Laface 2007 File system 2.1 Operating System Design Filesystem system calls buffer allocation algorithms getblk brelse bread breada bwrite iget iput bmap namei alloc freeialloc ifree Filesystem low level functions open creat dup pipe close open stat creat link chdir unlink chroot mknod chown mount chmod umount creat mknod link unlink chown chmod stat read write lseek mount umount chdir chroot Return a descriptor Use namei Allocate inode AttributesI/O File System Structure Management

Laface 2007 File system 2.2 Operating System Design open fd = open (pathname, flag, mode); O_RDONLY O_WRONLY O_RDWR O_NDELAY O_APPEND O_CREAT O_TRUNC O_EXCL...

Laface 2007 File system 2.3 Operating System Design File System structures File table Inode table User file descriptor table RC=1 Read RC=1 Write RC=1 RW RC=2 (/etc/passwd) RC=1 (local) fd1=open(“/etc/passwd”, O_RDONLY); fd2=open(“local”, O_RDWR); fd3=open(“/etc/passwd”, O_WRONLY);

Laface 2007 File system 2.4 Operating System Design File System structures User file descriptor table process A File table RC=1 RW RC=1 Read RC=1 Write RC=3 (/etc/passwd) RC=1 (local) RC=1 (private) Inode table process B fd1=open("/etc/passwd”, O_RDONLY); fd2=open("private”, O_WRONLY);

Laface 2007 File system 2.5 Operating System Design read number = read (fd, buffer, count); The I/O parameters are copied in the u-area :  moderead or write  countnumber of bytes to be read or written  offsetwhere (byte) to begin the I/O operation addresssource or destination flag kernel or user space Other information is u-area: –Current directory –possible changed root

Laface 2007 File system 2.6 Operating System Design read The while count cycle ends: –because count is satisfied –for EOF –for read error from the device –for error during the copy to the user buffer Reaching EOF is different than reading a block with zero pointer in its inode File and record locking for mutual exclusion access.

Laface 2007 File system 2.7 Operating System Design Sequential read #include main () { int fd; char lilbuf[20],bigbuf[1024]; fd = open("/etc/passwd", O_RDONLY); read(fd, lilbuf,20); read(fd, bigbuf,1024); read(fd, lilbuf, 20); } #include main () { int fd; char lilbuf[20],bigbuf[1024]; fd = open("/etc/passwd", O_RDONLY); read(fd, lilbuf,20); read(fd, bigbuf,1024); read(fd, lilbuf, 20); }

Laface 2007 File system 2.8 Operating System Design Read ahead While a process executes the system call read of two logical sequential blocks, the kernel assumes that all its successive calls will be sequential At every iteration of the reading cycle, the kernel stores the next logical block number in the inode in memory In the next iteration it tests if the current block number is equal to the saved one If they are equal, the kernel computes the physical block number for the read ahed and stores its value in the u-area so that it can be used by breada.

Laface 2007 File system 2.9 Operating System Design Concurrent read and write #include /* processo A */ main () { int fd; char buf[512]; fd = open("/etc/passwd", O_RDONLY); read(fd, buf, sizeof(buf)); /* read 1 */ read(fd, buf, sizeof(buf)); /* read 2 */ } #include /* processo A */ main () { int fd; char buf[512]; fd = open("/etc/passwd", O_RDONLY); read(fd, buf, sizeof(buf)); /* read 1 */ read(fd, buf, sizeof(buf)); /* read 2 */ } #include /*processo B */ main() { int fd,i; char buf[512]; for (i=0; i<sizeof(buf);i++) buf[i]='a'; fd = open("/etc/passwd", O_WRONLY); write(fd, buf, sizeof(buf)); /* write 1 */ write(fd, buf, sizeof(buf)); /* write 2 */ } #include /*processo B */ main() { int fd,i; char buf[512]; for (i=0; i<sizeof(buf);i++) buf[i]='a'; fd = open("/etc/passwd", O_WRONLY); write(fd, buf, sizeof(buf)); /* write 1 */ write(fd, buf, sizeof(buf)); /* write 2 */ }

Laface 2007 File system 2.10 Operating System Design Reading a file using two descriptors #include main () { int fd1, fd2; char buf1[512],buf2[512]; fd1 = open("/etc/passwd", O_RDONLY); fd2 = open("/etc/passwd", O_RDONLY); read(fd1, buf1, sizeof(buf1)); read(fd2, buf2, sizeof(buf2)); } #include main () { int fd1, fd2; char buf1[512],buf2[512]; fd1 = open("/etc/passwd", O_RDONLY); fd2 = open("/etc/passwd", O_RDONLY); read(fd1, buf1, sizeof(buf1)); read(fd2, buf2, sizeof(buf2)); }

Laface 2007 File system 2.11 Operating System Design write number = write (fd, buffer, count); If the write offset does not correspond to an already allocated block, the kernel allocates a new block and updates the inode pointer structure. – It may be necessary to allocate one or more indirect blocks If the kernel has to write only part of a block, it must read the block from disk. Delayed write is particularly suited to pipes and temporary files.

Laface 2007 File system 2.12 Operating System Design lseek position = lseek (fd, offset, reference); fd is the file descriptor offset is the offset inside the file Reference is a constant indicating if the offset refers to: 0 (SEEK_SET) : the beginning of the file 1 (SEEK_CUR): the current position 2 (SEEK_END): the end of file position is the final offset in bytes

Laface 2007 File system 2.13 Operating System Design lseek #include main (int argc, char **argv) { int fd; fd = open(argv[1], O_CREAT, 0755); if (fd == -1) exit(); lseek(fd, , SEEK_SET ); // 0 (SEEK_SET) // 1 (SEEK_CUR // 2 (SEEK_END write(fd, ” ”, 1); } #include main (int argc, char **argv) { int fd; fd = open(argv[1], O_CREAT, 0755); if (fd == -1) exit(); lseek(fd, , SEEK_SET ); // 0 (SEEK_SET) // 1 (SEEK_CUR // 2 (SEEK_END write(fd, ” ”, 1); }

Laface 2007 File system 2.14 Operating System Design lseek #include main (int argc, char **argv) { int fd, skval; char c; if (argc!=2)exit (); fd = open(argv[1], O_RDONLY); if (fd == -1) exit(); while ((skval = read(fd, &c, 1)) == 1) { printf("char %c\n", c); skval = lseek(fd, 1023L, 1); printf(“new offset %d\n", skval); } #include main (int argc, char **argv) { int fd, skval; char c; if (argc!=2)exit (); fd = open(argv[1], O_RDONLY); if (fd == -1) exit(); while ((skval = read(fd, &c, 1)) == 1) { printf("char %c\n", c); skval = lseek(fd, 1023L, 1); printf(“new offset %d\n", skval); }

Laface 2007 File system 2.15 Operating System Design close close(fd); If the inode reference count > 1, decrements the counter and return If the inode reference count = 1, the kernel releases (by means of iput ) – the inode allocated in memory by the open system call – the corresponding entry in the inode table – The entry in the user file description table When a process exits the kernel closes all its file descriptor still open.

Laface 2007 File system 2.16 Operating System Design creat fd = creat (pathname, mode); If the file does not exist, it is created with the specified name and mode. – The kernel analyzes the pathname by means of namei, and when the last component is parsed T it allocates a free inode T stores the name in the first free entry of the last parsed directory name T open the file If the file exists, parsing its pathname the kernel finds its inode – initializes the file dimension to 0 – releases all its data blocks

Laface 2007 File system 2.17 Operating System Design creat If the process calling creat has the write permission, and the file is exist, the file owner and the access permissions do not change. The kernel does not verify that the parent directory of the existing file has the write permission because the directory content does not change.

Laface 2007 File system 2.18 Operating System Design mknod mknod (pathname, type and mode, device); pathname is the file pathname, type and mode are the type and permissions of the created special file device specifies the major and minor number of the new device

Laface 2007 File system 2.19 Operating System Design chdir chdir ( pathname ); pathname is the directory path of the new current directory The kernel decrements the reference count and releases the old directory inode Stores the inode of the new directory in the u-area The current directory inode is released after the process exits or calls again chdir

Laface 2007 File system 2.20 Operating System Design chroot chroot (pathname); Changes the root directory of the filesystem The kernel keeps a global variable pointer to the root inode After a successful chroot the process and its descendants see pathname as the root directory

Laface 2007 File system 2.21 Operating System Design chown - chmod chown (pathname, owner, group); chmod (pathname, mode); These are operations that change the inode, not the file content To change the file owner the process must be the owner of the file or have supersuser privileges.

Laface 2007 File system 2.22 Operating System Design stat - fstat stat (pathname, buffer); fstat (fd, buffer); pathname is the filename fd is a file descriptor buffer is the address of the data structure defined in stat.h that includes all the relevant inode information

Laface 2007 File system 2.23 Operating System Design stat - fstat struct stat { dev_t st_dev; // ID of device containing file ino_t st_ino; // inode number mode_t st_mode; // protection nlink_t st_nlink; // number of hard links uid_t st_uid; // user ID of owner gid_t st_gid; // group ID of owner dev_t st_rdev; // device ID (if special file) off_t st_size; // total size, in bytes blksize_t st_blksize;// blocksize for filesystem I/O blkcnt_t st_blocks; // number of blocks allocated time_t st_atime; // time of last access time_t st_mtime; // time of last modification time_t st_ctime; // time of last status change };