Assume the following struct is declared for the permission list of a file in Linux. Each permission (u or g or o) is represented as an octal. For example, u=7 means rwx, u=5 means r-x.

typedef struct {
unsigned int uid; // owner id
unsigned int gid; // group id
unsigned char u; // owner's permission
unsigned char g; // group's permission
unsigned char o; // other's permission
} Permission;

The permission check procedure is

(1) A user requests an operation on a file.
(2) If the user is the owner of the file, the operation will be checked against the owner's permission of the file. The result is either grant or deny.
(3) Otherwise, if the user is not the owner but in the group of the file, the operation will be checked against the group's permission of the file. The result is either grant or deny.
(4) Otherwise, if the user is neither the owner nor in the group of the file, the operation will be checked against the other's permission of the file. The result is either grant or deny.

Write a C/C++ function accesscheck(unsigned int uid, unsigned int gid, unsigned char req, int fid) to enforce access control in Linux.