Archive for October, 2011

linux batch rename files

October 28th, 2011

Use this command to rename a set of files:

for file in * ; do mv $file `echo $file | sed 's/OLD_STRRING/NEW_STRING/'` ; done

And here is a batch file:


if [ $# -ne 3 ]; then
        echo "Usage: $0 'FilePattern' SearchString ReplaceString";
        echo "Eg: $0 'r-*.sh' 5 6";
        for file in $1 ; do mv $file `echo $file | sed s/$2/$3/` ; done

If the 2nd or 3rd parameter contains space, use ‘ ‘ to surround the parameter.

CPP (C++) time

October 21st, 2011

There are may ways to estimate how long a program (or a part of the program) elapsed.

Use clock()

clock() is a standard function defined in . Here is the usage:

#include <ctime>

int main() {
  clock_t start, end;
  start = clock();
  // do sth...
  end = clock();

  double elapsedSeconds = (end - start) / CLOCKS_PER_SEC;

Use gettimeofday()

clock() is a linux function defined in . Here is the usage: #include <sys/time.h> int main() { timeval start, end; gettimeofday(&start, 0); // the second parameter is the timezone. // do sth... gettimeofday(&end, 0); double elapsedSeconds = (end.tv_sec - start.tv_sec + 1e-6*(end.tv_usec - start.tv_usec)); }

solve the problem of no output in console on windows eclipse with MinGW

October 14th, 2011

If you are experiencing the problem of no output in console on windows eclipse with MinGW, here is the solution.

And actually the solution the is very simple, you only have to add a PATH variable to your MinGW bin directory in the run configuration.

Here are the steps:

From menu: Run -> Run Configuration…, open the run configuration window, and add the PATH variable to your MinGW bin directory (as shown in the picture below).

Differences between new/delete and malloc/free in C and CPP

October 4th, 2011

There is a very good post about this:

Here I’ll just list some of the differences concisely:

  • new & delete will call the object’s default constructor & destructor respectively; while malloc & free will not. This is also why dynamically array will call it’s element’s default constuctor. See
  • malloc returns a void*, so it’s result always needs cast. Such as: FooCls* t = (FooCls*) malloc(sizeof FooCls);
  • when using new to create an array, delete[] is needed. And never delete[] a single object, it's even worse.
  • realloc only exists with malloc/free pair. It's handy when resize an object to the same or less size.
  • new & delete are based on malloc & free. Malloc additionally allocate a header storing the size allocated, so free() knows the size to free. new[] not only keeps the malloc header, but also has an extra header storing the size of the array/vector, so delete[] knows how many objects are in the array/vector, and thus knows how many times to call the destructor.
    details of new[] & delete[] can be found at: Mismatching scalar and vector new and delete

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 =;
	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: " << << endl;
	cout << "connected: " << cn.connected() << endl;
		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 =;
	cout << "second query result: " << res[0][0] << endl;

	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());

set mysql connection idle timeout

October 3rd, 2011

in config file my.ini, under [mysqld] add these 2 lines:

# set timout to be 1h (3600 seconds)