snprintf question

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Bruno van Dooren - 14 Sep 2005 20:44 GMT

Hi,

this might be a strange question, but why is snprintf not guaranteed to nul
terminate a string?
i thought the whole point of those sn functions was to prevent these errors.

sure, they prevent buffer overwrites, but callers can still screw up bad if
they don't always make sure for themselves that the 0 is there before they
let someone else read the contents.

now you still have the problem that you have to make that check everywhere,
instead of not having to bother.

kind regards,
Bruno.

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William DePalo [MVP VC++] - 14 Sep 2005 20:57 GMT

> this might be a strange question, but why is snprintf not guaranteed to
> nul terminate a string?

History, mostly.

As I see it, there are two ways to go when the string being written exceeds
the space avaialble to it.

1) truncate and null terminate
2) just truncate

C has always had the mindset that the programmer knows what he is doing or
should know. So, it chooses option 2.

> i thought the whole point of those sn functions was to prevent these
> errors.

You may be thinking of the new "safe" string functions

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncode/html/sec
ure03102004.asp

> sure, they prevent buffer overwrites, but callers can still screw up bad
> if they don't always make sure for themselves that the 0 is there before
> they let someone else read the contents.

Yup. The docs do mention in their security note that

snprintf(s, ...)

should be followed by

s[n - 1] = 0;

where n = the number of characters in s.

> now you still have the problem that you have to make that check
> everywhere, instead of not having to bother.

Yup. Sadly, such is life. Since C "strings" are just arrays, and since
"strings" are passed by simple pointer, called functions have no
authoritative source for string length.

Regards,
Will

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Ronald Laeremans [MSFT] - 14 Sep 2005 20:59 GMT

> Hi,
>
[quoted text clipped - 11 lines]
> kind regards,
> Bruno.

Hi Bruno,

Take a look at the following link:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncode/html/sec
ure03102004.asp

Ronald Laeremans
Visual C++ team

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