/in/foo.c:1:1: warning: data definition has no type or storage class
 a = input("请输入正方形边长，是一个整数：")
 ^
/in/foo.c:1:1: warning: type defaults to 'int' in declaration of 'a' [-Wimplicit-int]
/in/foo.c:1:5: warning: implicit declaration of function 'input' [-Wimplicit-function-declaration]
 a = input("请输入正方形边长，是一个整数：")
     ^~~~~
/in/foo.c:1:5: error: initializer element is not constant
/in/foo.c:2:1: error: expected ',' or ';' before 'a'
 a = int(a)
 ^