/in/foo.c: In function 'encrypt':
/in/foo.c:27:18: error: expected expression before ']' token
    en(a[i]+5,8,s[]);
                  ^
/in/foo.c:29:19: error: expected expression before ']' token
    en(a[i]-5,16,s[]);
                   ^
/in/foo.c: At top level:
/in/foo.c:34:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
/in/foo.c: In function 'main':
/in/foo.c:37:10: error: expected expression before 'char'
  encrypt(char a[30],char b[30]);
          ^~~~
/in/foo.c:37:2: error: too few arguments to function 'encrypt'
  encrypt(char a[30],char b[30]);
  ^~~~~~~
/in/foo.c:20:6: note: declared here
 void encrypt(char a[],char b[])
      ^~~~~~~
/in/foo.c:36:14: warning: unused variable 'a' [-Wunused-variable]
  static char a[30]="No.1",b[100];
              ^
At top level:
/in/foo.c:36:14: warning: 'a' defined but not used [-Wunused-variable]