Expand integer macro inside quoted string - c

Question

While contributing to exim, I saw many values where hard-coded :
uschar filebuffer[256];
(void)sprintf(CS filebuffer, "%.256s.db", filename);
rc = lf_check_file(-1, filebuffer, S_IFREG, modemask, owners, owngroups,
"dbm", errmsg);
if (rc < 0) /* stat() failed */
{
(void)sprintf(CS filebuffer, "%.256s.dir", filename);
rc = lf_check_file(-1, filebuffer, S_IFREG, modemask, owners, owngroups,
"dbm", errmsg);
if (rc == 0) /* x.dir was OK */
{
(void)sprintf(CS filebuffer, "%.256s.pag", filename);
rc = lf_check_file(-1, filebuffer, S_IFREG, modemask, owners, owngroups,
"dbm", errmsg);
}
}
}
As the code isn’t windows specific, every256values should be converted toPATH_MAX.
I know that expanding macros inside quoted strings isn’t possible, but that string concatenation is trivial :
#define STR "string"
size_t len=strlen("part"STR"part 2");
However, things like :
"%."PATH_MAX".db"
Shouldn’t work becausePATH_MAXexpands to an integer, not a string.
So is there a way to do this without calling a function that convert integers to C strings ?

Answer 1

The right way to do this is to use a * in your format string, which will cause it to take the value from your argument list. For example:
printf("%.*s\n", 3, "abcde");
This is equivalent to:
printf("%.3s\n", "abcde");
That way you can use PATH_MAX or any other value to control the format without having to worry about how they're defined (e.g., whether they contain parentheses or addition operators, etc.)

Answer 2

You can stringify a macro argument by using the # operator. But you need an indirect macro invocation to expand the argument:
#define Q(x) Q_(x)
#define Q_(x) #x
So you could do something like:
char filebuffer[PATH_MAX + 10];
sprintf(filebuffer, "%." Q(PATH_MAX)"s.db", filename);
The existing code limits the string sibstitution to 256 characters but then adds a file extension (and a NUL terminator) which will be a buffer overflow when the length is close to 256. I used an arbitrary 10-byte overallocation above, but it would be better to use a checked-length sprintf like snprintf. That would have the additional advantage of not requiring macro games.