POSIX Shared Memory

• 3.5 plus Kerrisk chapter 54
  • Kerrisk also covers
  • System V Shared Memory chapter 48
  • Memory Mappings chapter 49
• We some how need to ask the kernel to allow us to share memory
• We then should be able to read/write to this memory.
• Plus we need a way to
  • Lock this memory for a unique write.
• Race Condition:
  • When two or more processes need access to a shared object
  • And it is unpredictable as to which one will gain access to that object.
  • A condition where the behavior of a system is dependent on the sequence of uncontrollable events, which results in unexpected or inconsistent results.
• We will do some simple shared memory computations for now, but we will need something to help us eliminate race conditions later.
• I will do a pi computation with n processes
  • The leader will fork n-1 times
  • workers will compute pi and store it in shared memory
  • The workers will exit, giving the leader an indication that work is done
  • At this point, it will compute the final sum.
  • Take a look at piStart.cpp.
• This avoids race conditions, as they can not occur.
• After making the notes, this is too complex to do in class, so we will look at the code in smemPi.cpp
• Adding shared memory
  • We need sys/mman.h, sys/stat.h and fcntl
  • >int shm_open(const char *name, int oflag, mode_t mode);
    • name is a c style string, the shared memory
    • oflag is the open flag
      • In this case we open it for read/write
      • And we open it creating a new file if one does not exist
      • There are other flags, but this will do.
    • mode gives the file permissions
      • In this case we do the equivalent of chmod u=rw, go-rwx
    • This returns a file descriptor
      • A small integer representing a file.
      • -1 means error
  • int shm_unlink(const char *name);
    • If you fail to call this, the "file" will not be removed
    • look at /dev/shm
• Shared memory objects are not closed when a fork occurs (unless specified to do so).
• Shared memory objects can be opened by other processes as well.
• we need to make the file the right size with ftruncate
  • int ftruncate(int fildes, off_t length);
• At this point, we could read, write, lseek , but that is
  • Three more commands not one
  • Not in the spirit of the thing.
  • so mmap
• Next we will map the shared memory to a pointer with mmap
  • void *mmap( void addr[length], size_t length, int prot, int flags, int fd, off_t offset);
  • This will return a pointer to the memory, not the file descriptor.
  • We can use this as an array.
• The man page says we can close the fd now, so I will.