Path: utzoo!attcan!uunet!auspex!guy
From: guy@auspex.UUCP (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: gets(3) nonsense
Message-ID: <502@auspex.UUCP>
Date: 23 Nov 88 21:56:43 GMT
References: <867@cernvax.UUCP> <645@quintus.UUCP> <339@igor.Rational.COM> <14447@mimsy.UUCP> <1643@solo11.cs.vu.nl> <1403@unisoft.UUCP> <644@scotty.UUCP>
Reply-To: guy@auspex.UUCP (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 43

>What about sprintf() & fprintf()?
>The user does not have *complete control* over these functions.

"fprintf()" is, as stated in another article, irrelevant; if the buffer
fills up, the standard I/O library will generally write it out and
continue.

As for "sprintf":

Users do have control over the format operations they use with these
functions.

The "%d", "%i", "%o", "%u", "%x", "%X", "%e", "%E", "%f", "%g", and "%G"
conversions, and their "l"-prefixed equivalents (and "h"-prefixed and
"L"-prefixed equivalents, in the dpANS upon which K&R Second Edition is
based), have, on most implementations, a maximum length of output that
they can generate; you can probably pick a number that's "big enough"
for all implementations you're likely to run into (e.g., "%d" is
unlikely to produce more digits than are in -2^64 - the "-" is for the
minus sign" and is extremely unlikely to produce more digits than are in
-2^128). 

"%s" can take a "precision" argument that specifies the *maximum* number
of characters to be produced.

"%c" only produces one character.

The only tricky one appears to be the dpANS's "%p", and you can
probably, in most implementations, just say something like "it's
unlikely to produce more than 128 characters"; if a pointer value takes
128 characters to dump, you may want to consider not dumping it....

Thus, if you don't use "%s" by itself, you can compute a "maximum
length" for the output of "sprintf" that should work, as stated, on most
implementations.  If you do use "%s" by itself, you do at least have
control over what argument matches it, so you can use "strlen" to find
out how many characters it will generate.

If the format string is generated at run time (e.g., inside an
interpreter for a programming language that includes a "printf"
construct), you can consider scanning the string, computing the maximum
length (which may, as indicated in the previous paragraph, require you
to scan the arguments that match "%s"s), and proceeding from there.