What happened if GCC search for libraries in C/C++

Hi all,
In some beautiful days, we wonder how GCC search for standard libraries like libstdc++.so. Cool! It really matters for those guys who works on developments of new standard libraries. They want to have both versions of libraries at the same time. It is a controversial topics. After I googled it, here is the summary I found.

There are two definitions denoted as ld you need to differentiate: linker in GCC and loader in linux. ld can refer to ld (GNU linker) (at compiling time). It can also refer to loader ld.so in linux (at running time).

How to know search path during linking and running time? 

http://stackoverflow.com/questions/9922949/how-to-print-the-ldlinker-search-path 

During linking of GCC, we can use this instruction:

1/ You can do this by executing the following command:

ld --verbose | grep SEARCH_DIR | tr -s ' ;' \\012

2/ gcc passes a few extra -L paths to the linker, which you can list with the following command:

gcc -print-search-dirs | sed '/^lib/b 1;d;:1;s,/[^/.][^/]*/\.\./,/,;t 1;s,:[^=]*=,:;,;s,;,;  ,g' | tr \; \\012

The answers suggesting to use ld.so.conf and ldconfig are not correct because they refer to the paths searched by the runtime dynamic linker (i.e. whenever a program is executed), which is not the same as the path searched by ld (i.e. whenever a program is linked).

The most important keynote: GCC search -L paths first, then the default library (i.e, 2 first, then 1).

During run-time:

ldconfig -v 2>/dev/null | grep -v ^$'\t'

what happened after # in C/C++

Hi all,
If you ever want to know what will happen after including a header, what does GCC look for, or where does it find these headers, or then can I read this header files in Linux. Awesom.......e!!!
If you feel awesome, you need to know a little bit about it.
Well, GCC will look in default "include" folder in Linux such as

     /usr/local/include
     libdir/gcc/target/version/include
     /usr/target/include
     /usr/include

If you mention -Idir option in the command line or makefile scripts, it will search your directory first, then the default folder. For example, if you want to search for headers in current working folder, then use "-I."
More information,
https://gcc.gnu.org/onlinedocs/cpp/Search-Path.html.

HOW-TO: things after fresh Ubuntu 12.04 Desktop installation as a computing server.

Hi all,
After installing Ubuntu 12.04 Desktop, you should install some packages which is very useful in long term. All of these tips, I collect from Internet. It's totally up to purpose of your machine. My machine is up to running experiments, workstation usages, not for entertainments.
1. If you occasionally build applications from source or make your own deb files, here are some basic packages you should install:

sudo apt-get install build-essential automake make checkinstall dpatch patchutils autotools-dev debhelper quilt fakeroot xutils lintian cmake dh-make libtool autoconf git git-core subversion bzr

2. I install Ubuntu on a workstation as a server. At home or office, you have a personal Windows machine. So, remote desktop logging into Linux workstation from a Windows computer is very important. I recommend you use X11rdp, because you can copy text or files between Windows and Linux computers. Other remote desktop protocols such as VNC don't have this feature. This feature is very useful for developments latter.

3. SSH connection.
If you are a fan of terminal and SSH, you should set up a SSH server on your workstation. Then, it is vital for a connection from Linux machine to a Linux machine w/o using remote desktop logging.

4. If you have a GPU, setting up NVIDIA driver and CUDA development kit are enlightened.

5. Sharing is important. Samba is the most useful way to share files between Linux and Linux machine. It is also compatible with Windows machine.
https://help.ubuntu.com/lts/serverguide/samba-fileserver.html

sudo apt-get install samba.

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HOW-TO: set up Java environment on Ubuntu.

Hi all,
I am kind of big moving from Windows to Ubuntu including infrastructures, softwares, and even cultures. However, I still use both Windows and Linux. I cannot live isolated with surrounding Windows machine around me in labs even my laptop. Windows are simple and convenient for normal users, but Ubuntu is just great for developers.
I definitely love Eclipse. In this blog, we provide some guide to start Java life in Ubuntu. C/C++ life in Ubuntu are great and simple. I supposed that Java life is also simple across different OSs.
First, you need to install Java Development Kit (JDK) which includes Java Runtime Environments (JRE). Going to Oracle and download the newest jdk1.7 [1]. For example, I downloaded jdk1.7.0_51. Then I untar it.

tar xzf jdk-7u51-linux-x64.gz

 As a admin user, you can add or change these two lines in your /etc/profile to point to the installation and it will affect system-wide. OR as a normal user, you can export an environment variable in ~ /.profile. Where JAVA_HOME is the place you just untar your jdk, for instance, /home/tranlaman/Downloads/jdk1.7.0_51.

export JAVA_HOME=/home/tranlaman/Downloads/jdk1.7.0_51

export PATH=$PATH:$JAVA_HOME/bin

 Then, you log out and in again to see effect of the environment variables. After that, you download and install Eclipse from [2].
Further, if you want to install C/C++ plugin in Eclipse to develop C/C++ programs, you can follow this instruction. Go to Help/Install New Softwares, then add the C/C++ Development Toolkit (CDT) link in the box as http://download.eclipse.org/tools/cdt/releases/kepler.
You can replace kepler as the version of Eclipse. Then press Enter.

[1] http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html
[2] https://www.eclipse.org/downloads/

HOW-TO: build OpenCV 2.0 on Ubuntu 64 bits 12.04 LT

Hi all,
Sometimes, you want to build an old version of OpenCV on Ubuntu 64 bits. It seems not to be easy as I think. I met this error. That was suck.

../include/opencv/cxoperations.hpp:1916:15: error: ‘ptrdiff_t’ does not name a type

../include/opencv/cxoperations.hpp:2465:31: error: ‘ptrdiff_t’ does not name a type

The reason is that library might lack of a header file [1]. Just add "#include <cstddef>" to corresponding headers file that generates errors including cxoperations.hpp. And hope it will solve your problem.

HOW-TO: know what I have passion on

Hi,
It is not a tech blog as usual. Instead, I write about how life could be complicated and what I think. Life is not as simple as it seems to be.

PATH: all you need to know

Hi all,
Previously, I write a blog about LD_LIBRARY_PATH [1]. Continuing this series, I write this blog about another important environment variable in Linux which is PATH.
PATH is the environment that contains paths to executable files. For example, /home/tranlaman/bin, /usr/local/sbin, /usr/local/bin/, /usr/sbin, or /bin.
Otherwise, LD_LIBRARY_PATH contains dynamic library for other programs.

MATLAB options file in Linux 64

Happy new year 2014,
I am writing this blog to notice you a trick to change option files in Matlab in Linux, which is very useful. When you want to use MEX to compile a C/Fortran source in MEX interface, you need to modify option files mexopts.sh in Linux because the default file contains errors in Linux. Why is it important so? The answer is that options files control which compiler to use, the compiler and link command options, and the runtime libraries to link against.
Then, where the file locate in Linux? The default file mexopts.sh locates in /home/tranlaman/.matlab/R2012a. But there is another default root file lie in /home/tranlaman/MATLAB/R2012a/bin which overrides the file in /home/tranlaman/.matlab/R2012a. So, modifying the mexopts.sh in /home/tranlaman/MATLAB/R2012a/bin would change everything.
Finally, how do we modify this file? The answer is to change some compiling and linking flag such as CC, CFLAGS, CLIBS, LD, LDFLAGS. Please refer to [1] as a complete flag for custom building MEX files.
Wish everyone has a good year.