lecture #9

OSSF

• Banker's algorithm

  • safety: given max requests & current allocation, determine whether there exists a sequence of allocations (assuming all programs eventually terminate) that will allow all processes to run to completion
    • system is in a safe state
  • allocation: determine whether allocation request, if granted, would keep the system in a safe state

• algorithm is worst-case: as if processes were run sequentially

  • actual allocation would be less inefficient

• a system in a safe state can never deadlock

  • a system in an unsafe state may or may not eventually hit deadlock
  • banker's algorithm is conservative

• peripheral devices: many properties

  • character/block
  • sequential/random access
  • read-only/write-only/both

• disk device: 3-tuple addressing (cylinder, platter, sector)

  • device driver: block 0..N-1

• filesystem: data structure on top of block sequence

• Unix-based systems: hierarchical system of directories & files

• file attributes: metadata (data about the data)

  • name, type, size, protection/permission, time/date, ownership
  • disk is shared by multiple users

• file formats

  • operating system dependent: complex, inflexible
  • Unix-derived systems: simplify, simplify
    • file is sequence of bytes--unstructured
    • imposed structure is application-specific
    • convention: text files have newline-delimited records (aka lines)

• file operations

  • open
  • close
  • read
  • write
  • tell
  • lseek
  • ioctl
  • fcntl
  • ...

• creat(2) vs. open(2)

  • creat(const char *pathname, int flags)
  • open(const char *pathname, int flags, mode_t mode)
    • third argument is used only if new file is created

• opening file is more complex than one might expect

  • open only if file already exists
  • open only if file does not already exist (create)
    • atomic operation: may be used for synchronization
      • prefer other synchronization mechanisms
  • open for reading
  • open for writing
    • truncate
    • append
  • open for async I/O ops
  • open for non-blocking I/O (pipes, devices)

• flags: symbolic values, powers of 2 (bitwise operations, values ored together)

  • open mode: O_RDONLY, O_WRONLY, O_RDWR
  • other flags: O_APPEND, O_CREAT, O_EXCL, O_TRUNC, ...

• mode: permission bits

  • same values as chmod(2)

File Interface (cont.)

• file operations

  • open: open file
  • close: close file, release kernel resources
  • read: read specified #bytes, advance file pointer
  • write: write specified #bytes, advance file pointer
  • tell: return current position of file pointer
  • lseek: reposition file pointer
  • ioctl: device-specific operation (general interface)
  • fcntl: file (kernel data structure) parameters
  • ...

• repositioning the file pointer: off_t lseek(int fd, off_t offest, int whence);

  • whence:
    • SEEK_SET (O): from beginning (positive offset)
    • SEEK_CUR (1): from current position (positive or negative offset)
    • SEEK_END (2): from end (negative offset)
  • llseek(2): obsolete system call for 32-bit systems to use 64-bit file addressing
    • 32-bit addressing: limits file size to 2 GB (signed addressing)

• int ioctl(int fd, unsigned long request, ...)

  • additional arguments are device-specific (usually pointers)
  • device drivers: section 4 of the manual
    • man 4 tty: teletype (dumb terminal: keyboard & screen)
    • man 4 sd: SCSI disks

• advisory file locks: flock(2)

• metadata syscalls

  • unlink (delete file)
  • rename
  • stat
  • utime

• higher-level File I/O (userland standard library)

  • buffering: efficiency
  • formatting: convenience

C (stdio library):

FILE *fopen(char *fname, char *mode);
fread(void *p, size_t size, size_t nmemb, FILE *stream);
fwrite(const void *p, size_t size, size_t nmemb, FILE *stream);
fclose(FILE *stream);

C++ (streams):

filestream f(fname, ios:in);
filestream f(fname, ios:out);
f << buf;
f >> buf;
f.close()

Java:

FileInputStream inputStream = new FileInputStream(fileName);
ObjectInputStream input = new OjectInputStream(inputStream.getInputStream());
Object obj = input.objectRead();

FileOputStream outputStream = new FileOutpuStream(filename);
ObjectOutputStream output = new ObjectOutputStream(outputStream.getOutputStream());
output.objectWrite(obj);

f.close()

Memory Mapped Files

• mmap(2)