write longblob to file from c++ mysql connector select

Question

I have a mysql table with a longblob field. I have successfully inserted the blob records into the table and extracted them back into files on disk using python.

Now I want to select the latest version blob from the table and write it to a file on disk using c++ and mysql connector. This is the code I have so far.

It appears to work for smaller blobs, but not enough for large blobs on my database. The max buffer size appears to be near 1000000.

How can I write large longblob files from the mysql select?

string appname = "rocket.exe"; 

driver = get_driver_instance();
con = driver->connect(mysql_server, mysql_user, mysql_password);
con->setSchema("app");
stmt = con->createStatement();

sqlstring =
    "select installer"
    "  from app_table"
    " where appname = '" + appname + "'"
    "   order by version desc "
    " limit 1";  

std::istream *blob;
res = stmt->executeQuery(sqlstring);
while (res->next()) {
    blob = res->getBlob("installer");
}

char buffer[1000000];
memset(buffer, '\0', 1000000);
blob->read((char*)buffer,1000000);   

std::ofstream outfile ("rocket2.exe",std::ofstream::binary);
outfile.write (buffer,1000000);
outfile.close();

delete res;   // resultset
delete stmt;  // statement
delete con;   // connection

my bounty is just looking for the snippet of code to replace in the above code that will make it work with much larger blobs. No need to provide the entire program. what I have above works, just not for very large blobs — panofish
– panofish, Commented Oct 9, 2013 at 20:07

Community · Accepted Answer · 2017-05-23 12:05:14Z

2

+25

That's a large buffer. You are probably running out of stack space. Allocate to the heap by adding some memory management or using vector. You said you can use about 1000000. The variability there is due to the stack space being used by the rest of the code.

#include <vector>;
std::vector<char> buffer(1000000, '\0');
blob->read(&buffer[0], buffer.size());

For more information on stack vs heap memory, look here, and here

edited May 23, 2017 at 12:05

CommunityBot

11 silver badge

answered Oct 13, 2013 at 19:32

cs_alumnus

1,68917 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

panofish Over a year ago

Thanks... I will check this when I have the chance. Hopefully next week.

Henno · Accepted Answer · 2013-10-14 15:06:09Z

0

Where buffer is defined put:

std::vector<char> buffer(1000000, '\0');
blob->read(&buffer[0], buffer.size());

answered Oct 14, 2013 at 15:06

Henno

4072 silver badges7 bronze badges

Collectives™ on Stack Overflow

write longblob to file from c++ mysql connector select

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related