snprintf vs. strcpy (etc.) in C

Question

For doing string concatenation, I\'ve been doing basic strcpy, strncpy of char* buffers. Then I learned about the snprintf and friends

Answer 1

sprintf has an extremely useful return value that allows for efficient appending.

Here's the idiom:

char buffer[HUGE] = {0}; 
char *end_of_string = &buffer[0];
end_of_string += sprintf( /* whatever */ );
end_of_string += sprintf( /* whatever */ );
end_of_string += sprintf( /* whatever */ );

You get the idea. This works because sprintf returns the number of characters it wrote to the buffer, so advancing your buffer by that many positions will leave you pointing to the '\0' at the end of what's been written so far. So when you hand the updated position to the next sprintf, it can start writing new characters right there.

Constrast with strcpy, whose return value is required to be useless. It hands you back the same argument you passed it. So appending with strcpy implies traversing the entire first string looking for the end of it. And then appending again with another strcpy call implies traversing the entire first string, followed by the 2nd string that now lives after it, looking for the '\0'. A third strcpy will re-traverse the strings that have already been written yet again. And so forth.

So for many small appends to a very large buffer, strcpy approches (O^n) where n is the number of appends. Which is terrible.

Plus, as others mentioned, they do different things. sprintf can be used to format numbers, pointer values, etc, into your buffer.