Working with the Unix OS

Processes I
dev_t st_rdev; /* device ID, for character or block special files */
off_t st_size; /* file size in bytes */
time_t st_atime; /* time of last file access */
time_t st_mtime; /* time of last file mod */
time_t st_ctime; /* time of last file status */
};
st_mode
#define S_IFMT 0170000 /* type of file */
#define S_IFREG 0100000 /* regular */
#define S_IFDIR 0040000 /* directory */
#define S_IFCHR 0020000 /* character special */
#define S_IFBLK 0060000 /* block special */
#define S_IFLNK 0120000 /* symbolic link */
#define S_IFSOCK 0140000 /* socket - BSD only */
#define S_IFIFO 0010000 /* fifo - System V only */
! File Access Permissions
Every process has four IDs associated with it
- real user ID
- real group ID
- effective user ID
- effective group ID
Every file has the following attributes
- owners user ID (16 bit integer)
- owners group ID (16 bit integer)
- user read, write, execute permission (3 bits)
- group read, write, execute permission (3 bits)
- other read, write, execute permission (3 bits)
- set user ID (1 bit)
- set group ID (1 bit)
- see file fstatus.c
Test to determine if process can access a file:
- if the effective user ID of process is zero (superuser)
- if the effective user ID of process matches the user ID of the file and the appropriate access permission bits are
set
- if the effective user ID of process does NOT match the user ID of the file and if the effective group ID of
process matches the group ID of the file and the appropriate access permission bits are set
- if the other access permission bits for the file are set then access is allowed
! File Access Mode Word
system calls: {access, chmod, creat, mknod, msgctl, open,
semctl, shmctl, stat, fstat & umask}
04000 set user ID on execution
02000 set group ID on execution
01000 save text image after execution "sticky bit"
00400 read by user
00100 write by user
00200 execute by user
00040 read by group
00020 write by group
00010 execute by group
00004 read by other
00002 write by other
00001 execute by other
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