m4: Mkstemp
13.5 Making temporary files
===========================
Commands specified to 'syscmd' or 'esyscmd' might need a temporary file,
for output or for some other purpose. There is a builtin macro,
'mkstemp', for making a temporary file:
-- Builtin: mkstemp (TEMPLATE)
-- Builtin: maketemp (TEMPLATE)
Expands to the quoted name of a new, empty file, made from the
string TEMPLATE, which should end with the string 'XXXXXX'. The
six 'X' characters are then replaced with random characters
matching the regular expression '[a-zA-Z0-9._-]', in order to make
the file name unique. If fewer than six 'X' characters are found
at the end of 'template', the result will be longer than the
template. The created file will have access permissions as if by
'chmod =rw,go=', meaning that the current umask of the 'm4' process
is taken into account, and at most only the current user can read
and write the file.
The traditional behavior, standardized by POSIX, is that 'maketemp'
merely replaces the trailing 'X' with the process id, without
creating a file or quoting the expansion, and without ensuring that
the resulting string is a unique file name. In part, this means
that using the same TEMPLATE twice in the same input file will
result in the same expansion. This behavior is a security hole, as
it is very easy for another process to guess the name that will be
generated, and thus interfere with a subsequent use of 'syscmd'
trying to manipulate that file name. Hence, POSIX has recommended
that all new implementations of 'm4' provide the secure 'mkstemp'
builtin, and that users of 'm4' check for its existence.
The expansion is void and an error issued if a temporary file could
not be created.
The macros 'mkstemp' and 'maketemp' are recognized only with
parameters.
If you try this next example, you will most likely get different
output for the two file names, since the replacement characters are
randomly chosen:
$ m4
define(`tmp', `oops')
=>
maketemp(`/tmp/fooXXXXXX')
=>/tmp/fooa07346
ifdef(`mkstemp', `define(`maketemp', defn(`mkstemp'))',
`define(`mkstemp', defn(`maketemp'))dnl
errprint(`warning: potentially insecure maketemp implementation
')')
=>
mkstemp(`doc')
=>docQv83Uw
Unless you use the '--traditional' command line option (or '-G',
⇒Invoking m4 Limits control.), the GNU version of 'maketemp' is
secure. This means that using the same template to multiple calls will
generate multiple files. However, we recommend that you use the new
'mkstemp' macro, introduced in GNU M4 1.4.8, which is secure even in
traditional mode. Also, as of M4 1.4.11, the secure implementation
quotes the resulting file name, so that you are guaranteed to know what
file was created even if the random file name happens to match an
existing macro. Notice that this example is careful to use 'defn' to
avoid unintended expansion of 'foo'.
$ m4
define(`foo', `errprint(`oops')')
=>
syscmd(`rm -f foo-??????')sysval
=>0
define(`file1', maketemp(`foo-XXXXXX'))dnl
ifelse(esyscmd(`echo \` foo-?????? \''), ` foo-?????? ',
`no file', `created')
=>created
define(`file2', maketemp(`foo-XX'))dnl
define(`file3', mkstemp(`foo-XXXXXX'))dnl
ifelse(len(defn(`file1')), len(defn(`file2')),
`same length', `different')
=>same length
ifelse(defn(`file1'), defn(`file2'), `same', `different file')
=>different file
ifelse(defn(`file2'), defn(`file3'), `same', `different file')
=>different file
ifelse(defn(`file1'), defn(`file3'), `same', `different file')
=>different file
syscmd(`rm 'defn(`file1') defn(`file2') defn(`file3'))
=>
sysval
=>0
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