Posts Tagged ‘programming’

mysqlpp connection timeout problem

October 3rd, 2011

If you have a application having long time connection with mysql using mysqlpp. You may at first think of using Connection::connected() to determine whether the connection is still valid. But actually this doesn’t work if the connection gets timeout.

After checking the reference, I found out Connection::ping() would be a good way to determine timeout.

Here is a piece of test code:

#include <iostream>
#include <string>
#include <mysql++.h>

using namespace std;

int main() {
	string db_schema("DB_SCHEMA");
	string db_host("DB_HOST");
	string db_user("DB_USER");
	string db_pass("DB_PASS");
	string qry_str("SELECT MAX(user_id) FROM users");

	mysqlpp::Connection cn;
	cn.connect(db_schema.c_str(), db_host.c_str(), db_user.c_str(), db_pass.c_str());
	mysqlpp::Query qry = cn.query(qry_str);
	mysqlpp::StoreQueryResult res = qry.store();
	cout << "frist query result: " << res[0][0] << endl;

	string tmp;
	cout << "Wait for a period of time to let the connection timeout, and then input whatever a string to continue: " << endl;
	cin >> tmp;

	cout << "ping result: " << cn.ping() << endl;
	cout << "connected: " << cn.connected() << endl;
	
	if(!cn.ping())
		cn.connect(db_schema.c_str(), db_host.c_str(), db_user.c_str(), db_pass.c_str());
	mysqlpp::Query qry2 = cn.query(qry_str);
	res = qry2.store();
	cout << "second query result: " << res[0][0] << endl;

	cn.disconnect();
	
	return 0;	
}

But of course, I’m still new to mysqlpp. There may be a better way to do this. Please comment below if you have a better way.

NOTE: Here is how to change mysql connection idle timeout.

UPDATE: Just found another way to handle the timeout problem: use Connection::set_option(new mysqlpp::ReconnectOption(true));. This is much more elegant.
Here is a piece of sample code:

mysqlpp::Connection cn;
cn.set_option(new mysqlpp::ReconnectOption(true));
cn.connect(db_schema.c_str(), db_host.c_str(), db_user.c_str(), db_pass.c_str());

System V message queue example (msgget, msgctl, msgsnd, msgrcv)

September 21st, 2011

Found a very good example of message queue showing how to use msgget, msgctl, msgsnd, and msgrcv at http://www.dps.uibk.ac.at/~tf/lehre/ss04old/bs/tutorials/prozesse-syscalls/29.htm. Here I list the code with tiny fix and some of my comments.

The linux man page of msgsnd & msgrcv can be found here: http://linux.die.net/man/2/msgrcv.

———————————————-
msg_queue.c
———————————————-

#include<string.h>
#include<time.h>
#include<sys/ipc.h>
#include<sys/msg.h>
#include<sys/wait.h>
#include<sys/errno.h>
     
extern int errno;       // error NO.
#define MSGPERM 0600    // msg queue permission
#define MSGTXTLEN 128   // msg text length

int msgqid, rc;
int done;

struct msg_buf {
  long mtype;
  char mtext[MSGTXTLEN];
} msg;

int main(int argc,char **argv)
{
  // create a message queue. If here you get a invalid msgid and use it in msgsnd() or msgrcg(), an Invalid Argument error will be returned.
  msgqid = msgget(IPC_PRIVATE, MSGPERM|IPC_CREAT|IPC_EXCL);
  if (msgqid < 0) {
    perror(strerror(errno));
    printf("failed to create message queue with msgqid = %d\n", msgqid);
    return 1;
  }
  printf("message queue %d created\n",msgqid);
  
  // message to send
  msg.mtype = 1; // set the type of message
  sprintf (msg.mtext, "%s\n", "a text msg..."); /* setting the right time format by means of ctime() */

  // send the message to queue
  rc = msgsnd(msgqid, &msg, sizeof(msg.mtext), 0); // the last param can be: 0, IPC_NOWAIT, MSG_NOERROR, or IPC_NOWAIT|MSG_NOERROR.
  if (rc < 0) {
    perror( strerror(errno) );
    printf("msgsnd failed, rc = %d\n", rc);
    return 1;
  }

  // read the message from queue
  rc = msgrcv(msgqid, &msg, sizeof(msg.mtext), 0, 0); 
  if (rc < 0) {
    perror( strerror(errno) );
    printf("msgrcv failed, rc=%d\n", rc);
    return 1;
  } 
  printf("received msg: %s\n", msg.mtext);

  // remove the queue
  rc=msgctl(msgqid,IPC_RMID,NULL);
  if (rc < 0) {
    perror( strerror(errno) );
    printf("msgctl (return queue) failed, rc=%d\n", rc);
    return 1;
  }
  printf("message queue %d is gone\n",msgqid);

  return 0;
}

————– Update 14/Dec/2011 ——————
A note about the struct msg_buf. From: “The Linux Programmer’s Guide” http://tldp.org/LDP/lpg/node30.html.

The ability to assign a given message a type, essentially gives you the capability to multiplex messages on a single queue. For instance, client processes could be assigned a magic number, which could be used as the message type for messages sent from a server process. The server itself could use some other number, which clients could use to send messages to it. In another scenario, an application could mark error messages as having a message type of 1, request messages could be type 2, etc. The possibilities are endless.

On another note, do not be misled by the almost too-descriptive name assigned to the message data element (mtext). This field is not restricted to holding only arrays of characters, but any data, in any form. The field itself is actually completely arbitrary, since this structure gets redefined by the application programmer. Consider this redefinition:


struct my_msgbuf {
long mtype; /* Message type */
long request_id; /* Request identifier */
struct client info; /* Client information structure */
};

Here we see the message type, as before, but the remainder of the structure has been replaced by two other elements, one of which is another structure! This is the beauty of message queues. The kernel makes no translations of data whatsoever. Any information can be sent.

.htaccess rewrite to redirect non-www to www

August 25th, 2011

htaccess redirect www to non-www


#redirect www to non-www
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]

htaccess redirect non-www to www


#redirect non-www to www
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

htaccess redirect from one site to another


RewriteEngine on
RewriteCond %{HTTP_HOST} ^OLD_DOMAIN\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.OLD_DOMAIN\.com$
RewriteRule ^(.*)$ "http\:\/\/NEW_DOMAIN\.com\/$1" [R=301,L]