp
but allows specifying additional
.Xr open 2
flags (defined in
.In fcntl.h ) .
The permitted flags are
.Dv O_APPEND ,
.Dv O_DIRECT ,
.Dv O_SHLOCK ,
.Dv O_EXLOCK ,
.Dv O_SYNC ,
.Dv O_CLOEXEC
and
.Dv O_CLOFORK .
.Pp
The
.Fn mkstemps
and
.Fn mkostemps
functions act the same as
.Fn mkstemp
and
.Fn mkostemp
respectively,
except they permit a suffix to exist in the template.
The template should be of the form
.Pa /tmp/tmpXXXXXXXXXXsuffix .
The
.Fn mkstemps
and
.Fn mkostemps
function
are told the length of the suffix string.
.Pp
The
.Fn mkostempsat
function acts the same as
.Fn mkostemps
but takes an additional directory descriptor as a parameter.
The temporary file is created relative to the corresponding
directory, or to the current working directory if the special
value
.Dv AT_FDCWD
is specified.
If the template path is an absolute path, the
.Fa dfd
parameter is ignored and the behavior is identical to
.Fn mkostemps .
.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 either call fails an error code is placed in the global variable
.Va errno .
.Sh ERRORS
The
.Fn mkstemp ,
.Fn mkostemp ,
.Fn mkstemps ,
.Fn mkostemps
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 mkostemp
and
.Fn mkostemps
functions
may also set
.Va errno
to the following value:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa oflags
argument is invalid.
.El
.Pp
The
.Fn mkstemp ,
.Fn mkostemp ,
.Fn mkstemps ,
.Fn mkostemps
and
.Fn mkdtemp
functions
may also set
.Va errno
to any value specified by the
.Xr stat 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 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.XXXXXXXXXX
will result in a core dump due to
.Fn mkstemp
attempting to modify the string constant that was given.
.Pp
The
.Fn mkdtemp ,
.Fn mkstemp
and
.Fn mktemp
function prototypes are also available from
.In unistd.h .
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr getpid 2 ,
.Xr mkdir 2 ,
.Xr open 2 ,
.Xr stat 2
.Sh STANDARDS
The
.Fn mkstemp
and
.Fn mkdtemp
functions are expected to conform to
.St -p1003.1-2008 .
The
.Fn mktemp
function is expected to conform to
.St -p1003.1-2001
and is not specified by
.St -p1003.1-2008 .
The
.Fn mkostemp
function conforms to
.St -p1003.1-2024 .
The
.Fn mkstemps ,
.Fn mkostemps
and
.Fn mkostempsat
functions do not conform to any standard.
.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 .
The
.Fn mkostemp
and
.Fn mkostemps
functions appeared in
.Fx 10.0 .
The
.Fn mkostempsat
function appeared in
.Fx 13.0 .
.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 ,
.Fn mkostemp
or
.Fn mkostempsat
should be used instead, since they do 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.