Last modified: 17 April 2012
Name: H5Fget_file_image
Signature:
ssize_t H5Fget_file_image( hid_t file_id, void *buf_ptr, size_t *buf_len )

Purpose:
Retrieves a copy of the image of an existing, open file.

Motivation:
H5Fget_file_image and other elements of HDF5 are used to load an image of an HDF5 file into system memory and open that image as a regular HDF5 file. An application can then use the file without the overhead of disk I/O.

Recommended Reading:
This function is part of the file image operations feature set. It is highly recommended to study the guide “HDF5 File Image Operations” before using this feature set.

See the “See Also” section below for links to other elements of HDF5 file image operations.

Description:
H5Fget_file_image retrieves a copy of the image of an existing, open file. This routine can be used with files opened using the SEC2 (or POSIX), STDIO, and Core (or Memory) virtual file drivers (VFDs).

If the return value of H5Fget_file_image is a positive value, it will be the length in bytes of the buffer required to store the file image. So if the file size is unkown, it can be safely determined with an initial H5Fget_file_image call with buf_ptr set to NULL. The file image can then be retrieved with a second H5Fget_file_image with buf_len set to the initial call’s return value.

While the current file size can also be retrieved with H5Fget_filesize, that call may produce a larger value than is needed. The value returned by H5Fget_filesize includes the user block, if it exists, and any unallocated space at the end of the file. It is safe in all situations to get the file size with H5Fget_file_image and it often produces a value that is more appropriate for the size of a file image buffer.

Parameters:
hid_t file_id      IN: Target file identifier
void *buf_ptr   IN: Pointer to the buffer into which the image of the HDF5 file is to be copied

If buf_ptr is NULL, no data will be copied but the function’s return value will still indicate the buffer size required (or a negative value on error).

size_t *buf_len   IN: Size of the supplied buffer

Returns:
If successful, returns the size in bytes of the buffer required to store the file image if successful; otherwise returns a negative value.

Failure Modes:
H5Pget_file_image will fail, returning a negative value, if the file is too large for the supplied buffer.

See Also:
H5LTopen_file_image
 
H5Pset_file_image
H5Pget_file_image
 
H5Pset_file_image_callbacks
H5Pget_file_image_callbacks
     HDF5 File Image Operations
     in Advanced Topics in HDF5
 
Within H5Pset_file_image_callbacks:
Callback struct   H5_file_image_callbacks_t
Callback ENUM   H5_file_image_op_t

History:
Release     Change
1.8.9 C function introduced in this release.