Topics

• Orthogonality
• Processes
• Races

Orthogonality

• How programs can access files and processes?
• Programs should be: modular, composable
• Should be independent
• Example:
  1. Open a file
  2. Remove file
  3. Process was reading, and currently is writing
  • What happens?
  • Could choose to let I/O continue
  • Or tell I/O to stop
    • Not orthogonal since removing a file conflicts with reading/writing

Processes

• Goal of processes:

  • Multitasking (lots of apps at once)
  • Performance
  • Isolation
    • Reliability
    • Security (privacy on a multi-user system)
  • How much isolation?
    • Still want controlled comms. between processes

• IPC

  • Files cat >f head <f
  • Message passing cat | head
  • Shared memory
    • Issues: faster, so garbage, error and race errors
  • Alter/control other processes
    • Signals, create, weait
  • Backdoors

• Modeling OS resources

  • OOP?
    • No functional programming

  struct $file\\to cpp

  • Opaque file processes
    • (Controlled file access)
    • int fstat(fd, struct stat *)
      • Get metadata
    • Added protection, no races, but slow and complicated

• int open(char const* element, flags, mode_t, mode)

  • O_RDONLY
  • O_WRONLY
  • O_RDWR
  • O_PATH — what if want to open, but not read or write (check size)?
  • IO_CREAT, over write, or create or don’t create if don’t exist
  • On fail? returns -1, sets errno

• int close(int fd)

  • Close fail, EBADF
  • char* strerror(int e)

• ssize_t read(int fd, void* buf, size_t sizebuffsize)

  • Returns 0 on EOF
  • Returns 0 to buf size
    • How much read

• ssize_t write(void const* buf)

  • Returns how much written

• off_t lseek(int fd, off_t offset, int whence)

  • Whence:
    • SEEK_SET: cur = offset
    • SEEK_CUR: cur += offset
    • SEEK_END: cur = filesize + offset
  • On pipe? returns -1, errno set

• pread, pwrite—same, but no lseek

• Note that there is a distinction between storage and stream devices in API