преобразование форматированного вывода (formatted output conversion)
Возвращаемое значение (Return value)
Upon successful return, these functions return the number of
characters printed (excluding the null byte used to end output to
strings).
The functions snprintf() and vsnprintf() do not write more than
size bytes (including the terminating null byte ('\0')). If the
output was truncated due to this limit, then the return value is
the number of characters (excluding the terminating null byte)
which would have been written to the final string if enough space
had been available. Thus, a return value of size or more means
that the output was truncated. (See also below under NOTES.)
If an output error is encountered, a negative value is returned.
