Problems with shared memory and lists Post: 302329708

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Top Forums Programming Problems with shared memory and lists Post 302329708 by mgessner on Monday 29th of June 2009 09:00:38 AM

06-29-2009

Registered User

Here's the problem: the pointers that malloc() returns are local to the address space of the process that called malloc(). Those values won't mean anything in the context of another process. Hence putting those numbers in shared memory and using them from another process will invoke undefined behaviour. If you're lucky, your program will then crash. If you're not lucky, you might cause a supernova somewhere (a bit of C humour).

So what you need to do is to allocate a separate chunk of shared memory, and write a "malloc" and "free" that use that pool of memory. In other words, you have to write a small memory allocator/deallocator that uses this shared memory pool, and then passes back offsets from. Since all of your objects are the same, this will be pretty trivial.

Notice I said "offsets" and not "addresses". I don't recall anywhere in anything I've ever read that guarantees that the shared memory will be assigned the same address in different processes. Assuming so, and using those values in your lists will likely lead you back to your original problem. So you will have to handle that problem in an intelligent way.

You can write this to be general (will handle any size allocation) or specific (only handles things of your struct Obj type). Either way is fine, but I think the exercise of writing the general will teach you more.

This is a fairly easy task, but there are some things to watch out for. I will give you two:

a) Only ONE process can initialize the shared memory pool. Start that process FIRST, initialize the pool, and THEN start the others. Otherwise, you'll potentially have multiple processes trashing your pool.

b) What happens if a process that is allocating something from this shared pool gets interrupted during the middle of an allocation, and another process also tries to allocate something from this shared pool. Warning: if you do this wrong, Bad Things will happen, and your mother will laugh at you. Note that accessing the list can ALSO lead to the same kinds of problems, because in the general case you don't necessarily know if another cooperating process has manipulated the list.

One other thing: in your post, you had:

Code:

*sh = *head;

This is wrong: a) you don't want to change the contents of sh, and b) you want head to point to sh, not the other way around.

HTH

mgessner

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LEARN ABOUT X11R4

shm_overview

SHM_OVERVIEW(7) 					     Linux Programmer's Manual						   SHM_OVERVIEW(7)

NAME

       shm_overview - overview of POSIX shared memory

DESCRIPTION

       The POSIX shared memory API allows processes to communicate information by sharing a region of memory.

       The interfaces employed in the API are:

       shm_open(3)    Create and open a new object, or open an existing object.  This is analogous to open(2).	The call returns a file descriptor
		      for use by the other interfaces listed below.

       ftruncate(2)   Set the size of the shared memory object.  (A newly created shared memory object has a length of zero.)

       mmap(2)	      Map the shared memory object into the virtual address space of the calling process.

       munmap(2)      Unmap the shared memory object from the virtual address space of the calling process.

       shm_unlink(3)  Remove a shared memory object name.

       close(2)       Close the file descriptor allocated by shm_open(3) when it is no longer needed.

       fstat(2)       Obtain a stat structure that describes the shared memory object.	Among the  information	returned  by  this  call  are  the
		      object's size (st_size), permissions (st_mode), owner (st_uid), and group (st_gid).

       fchown(2)      To change the ownership of a shared memory object.

       fchmod(2)      To change the permissions of a shared memory object.

   Versions
       POSIX shared memory is supported since Linux 2.4 and glibc 2.2.

   Persistence
       POSIX  shared  memory  objects  have kernel persistence: a shared memory object will exist until the system is shut down, or until all pro-
       cesses have unmapped the object and it has been deleted with shm_unlink(3)

   Linking
       Programs using the POSIX shared memory API must be compiled with cc -lrt to link against the real-time library, librt.

   Accessing shared memory objects via the filesystem
       On Linux, shared memory objects are created in a (tmpfs(5)) virtual filesystem, normally mounted  under	/dev/shm.   Since  kernel  2.6.19,
       Linux supports the use of access control lists (ACLs) to control the permissions of objects in the virtual filesystem.

NOTES

       Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.

       System  V  shared  memory  (shmget(2),  shmop(2),  etc.) is an older shared memory API.	POSIX shared memory provides a simpler, and better
       designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older  systems)  than  System  V
       shared memory.

SEE ALSO

       fchmod(2),  fchown(2),  fstat(2), ftruncate(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_over-
       view(7)

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2016-12-12							   SHM_OVERVIEW(7)

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