Archive for September, 2011

bash script for killing all mysql processes / connections

September 29th, 2011

Here is a small bash script for killing all mysql processes (or connections).

The basic idea is to select all process IDs into a tmp file (each process ID a line), and then read the file line by line and kill the process, finally remove the tmp file.

This bash only works for MYSQL >= 5.1.7. Becuase before 5.1.7 process list is not stored in the table INFORMATION_SCHEMA.PROCESSLIST.


  • either by providing the MYSQL root password as a parameter to the bash as:
    user> MysqlRootPass
  • or by editing the password the line mysqlpass=$1 as:
    mysqlpass=MysqlRootPass and then excute the bash without parameter:

    Of course, you can also edit the other variables in the bash file.



# mysql host/user/pass who has the privilege to read schema INFORMATION_SCHEMA
# tmp file path

# select all process IDs into the tmp file. each line has a process id.
echo "SELECT id FROM INFORMATION_SCHEMA.PROCESSLIST INTO OUTFILE '$tmpprocesslistfile';" | mysql -h$mysqlhost -u$mysqluser -p$mysqlpass INFORMATION_SCHEMA

# read the file line by line and kill the process
while read qryid
echo "kill $qryid;" | mysql -h$mysqlhost -u$mysqluser -p$mysqlpass
done < "$tmpprocesslistfile"

# remove the tmp file
echo 'All connections have been killed.';
echo 'Please enter sudo password to remove the tmp file if necessary.';
sudo rm $tmpprocesslistfile

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 Here I list the code with tiny fix and some of my comments.

The linux man page of msgsnd & msgrcv can be found here:


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.
  if (msgqid < 0) {
    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
  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”

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.

Windows 7 cannot find a specific wireless network

September 18th, 2011

After an accidently reboot of the router, my win 7 laptop could not find this wireless network anymore. I tried to reboot the router and my computer. But either worked. All the other computers (and phones) can find this network and connect to it. And this computer can found other wireless networks and connect to them, but just not this. I also tried adding this network manually, however without luck. Searching on the internet didn’t give me any clue either.

Finally I thought reinstalling the network adapter’s driver might work. So I went to Device Manager, found the adaptoer, and updated the driver software. Bingo! It worked! Thank god, I don’t have to use someone else’s weak unsecured wireless network any more.