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Full Discussion: Shared memory
Top Forums Programming Shared memory Post 9287 by Neo on Wednesday 24th of October 2001 08:44:05 PM
Old 10-24-2001
Only one process (at a time) may have an open file descriptor for a shared memory segment. The interprocess communication that is used to insure proper file access is the semaphore.

When a semaphore is used correctly (and it can be tricky!) the problem with shared memory segmentation error will be solved (more than likely, unless you have a pointer or other C programming error).

Here is a basic algorithm:
[list=1][*]Create a shared memory segment.[*]Create a semaphore.[*]Process A acquires the semaphore used for the shared memory segment.[*]Process A opens the shared memory.[*]Process A does cool things with shared memory.[*]Process A closed the shared memory segment.[*]Process A releases the semaphore.[*]Process B has been waiting for the semaphore to be released.[*]Now that semaphore has been released......[*]Process B acquires the semaphore used for the shared memory segment.[*]Process B opens the shared memory.[*]Process B does cool things with shared memory.[*]Process B closed the shared memory segment.[*]Process B releases the semaphore.[/list=1]

Yadda, yadda, yadda....... does this help?
 

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class_init(3)						     Library Functions Manual						     class_init(3)

NAME
class_init - Initialize the class scheduler database LIBRARY
Standard C Library (libclass.a) SYNOPSIS
#include <sys/class.h> class_init(void) DESCRIPTION
Prepare the class scheduler database for use by: opening the disk file (if any) creating (or attaching to) a shared memory section creating (or attaching to) a semaphore Before class scheduling can be used, the database must be initialized. At the first call of class_init() since the last boot, the function creates a shared memory segment for the database and, if it exists on disk, reads it into the shared memory. A semaphore is also created that is used to synchronize potential simultaneous database updates (for example, from class_admin(8) or runclass(1)). On subsequent calls to class_init(), the function attaches to the existing shared memory and semaphore. Note that the calling process must have root access privileges RETURN VALUES
Operation successfully completed. Not super user - permission denied. Failed to access disk file, if it exists. Failed to create or attach to shared memory. Failed to create or attach to semaphore. SEE ALSO
Functions:class_add(3), class_change(3), class_create(3), class_close(3), class_configure(3), class_get_config_stats(3), class_delete(3), class_destroy(3), class_disable(3), class_enable(3), class_save_database(3), class_restore_database(3), class_load_database(3), class_change_name(3), class_database_file_exists(3), class_database_name(3), class_database_modified(3), class_get_classes(3), class_get_class_members(3), Commands:class_admin(8), runclass(1), class_scheduling(4) Other: The System Administration guide. delim off class_init(3)
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