//data = file in []byte
length := len(data)
length is in type int. I need to find the length in int64. I only have the []byte data of the file. How can I find the size of data in int64?
Convert the int to an int64:
l := int64(len(data))
The length of a slice is guaranteed to fit in an int. If the file is larger than what can be represented by an int, then it's not possible to read the entire file into a []byte.