c review beginning

1. The cognition of main function: the entrance of a program, there is only one

2. The specific differences of data types:
   the distinction between short/int/long and float/double in integer types is determined by the size of the memory allocation space.

3. The difference between local variables and global variables is the connection: the
   scope of the scope is different, local variables can only be meaningful within the code block (ie within {}), while the life cycle of global variables exists in the entire project, even if they are of different sources Files, can use the same global variable