shm_open

Arguments

Returns

A handle to the shared memory object, or nil on failure.

Description

This function is a wrapper for the C function shm_open. It is currently only available in QNX 4.

The name of the shared memory object is usually a name found under the /dev/shmem directory. Direct shared memory access to devices is acheived through a shm_open call to the existing Physical shared memory.

If you are accessing the existing Physical shared memory region (/dev/shmem/Physical) DO NOT use the size argument, as you may inadvertantly resize this shared memory. The size argument is added as a convenience, and can be used to specify the size of a newly created object.

Valid open-flags are OR-ed combinations of:

• O_RDONLY open for read-only

• O_RDWR open for read and write access

• O_CREAT create a new shared memory segment with access privledges governed by the create_mode parameter

• O_EXCL Exclusive mode. If O_EXCL and O_CREAT are set then shm_open will fail if the shared memory segment exists.

• O_TRUNC If the shared memory object exists, and it is successfully opened O_RDWR, the object is truncated to zero length and the mode and owner are unchanged.

The creation mode is usually an octal number in the range 0o000 - 0o777 defining the access privledges for the shared memory object. Require the 'const/filesys' file to load constants to make this arg easier

Possible errno values are:

• EACCESS permission to create the shared memory object denied

• EEXIST O_CREAT and O_EXCL are set and the named shared memory object already exists

• EINTR The function call was interrupted by a signal

• EMFILE Too many file descriptors in use by this process

• ENAMETOOLONG The lengthof the name arg is too long

• ENFILE To many shared memory objects are currently open in the system

• ENOENT O_CREAT is not set and the named shared memory object does not exist, or O_CREAT is set and either the name prefix does not exist or the name arg is an empty string

• ENOSPC Not enough space for the creation of the new shared memory object

• ENOSYS This function is not supported by this implementation.

Example

    
    //This code maps the first 1000 bytes from video
    //memory (0xA0000) into a buffer named buf.
    
    require_lisp("const/filesys");
    require_lisp("const/mman");
    fd = shm_open("Physical",O_RDONLY, 0o777);
    buf = mmap(1000, PROT_READ , MAP_SHARED, fd, 0xA0000);
    
    
    		

See Also

mmap, shm_unlink