Lecture 18
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Log into Linux. Copy directory
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Reminder: Project 5 due on Tuesday.
Questions?
Thursday, October 29
CS 375 UNIX System Programming - Lecture 18
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Outline
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System V Shared Memory
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Shared Memory via mmap( )
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POSIX Shared Memory
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CS 375 UNIX System Programming - Lecture 18
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System V Shared Memory
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The System V shared memory facility allows
unrelated processes to access the same
segment of memory. It is the fastest means of
transferring data between two processes.
The shared memory routines map the physical
address space of the shared memory segment
into the logical address space of each process.
As noted in last class, the shared memory
routines are similar in format to the semaphore
routines.
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CS 375 UNIX System Programming - Lecture 18
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System V Shared Memory
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The shmget( ) routine is used to create a
shared memory segment:
shmget(key_t key, size_t size, int flags);
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An id is returned that is used in other calls:
// Create named shm block
id = shmget(123, 2048, IPC_CREAT|0660);
// Access existing block by name
id = shmget(123, 0, 0);
// Create unnamed shm block
id = shmget(IPC_PRIVATE, 2048, 0660);
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System V Shared Memory
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The shmat( ) routine maps the shared
segment to the process address space:
void *shmat(id, addr, flags);
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The addr argument allows us to specify a local
address at which the shared memory can be
accessed, it should normally be NULL to allow
the system to choose an address. flags is
usually 0. shmat( ) returns a pointer to the first
byte of shared memory.
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System V Shared Memory
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The shmdt( ) routine detaches the shared
memory segment from the current process.
Detaching the segment does not delete the
segment or clear the segment contents. The
segment can be reattached later:
// Attach shared memory segment
shmaddr = shmat(shmid, (void *)0, 0);
// Detach shared memory segment
ret = shmdt(shmaddr);
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System V Shared Memory
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The shmctl( ) routine is used to change
permissions on a shared memory segment, get
info about a segment and delete a segment:
// Remove the segment from the system
ret = shmctl(shmid, IPC_RMID, 0);
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The segment may not be deleted until
sometime later, after all processes using the
segment have detached it.
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System V Shared Memory
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As noted last time, the ipcs command can be
used to display information (key, id, owner,
permissions, size, and number of attaching
processes) about System V shared memory
segments.
System V shared memory segments have
kernel persistence. The server program should
take responsibility for deleting them. They can
be deleted from the command line via the
ipcrm command, if necessary.
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In-class Exercise
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The program in shm_srvr.cpp demonstrates a
server program that creates, attaches,
detaches, and removes a System V shared
memory segment. It displays the shared
memory id so that it can be used as an
argument when starting the client program
Complete the unfinished client program in
shm_clnt.cpp, then run it with the id given by
the server.
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Shared Memory Via mmap( )
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The mmap( ) routine has traditionally been
used to map a file into memory. It can be used
by a single process for fast file I/O.
mmap( ) also can be used to share memory
between processes. It typically is preferred
over System V shared memory in modern
operating systems.
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Shared Memory Via mmap( )
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mmap( ) supports two types of mappings: file
mappings and anonymous mappings.
A file mapping maps a file (or a portion of a file)
into memory. The file must first be opened and
the file descriptor passed to mmap( ). Other
(related) processes that inherit the file
descriptor can use mmap( ) to map to the same
memory segment.
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Shared Memory Via mmap( )
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An anonymous mapping is not associated with
a file. Anonymous mappings are preserved
across a fork( ) (as are file mappings), but (of
course) not across an exec( ) call.
Anonymous mappings are similar to System V
shared memory segments, but they can not be
used by unrelated processes.
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POSIX Shared Memory
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POSIX defines memory objects that can be
mapped into a processes' memory space by
mmap( ).
Objects are created with shm_open( ):
int shm_open(const char *name, int oflag, mode_t mode);
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name is the name of the object. It should have
a leading '/'. shm_open( ) returns a file
descriptor.
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POSIX Shared Memory
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When a POSIX shared memory object is first
created (oflag contains O_CREAT), the file
descriptor returned from shm_open( ) should
be first passed to ftruncate( ) to set the
memory size and then to mmap( ).
The file descriptor of an existing memory
segment may be passed directly to mmap( ).
POSIX memory objects can be shared by
unrelated processes
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POSIX Shared Memory
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An mmap( ) memory segment can be
unmapped using munmap( ).
A POSIX shared memory object should be
deleted using shm_unlink( ). The segments
will “live” until the next kernel reboot if they are
not deleted. As noted last time, they can be
deleted from the command line using “rm”.
They have corresponding file names in the
/dev/shm directory.
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In-Class Exercise
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The programs in the posix subdirectory are
versions of the example programs modified to
use POSIX shared memory. Complete the
unfinished client program.
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Shared Memory Synchronization
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When using shared memory segments you
need to take precautions to synchronize access
to the segment.
The example programs demonstrate using a
shared boolean variable for exclusive access.
This works only for two processes sharing a
segment.
Other methods (e.g., semaphores) are more
efficient and must be used when more than two
processes are involved.
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In-class Exercise
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Modify the example programs to use a pair of
semaphores for synchronization, instead of
polling the written_by_you flag. (System V
semaphores and POSIX semaphores,
respectively.)
The semaphores are initialized to 1 and 0. The
producer acquires the first before writing data
and releases the second when it is done, while
the consumer does the opposite.
Try running multiple clients.
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