c - Since we have snprintf, why we don't have a snscanf?

Question

I have snprintf and it can avoid a buffer overflow, but why there is no function called snscanf?

Code:

int main()
{
     char * src = "helloeveryone";
     char buf1[5];
     sscanf(src,"%s",buf1); // here is a  array out of bounds

}

So, I think a snscanf is also needed. Why do we have only have snprintf?

Answer 1

The controversial (and optional) Annex K to C11 adds a sscanf_s function which takes an additional argument of type rsize_t (also defined in Annex K) after the pointer argument, specifying the size of the pointed-to array. For better or worse, these functions are not widely supported. You can achieve the same results by putting the size in the conversion specifier, e.g.

char out[20];
sscanf(in, "%19s", out);

but this is awkward and error-prone if the size of the destination object may vary at runtime (you would have to construct the conversion specifier programmatically with snprintf). Note that the field width in the conversion specifier is the maximum number of input characters to read, and sscanf also writes a terminating null byte for %s conversions, so the field width you pass must be strictly less than the size of the destination object.