Why in msvc++ we have _snprintf while other compilers allows snprintf

Adding to the above,

there are proper C99 snprintf() and vsnprintf() available since Visual Studio 2015

and they do not even trigger the well known unsafe function deprecation warning.
_snprintf_s() or _vsnprintf_s(), while provided, are "secure variants" of the MSVC-specific functions, with the non-C99 behavior.

Aside from a different return value in case of insufficiently large buffer (described in David's answer), functions from _sn... group differ from the standard snprintf in another important regard. If the target buffer is too short by just one character, the _sn... functions consider this situation a "success".

In more detail, if the target buffer is long enough to store the whole resultant sequence but without the terminating zero character, the _sn... functions do not truncate the result, do not write the terminating zero into the target buffer and return the size of the buffer as a result. So, in general case the result is not guaranteed to be zero-terminated.

In the same situation, snprintf will discard the last character of the resultant sequence and write zero terminator in its place. The result of snprintf is always zero-terminated.