Is sscanf considered safe to use?

Question

I have vague memories of suggestions that sscanf was bad. I know it won't overflow buffers if I use the field width specifier, so is my memory just playing tricks with me?

Answer 1

I think it depends on how you're using it: If you're scanning for something like int, it's fine. If you're scanning for a string, it's not (unless there was a width field I'm forgetting?).

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Edit:

It's not always safe for scanning strings.

If your buffer size is a constant, then you can certainly specify it as something like %20s. But if it's not a constant, you need to specify it in the format string, and you'd need to do:

char format[80]; //Make sure this is big enough... kinda painful
sprintf(format, "%%%ds", cchBuffer - 1); //Don't miss the percent signs and - 1!
sscanf(format, input); //Good luck

which is possible but very easy to get wrong, like I did in my previous edit (forgot to take care of the null-terminator). You might even overflow the format string buffer.

Answer 2

The reason why sscanf might be considered bad is because it doesnt require you to specify maximum string width for string arguments, which could result in overflows if the input read from the source string is longer. so the precise answer is: it is safe if you specify widths properly in the format string otherwise not.