Comparing and writing out BLOBS

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Comparing and writing out BLOBS

Comparing and writing out BLOBS

2004-03-19 - By John Ling

Back
I had posted the following code in the plusplus mailing list but it was
suggested I post this question in this list instead.

I 've been writing a test program using the MySQL C API to test the
reading and writing of BLOB data in and out of the database. The file I
read into the database is a small binary program. I then write out this
BLOB to disk as another file. There is obviously something wrong as the
newly written out file does not execute when run:

"./a.out: Exec format error. Binary file not executable. "

Permissions are executable. I think the code that I use to read in the
BLOB is correct. But I am not sure if I am retrieving the BLOB
properly. I want to do a comparison between my input buffer and what I
get back from a mysql_fetch_row() but I 'm not sure if I am doing it
correctly or not. If I am doing the comparison correctly, then there is
something wrong because the comparison between the two buffers fails at
some point (the first 630 some characters are the same but then a
failure), in which case, what am I doing wrong? How do I read out the
program that I stored in the database?

I give my code here and I would much appreciate it if someone could
point out what it is I am not doing correctly.

// First read in the BLOB into the database
ifstream is;
is.open(inputFilename,ifstream::in|ifstream::binary);
is.seekg(0, ifstream::end);
long bufferSize = is.tellg();
is.seekg(0, ios::beg);
char buffer[bufferSize];

is.read(buffer, bufferSize);
is.close();

// Set query string
char mySql[55000] = "INSERT into Blob_file (blob_file) values ( ' ";
char* tail;

tail = mySql + strlen(mySql);

if ((tail + 2*bufferSize) + 3 > mySql + sizeof(mySql)) {
cerr < < "Binary too big " < < endl;
return 1;
}

tail += mysql_escape_string(tail, buffer, bufferSize);

(void) strcpy (tail, " ') ");
mysql_real_query(myConnection, mySql, strlen(mySql));

// Now try retrieve the BLOB out of the database
char getMySql[1024] = "select blob_file from Blob_file where
blob_file_id = 79435 "; // assuming the insert created entry 79435

mysql_real_query(myConnection, getMySql, strlen(getMySql));

MYSQL_RES* Res;
MYSQL_ROW Row;
Res = mysql_store_result(myConnection);
cout < < mysql_num_rows(Res) < < endl;
Row = mysql_fetch_row(Res);

unsigned long *lengths;
lengths = mysql_fetch_lengths(Res);

// Is what I retrieved the same as what I put in?
for (int j=0;j <=lengths[0]-1;j++) {
cout < < ". ";
if (Row[0][j] != buffer[j]) {
cout < < "BAD " < < endl;
return 1; // I end up here and terminate so something wrong
};
}

ofstream os;
os.open(outputFilename,ofstream::binary);
os.write(Row[0],bufferSize);
os.close();

// close database connection
myDatabase.disconnect();

return 0;

Thank You,
John

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