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8.2.5 Notes about the shmem filetype:

Shared memory files are currently supported on most Unix platforms, where the shared memory segments are managed by the operating system kernel and `live' independently of processes. They are not deleted (by default) when the process which created them terminates, although they will disappear if the system is rebooted. Applications can create shared memory files in CFITSIO by calling:

   fit_create_file(&fitsfileptr, "shmem://h2", &status);
where the root `file' names are currently restricted to be 'h0', 'h1', 'h2', 'h3', etc., up to a maximum number defined by the the value of SHARED_MAXSEG (equal to 16 by default). This is a prototype implementation of the shared memory interface and a more robust interface, which will have fewer restrictions on the number of files and on their names, may be developed in the future.

When opening an already existing FITS file in shared memory one calls the usual CFITSIO routine:

   fits_open_file(&fitsfileptr, "shmem://h7", mode, &status)
The file mode can be READWRITE or READONLY just as with disk files. More than one process can operate on READONLY mode files at the same time. CFITSIO supports proper file locking (both in READONLY and READWRITE modes), so calls to fits_open_file may be locked out until another other process closes the file.

When an application is finished accessing a FITS file in a shared memory segment, it may close it (and the file will remain in the system) with fits_close_file, or delete it with fits_delete_file. Physical deletion is postponed until the last process calls ffclos/ffdelt. fits_delete_file tries to obtain a READWRITE lock on the file to be deleted, thus it can be blocked if the object was not opened in READWRITE mode.

A shared memory management utility program called `smem', is included with the CFITSIO distribution. It can be built by typing `make smem'; then type `smem -h' to get a list of valid options. Executing smem without any options causes it to list all the shared memory segments currently residing in the system and managed by the shared memory driver. To get a list of all the shared memory objects, run the system utility program `ipcs [-a]'.


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