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Re: syslog()/snprintf(): beware of functions with fuzzy specs
From: casper () Holland Sun COM (Casper Dik)
Date: Wed, 6 Sep 1995 16:39:10 +0200


BSD4.4 snprintf()s return the number of characters they would have
written had the buffer been infinite.  This is despite the manual page
saying they return the number of characters actually written.

This should be fixed.  It requires the *snprintf() code to parse
and examine all arguments: that isn't necessary.

And p += snprintf(p, &buf[sizeof(buf)] -p, ...)
won't work as expected.

Number of characters written does not include the NULL.

(Aside: I prefer NUL to refer to '\0' and NULL to refer to the null pointer)

GNU snprintf()s return the number of characters actually written (minus
the NULL).  On a overflowed buffer, the GNU documentation conflicts with
itself.  At one point it says, (bufsiz - 1) is returned, and in another
example it says (bufsiz) is returned (but (bufsiz - 1) plus a NULL
character are written).  I'm not sure what it does for real.

I'm pretty sure we want a terminated string.  I.e., return "bufsiz - 1".

I've seen a few other cobbled version of *nprintf.  I'm best off with a
dart board if I needed to apriori determine what they do on the boundary
cases.

Some important boundary cases:

vsnprintf (buf, bufsiz, fmt, ap)

 bufsiz = 0
 bufsiz = 1
 bufsiz = amount of chars sprint will use, less one for the null
 bufsiz = amount of chars sprint will use, no room for null
 bufsiz = less than amount of chars sprint will use

Which return values of -1, 0, bufsiz - 1, bufsiz, or >bufsiz (heaven
forbid a core dump)?  Do they write on buf[bufsiz]?  What do they write
to buf?  What is the state of NULL terminatation on each of the
sitations above?  Many different answers have I seen.

snsprintf should never write buf[bufsize], it can write buf[bufsize-1].

I prefer the following:

        bufsiz          strsize                 return value
         0               N/A                     0  (can argue in favour of -1)
         1               N/A                     0
         >1              <bufsiz                 strsize
         >1              >=bufsiz                bufsiz-1

It can be argued that snprintf(buf, 0, ...) should return -1: there's
no room for the '\0'.

But you also want p += snprintf(....) to continue to work.

(But note that if you do that properly, the remaining number of bytes
will never drop below 1)

For those of you working on syslog patches using *nprintf, you will do
the world a favor if you make it explicitly clear the semantics you
expect.

Or ignore the return value and use strlen().  (In which case you
depend on snprintf to return '\0' terminated strings.)

Casper



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