Difference between revisions of "Private:Technical"
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== C/C++ == | == C/C++ == | ||
| − | * [http://antonym.org/2009/05/threading-with-boost---part-i-creating-threads.html | + | * Threading with Boost - [http://antonym.org/2009/05/threading-with-boost---part-i-creating-threads.html Part I: Creating Threads], [http://antonym.org/2010/01/threading-with-boost---part-ii-threading-challenges.html Part II: Threading Challenges] |
* [http://opensourceforge.tistory.com/3 A Quick Tour of the Boost Graph Library] | * [http://opensourceforge.tistory.com/3 A Quick Tour of the Boost Graph Library] | ||
Revision as of 15:05, 16 March 2010
Java
- The "JNI HOW-TO" by Udo Schuermann
If you are using the Ant Eclipse plug-in and get the following error message
[javac] BUILD FAILED: file:C:/[ECLIPSE_DIR]/workspace/[PROJECT_DIR]/build.xml:32: Unable to find a javac compiler; com.sun.tools.javac.Main is not on the classpath. Perhaps JAVA_HOME does not point to the JDK
you are using the wrong Java Virtual Machine (JVM) with Eclipse. Ant goes ahead and uses the javac from the JVM Eclipse is using no matter what you put in the compiler attribute. This is bad because Eclipse uses the first Java VM it finds on your computer's PATH variable.
So you have to tell Eclipse to run using the SDK so that Ant can use the SDK. To do this make a shortcut to eclipse.exe and change the target to:
[ECLIPSE_DIR]\eclipse.exe -vm [SDK_DIR]\bin\javaw.exe
Where [ECLIPSE_DIR] and [SDK_DIR] are the full paths to the Eclipse and Java SDK directories respectively.
Note that for Linux, there's no corresponding executable to javaw.exe. You should use java instead.
C/C++
- Threading with Boost - Part I: Creating Threads, Part II: Threading Challenges
SSH and SCP
Linux
A file descriptor is a data structure used by a program to get a handle on a file, the most well known being 0,1,2 for standard in, standard out, and standard error. You can set the open file descriptors limit each time you boot, for each user. If you are user X, you can see the current limit for you with:
ulimit -n
If you have not used the ulimit command in your user profile, .bashrc file or anything to tweak the maximum number of open file descriptors, then you should see the value of INR_OPEN as set in the includes/linux/fs.h file, that is: 1024.
You can determine how many open files you have using "lsof" command:
lsof -u chris
This will show you all files that are open by user chris. You can pipe the output of lsof to the wc (word count) command, to get the number of open files by user chris, as follows:
lsof -u chris | wc -l
Note that 'lsof' gives the number of open files. An open file may be a regular file, a directory, a block special file, a character special file, an executing text reference, a library, a stream or a network file. Though a file is open, it might not have a file descriptor associated with it - such as current working directories, memory mapped files and executable text files. 'lsof | wc -l' gives the current number of open files. Therefore, there will be a difference in the number of current open files and the number of current file descriptors/handles.
To increase the maximum number of file descriptors allowed for a user, just put the following in his profile file, or .bashrc.
ulimit -n 32768
Now, you have to be careful here: if you are the only user on your system (not counting users needed by some programs, like your Servlet container (Tomcat) or Web server (wwwrun) etc.) and if you want to be sure that you have enough file descriptors, then go with such a large number as 32768 above. But setting a limit that high for all users in your system, may lead to system degradation.
This doesn't mean you can set the limit as you please. The /etc/security/limits.conf file contains the absolute maximums a user can set for himself with the ulimit command. For the file descriptors, for example, you can set the "nofile" option as follows in my /etc/security/limits.conf:
* soft nofile 1024 * hard nofile 2048 root soft nofile 2048 root hard nofile 32768 chris soft nofile 1024 chris hard nofile 32768 tomcat soft nofile 2048 tomcat hard nofile 8192
User chris will start with a maximum of 1024 (that is the "soft" line), but if he likes, he can set it up to 32768 with a ulimit command (in his profile, .bashrc file, on the command line and so on). User tomcat will start with a maximum of 2048 (just to be sure all those webapps get what they need) and can even increase it with a ulimit command up to a hard limit of 8192.
But again, even if you are root and can edit the /etc/security/limits.conf file, there is a limit. Your "limit of limits" in this case is the value you got when you did at the start of this discussion.
cat /proc/sys/fs/file-max
This is what your kernel uses. To change the kernel parameter on the fly, you can "echo" the number you wish to /proc/sys/fs/file-max as follows (do it with caution please):
echo "300000" > /proc/sys/fs/file-max
Now test again to make sure that the new limit is set.
cat /proc/sys/fs/file-max
To know how many file descriptors are being used, do a
cat /proc/sys/fs/file-nr
You get an output like this.
8667 3145 288217 | | |__ maximum number of file descriptors allowed on the system | | | |__ total free allocated file descriptors | |__ total allocated file descriptors
- Linux console Colors And Other Trick's By Jay Link
This is made possible by \033 , the standard console "escape" code, which is equivalent to ^[ , or 0x1B in hex. When this character is received, the Linux console treats the subsequent characters as a series of commands. These commands can do any number of neat tricks, including changing text colors.
Here's the actual syntax:
\033 [ <command> m
(In practice, you can't have any spaces between the characters; I've just inserted them here for clarity).
Anything following the trailing "m" is considered to be text. It doesn't matter if you leave a space behind the "m" or not. So this is how you turn your text to a deep forest green:
echo -e "\033[32mRun, forest green, run."
Note that the "-e" argument to "echo" turns on the processing of backslash-escaped characters; without this, you'd just see the full string of text in gray, command characters and all. Finally, the command "0" turns off any colors or otherwise funkified text:
\033[0m
Without the "0" command, your output will continue to be processed, like so:
echo -e "\033[32mThis is green." echo "And so is this." echo "And this." echo -e "\033[0mNow we're back to normal."
Running a command that uses console colors (e.g., ls) will also reset the console to the standard gray on black.
Programming Console Colors
Of course, escape sequences aren't limited to shell scripts and functions. Let's see how the same result can be achieved with C and Perl:
C:
printf("\033[34mThis is blue.\033[0m\n");
Perl:
print "\033[34mThis is blue.\033[0m\n";
Available Colors
Now, how do you know which codes do what? The first eight basic EGA colors are defined as follows:
30 black foreground 31 red foreground 32 green foreground 33 brown foreground 34 blue foreground 35 magenta (purple) foreground 36 cyan (light blue) foreground 37 gray foreground
So, if I wanted the word "ocean" to appear in light blue, I could type the following:
echo -e "The \033[36mocean\033[0m is deep."
Combining Commands
Multiple console codes can be issued simultaneously by using a semicolon (";"). One useful command is "1", which sets text to bold. The actual effect is a lighter shade of the chosen color. So, to get a light magenta (purple) as shown in the first example, you would do this:
echo -e "\033[35;1mCombining console codes\033[0m"
This bolding feature allows you to access the other half of the standard 16 EGA colors. Most notably, brown turns into yellow, and gray turns into bright white. The other six colors are just brighter versions of their base counterparts.
Backgrounds
Text backgrounds can also be set with console codes, allowing you to have white on top of red (for example). Here is the full list of available background options:
40 black background 41 red background 42 green background 43 brown background 44 blue background 45 magenta background 46 cyan background 47 white background
Finally, here are some other noteworthy command codes:
0 reset all attributes to their defaults 1 set bold 5 set blink 7 set reverse video 22 set normal intensity 25 blink off 27 reverse video off
Unfortunately, these techniques are limited to the console, as they don't display over telnet (unless the remote interface is also a Linux console).
Note that the codes given here are known as ECMA-48 compliant. That is, they work on systems other than Linux. (In case you're interested, ECMA is the European Computer Manufacturers Association, a standards body similar to the ISO). Any system with a VT-102 capable console can use the color codes demonstrated above.
For loop uses $IFS variable to determine what the field separators are. By default $IFS is set to the space character. To read an entire line as a single element, you should change the separator,
OLD_IFS=$IFS IFS=$'\n' ''for loop goes here..'' IFS=$OLD_IFS
In short, something is wrong with if condition, it is expecting a single value but gets multiple values.
This can be done using either the groups command or the id command.
Use one of the greps (grep, egrep, fgrep). You want to use the -r "recursive" option to search a directory and all files included within it.
grep -r -i ONOCR /usr/include [nsl@nsl]$ grep -r -i ONOCR /usr/include /usr/include/asm/termbits.h:#define ONOCR 0000020 /usr/include/linux/cdk.h:#define FO_ONOCR 0x8 /usr/include/linux/tty.h:#define O_ONOCR(tty) _O_FLAG((tty),ONOCR) /usr/include/bits/termios.h:#define ONOCR 0000020
The '-i' means ignore case. So if you only wanted it in all capitals you would not use it. When used it will find any combination of upper and lower case.
OS Compatibility Issues
