Question

What “_” means? Why Microsoft adds this mark at the beginning?

Answer 1

Identifiers in the global namespace starting with _ are reserved for the implementation. _snprintf is just a function that the implementation (Visual Studio) has provided. As to the rationale for that, Visual Studio implements C89, and snprintf is part of a later C99 standard.

Besides that, the semantics of both functions are different in the return type, which in snprintf is always the number of characters that the formatted string takes (whether there was enough space in the buffer or not, while _snprintf will return a negative number if there is not enough space in the buffer.

That is, to allocate a buffer just large enough for the output you can do:

int size = snprintf( 0, 0, "%s %d\n", str, i );
char * buffer = malloc( size+1 );
snprintf( buffer, size+1, "%s %d\n", str, i );

You cannot do that with _snprintf as the only information that the function yields back is that the current size is not enough.