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Is there an example using a Blob ?
Posted by: Jean-Denis Muys
Date: September 22, 2009 02:40AM

Hi,

As far as I can see, none of the examples for Connector/C++ demonstrate how to send and/or receive a blob to/from mySQL.

Where could I find such an example?

The difficulty to me stems from the fact that sending a blob involves a std::istream:

virtual void PreparedStatement:setBlob(unsigned int parameterIndex, std::istream * blob);

And given a blob in memory (an opaque block whose origin could be, say, malloc), I have a hard time making a std::istream out of it, in a way that does NOT imply copying the memory block.

Here is what I tried, questions follow:

std::string insertSQL = "INSERT INTO RKTable (RKBlob, RKModifStamp) VALUES (?, now())";
std::auto_ptr<sql::PreparedStatement> prep_stmt(this->_mySQL->prepareStatement(insertSQL));

size_t packageLength;
void * package;
while ((package = getNextPackage(&packageLength)) != NULL) {

try {
std::strstream tmp_blob(static_cast<char *> (package), packageLength);
prep_stmt->setBlob(1, &tmp_blob);
int num_rows = prep_stmt->executeUpdate();
tmp_blob.freeze(true);
log << "insert into mySQL affected " << num_rows << endl;
} catch (...) { snip exception processing }

free(package);
}

I am very nervous using std::strstream as a class as the char * streams are deprecated, but I could not find a way using any other standard stream class.

Note that, as documented, you can't rely on the char * stream to release the memory buffer. That's why I have my call to free. Of course, I know that getNextPackage allocated using malloc().

Since I want to free the blob myself, I need to freeze the stream. Otherwise, the stream thinks it manages the memory itself.

Since the istrstream doesn't have a freeze function, I use a strstream.

Note also that the stream is constructed and destructed every time through the loop. This might be a little wasteful, but should not be otherwise detrimental.

Here are my questions:

- Has anybody some working code to share that works with blobs?
- Is memory managed correctly in this code snippet, as far as the stream is concerned?
- How would I modify the snipped to construct and destruct the stream only once?
- Is there a workable way to change this snippet to use a istrstream instead?
- Is there a way to use not-deprecated stream class instead?

Thanks for any comment.

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