Blobs: GNOME Data Access 5 manual

Blobs

Blobs — Binary data and BLOBs handling

Stability Level

Stable, unless otherwise indicated

Functions

glong	gda_blob_op_get_length ()
glong	gda_blob_op_read ()
gboolean	gda_blob_op_read_all ()
glong	gda_blob_op_write ()
gboolean	gda_blob_op_write_all ()

Types and Values

GdaBlobOp

Object Hierarchy

    GObject
    ╰── GdaBlobOp

Includes

#include <libgda/gda-blob-op.h>

Description

This object is a base class for individual database providers which support BLOB types. It supports operations to read and write data in a BLOB value (of type GDA_BLOB_TYPE).

Libgda offers two methods to manipulate binary values as two containers: GdaBinary and GdaBlob:

When reading from a data model returned by Libgda binary data will often be in a GdaBlob object, and the associated GdaBlobOp object can be used to manipulate the binary object (in a database for example)
When the binary value is created by the user, then there is no point in using a GdaBlob as there can not be any GdaBlobOp object, so the GdaBinary container is enough.

Note that a GdaBlob value (the "data" attribute) will often not contain any data (or only some part of the actual BLOB) and that it's up to the user to use the associated GdaBlobOp object to "load" the data into the container (into the actual process heap).

For example to load the 1st 40 bytes of a blob:

GValue *blob_value = ...
GdaBlob *blob;

blob = (GdaBlob*) gda_value_get_blob (blob_value);
gda_blob_op_read (blob->op, blob, 0, 40);

Another example is to write the contents of a blob to a file on disk, using a special

GdaBlobOp object (internal to Libgda which interfaces

with a file in a filesystem):

GValue *blob_value; // value to copy from
GValue *tmp_value;
GdaBlob *file_blob;

GValue *blob_value = ...
tmp_value = gda_value_new_blob_from_file ("MyFile.bin");
file_blob = (GdaBlob*) gda_value_get_blob (tmp_value);

if (! gda_blob_op_write_all (file_blob->op, gda_value_get_blob (blob_value))) {
      // error
}
else {
      gsize size;
      size = gda_blob_op_get_length (file_blob->op);
      g_print ("Wrote %s, size = %d\n", filename, size);
}
gda_value_free (tmp_value);

For further information, see:

the section about Binary large objects (BLOBs)'s abstraction
Virtual methods for Blob operations section for more information about how to implement the virtual methods when creating a database provider

Functions

gda_blob_op_get_length ()

glong
gda_blob_op_get_length (GdaBlobOp *op);

Parameters

an existing GdaBlobOp

Returns

the length of the blob in bytes. In case of error, -1 is returned and the provider should have added an error (a GdaConnectionEvent) to the connection.

gda_blob_op_read ()

glong
gda_blob_op_read (GdaBlobOp *op,
                  GdaBlob *blob,
                  glong offset,
                  glong size);

Reads a chunk of bytes from the BLOB accessible through op into blob .

Parameters

op	a GdaBlobOp
blob	a GdaBlob to read data to
offset	offset to read from the start of the blob (starts at 0)
size	maximum number of bytes to read.

Returns

the number of bytes actually read. In case of error, -1 is returned and the provider should have added an error to the connection.

gda_blob_op_read_all ()

gboolean
gda_blob_op_read_all (GdaBlobOp *op,
                      GdaBlob *blob);

Reads the whole contents of the blob manipulated by op into blob

Parameters

op	a GdaBlobOp
blob	a GdaBlob to read data to

Returns

TRUE if blob->data contains the whole BLOB manipulated by op

gda_blob_op_write ()

glong
gda_blob_op_write (GdaBlobOp *op,
                   GdaBlob *blob,
                   glong offset);

Writes a chunk of bytes from a blob to the BLOB accessible through op , blob is unchanged after this call.

If blob has an associated GdaBlobOp (ie. if blob->op is not NULL) then the data to be written using op is the data fetched using blob->op .

Parameters

op	a GdaBlobOp
blob	a GdaBlob which contains the data to write
offset	offset to write from the start of the blob (starts at 0)

Returns

the number of bytes written. In case of error, -1 is returned and the provider should have added an error to the connection.

gda_blob_op_write_all ()

gboolean
gda_blob_op_write_all (GdaBlobOp *op,
                       GdaBlob *blob);

Writes the whole contents of blob into the blob manipulated by op . If necessary the resulting blob is truncated from its previous length.

Parameters

op	a GdaBlobOp
blob	a GdaBlob which contains the data to write

Returns

TRUE on success

Types and Values

GdaBlobOp

typedef struct _GdaBlobOp GdaBlobOp;