Question 12.21

How can I tell how much destination buffer space I'll need for an arbitrary sprintf call? How can I avoid overflowing the destination buffer with sprintf?

There are not (yet) any good answers to either of these excellent questions, and this represents perhaps the biggest deficiency in the traditional stdio library.

When the format string being used with sprintf is known and relatively simple, you can usually predict a buffer size in an ad-hoc way. If the format consists of one or two %s's, you can count the fixed characters in the format string yourself (or let sizeof count them for you) and add in the result of calling strlen on the string(s) to be inserted. You can conservatively estimate the size that %d will expand to with code like:

#include <limits.h>
char buf[(sizeof(int) * CHAR_BIT + 2) / 3 + 1 + 1];
sprintf(buf, "%d", n);

(This code computes the number of characters required for a base-8 representation of a number; a base-10 expansion is guaranteed to take as much room or less.)

When the format string is more complicated, or is not even known until run time, predicting the buffer size becomes as difficult as reimplementing sprintf, and correspondingly error-prone (and inadvisable). A last-ditch technique which is sometimes suggested is to use fprintf to print the same text to a bit bucket or temporary file, and then to look at fprintf's return value or the size of the file (but see question 19.12).

If there's any chance that the buffer might not be big enough, you won't want to call sprintf without some guarantee that the buffer will not overflow and overwrite some other part of memory. Several stdio's (including GNU and 4.4bsd) provide the obvious snprintf function, which can be used like this:

	snprintf(buf, bufsize, "You typed \"%s\"", answer);

and we can hope that a future revision of the ANSI/ISO C Standard will include this function.

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