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
.Fd #include <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 unique and 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 .
The trailing
.Ql X Ns s
are replaced with the current process number and/or a
unique letter 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
testing roughly 26 ** 6 combinations.
.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 .
.Fn mkstemps
is told the length of the suffix string.
.Pp
The
.Fn mkdtemp
function makes the same replacement to the template as in
.Xr mktemp 3
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 [ENOTDIR]
.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 -ansiC
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 BUGS
This family of functions produces filenames which can be guessed.
This makes the race in
.Fn mktemp ,
between testing for a file's existence and opening it for use,
particularly dangerous from a security perspective.
Whenever it is possible,
.Fn mkstemp
should be used instead.
.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 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 .