Saturday, November 22, 2014

Undefined References to Boost Libraries - Solution

Yesterday, I was trying to compile a C++ source that uses Gnuplot-iostream interface. I issued the following compilation command:
$ g++ -std=c++11 -lboost_iostreams -lboost_system -lboost_filesystem ProcessScheduler.cpp 
/tmp/cchNNC8H.o: In function `__static_initialization_and_destruction_0(int, int)':
ProcessScheduler.cpp:(.text+0xc52): undefined reference to `boost::system::generic_category()'
ProcessScheduler.cpp:(.text+0xc5e): undefined reference to `boost::system::generic_category()'
ProcessScheduler.cpp:(.text+0xc6a): undefined reference to `boost::system::system_category()'
collect2: error: ld returned 1 exit status
I even had libboost-system-dev, libboost-filesystem-dev and libboost-iostreams-dev installed.
After searching for over an hour, I found that the problem was with how new g++ treated the command-line options.

The solution was to change the position of arguments.
$ g++ -std=c++11  ProcessScheduler.cpp  -lboost_iostreams -lboost_system -lboost_filesystem
So, adding linker parameters after source file worked.
The page at Link Options - GCC states that:
-l library [...] It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file foo.o but before bar.o. If bar.o refers to functions in ‘z’, those functions may not be loaded. [...]

Saturday, June 28, 2014

Streaming USB webcam on Raspberry PI

I had a USB webcam lying around and a Raspberry Pi. And, I wanted to stream it over my local WiFi network. To do this, I picked up MJPEG-Streamer. Connecting the webcam and issuing 'lsmod' showed this:
$ lsmod
Bus 001 Device 004: ID 0ac8:3420 Z-Star Microelectronics Corp. Venus USB2.0 Camera
The device is supported by UVC driver.
ls /dev/vid*
So, we have '/dev/video0' as our webcam.
Let's install some required packages.
On a RPi, I have faced multiple issues trying to install packages without first upgrading the system to the latest state. So, let's upgrade the system first.
$ sudo apt-get update && sudo apt-get upgrade
And, some requirements for MJPEG-Streamer
$ sudo apt-get install subversion imagemagick libjpeg8-dev
Now, let's download the latest source
$ svn co mjpg-streamer
$ make
When it finishes, it leaves "mjpeg-streamer" binary to the folder. This may be added to path variable. Now, let's stream. 15fps with hardware MJPEG Compression
$ ./mjpg_streamer -i "./ -d /dev/video0 -f 15 " -o "./ -w ./www"
If you get the error like this:
The input device does not supports MJPEG mode
You may also try the YUV mode (-yuv option), but it requires a much more CPU power
 Init v4L2 failed !! exit fatal 
 i: init_VideoIn failed
Then we need to add '-y' argument to fallback to software mode.
$ ./mjpg_streamer -i "./ -d /dev/video0 -y -f 15" -o "./ -w ./www"
You can access the web interface at http://<ip>:8080/ where 'ip' is the IP of the Raspberry Pi server. The frame rate can also be changed by specifying the custom value instead of 15. To stream at different resolution with 30fps:
$ ./mjpg_streamer -i "./ -d /dev/video0 -f 15 -r 960x720" -o "./ -w ./www"
The best part of MJPEG-Streamer is that you can also use VLC or any other streaming player to watch the stream and you are not limited to a web interface.
Happy streaming.

Saturday, January 5, 2013

WordPress SEO by Yoast and Nginx

Wordpress-SEO by Yoast is a great plugin for search engine optimization. It can generate XML sitemaps on the fly. While on Nginx, I found that I was getting 404 errors when trying to load the sitemap.

The solution was simple. We just need to pass the "sitemap name" as a URI to "index.php" which can be done by simply adding this to "server block":

rewrite ^/sitemap_index\.xml$ /index.php?sitemap=1 last;
rewrite ^/([^/]+?)-sitemap([0-9]+)?\.xml$ /index.php?sitemap=$1&sitemap_n=$2 last;
If you are already using Nginx with Wordpress friendly URLs, these steps are not necessary and redundant.

Wednesday, August 22, 2012

Allow only SCP/SFTP transfers with 'scponly'

scponly restricts users to SCP and SFTP logins by acting itself as a shell and this disables SSH logins. This allows using Filezilla or 'scp' tool to transfer files but restrict SSH logins via SSH clients. Debian has this package as 'scponly'. TO install it:
# apt-get install scponly
Since, we will not be chrooting users, suid binary is not required. So, select no to: Install the chrooted binary /usr/sbin/scponlyc SUID root? <-- No For existing users, do:
# usermod -s /usr/bin/scponly username
To create a new user with scponly:
# useradd -m -s /usr/bin/scponly username
# passwd username