scanf("%ld",a->num);

The syntax of this sentence is wrong~

It should be like this: scanf("% ld", &a->num);

See:/neicole/article/details/7198554

----30162. Use of scanf.

----------#include

----------int scanf( const char *format, ... );

----------In fact, the use of scanf and printf are similar. The general form of the scanf function is: scanf (format control, address table)

-- -------It should be noted that the address table column here is the address that needs to reference the variable.

------------For example: scanf("a = %f, b = %f, c = %f", &a, &b, &c);

------------In addition, when inputting, in addition to entering variables, you must also input according to the char *format in scanf.

--- ---------That is, the correct input method for this scanf example is: "a = 2.3, b = 2, c = 23 and press Enter" and the characters a= and b= must also be entered.