JNLP and JNI native libraries

Topic: How to use JNLP to load jar files which include native libraries ?

1. Build native libraries

* HelloWorld.dll for Windows

* libHelloWorld.so for Linux (library name must be libXXX.so in Linux !!)

(How to build those native libraries ? see jni book)

2. Pack native libraries into jar

jar -cvf Linux32_lib.jar libHelloWorld.so

jar -cvf Win32_lib.jar HelloWorld.dll

3. Add tags in jnlp

(please replace ( with <)

(resources os="Windows">

(j2se version="1.5+"/>

(nativelib href="native/Win32_lib.jar"/>

(/resources>

(resources os="Linux">

(j2se version="1.5+"/>

(nativelib href="native/Linux32_lib.jar"/>

(/resources>

4. In code, explicitly load the library

System.loadLibrary("HelloWorld");

(windows->HelloWorld.dll ; linux -> libHelloWorld.so)

TCP Keepalive mechanism

RFC1122 says

"Implementors MAY include "keep-alives" in their TCP implementations, although this practice is not universally accepted. If keep-alives are included, the application MUST be able to turn them on or off for each TCP connection, and they MUST default to off." [1]

In C, use setsockopt() to set the following arguments on a specific socket [2]

* SO_KEEPALIVE: enable tcp keepalive

* TCP_KEEPCNT: overrides tcp_keepalive_probes

* TCP_KEEPIDLE: overrides tcp_keepalive_time

* TCP_KEEPINTVL: overrides tcp_keepalive_intvl

These settings are only for the current socket.

In Linux, to see the default values (OS-wide variables) of the later 3 options:

# cat /proc/sys/net/ipv4/tcp_keepalive_time

7200

# cat /proc/sys/net/ipv4/tcp_keepalive_intvl

75

# cat /proc/sys/net/ipv4/tcp_keepalive_probes

9

This means "wait for two hours (7200 secs) before sending the first keepalive probe, and then resend it every 75 seconds. If no ACK response is received for nine consecutive times, the connection is marked as broken." [2]

Ref:

[1] Wiki: http://en.wikipedia.org/wiki/Keepalive

[2] http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/

[socket] set read/write timeout values for blocking socket

Set timeout to 10 seconds when doing read () or write () a blocking sock :

#define SOCK_RCV_TIMEOUT 10

#define SOCK_SND_TIMEOUT 10

void set_rw_timeout (int sock, int clear_timeout)

{

struct timeval rw_timeout;

memset (&rw_timeout, 0, sizeof (struct timeval));

rw_timeout.tv_sec = clear_timeout ? 0 : SOCK_RCV_TIMEOUT;

if (setsockopt (sock, SOL_SOCKET, SO_RCVTIMEO, &rw_timeout, sizeof (struct timeval))) {

fprintf (stderr, "setsockopt SO_RCVTIMEO failed\n");

}

memset (&rw_timeout, 0, sizeof (struct timeval));

rw_timeout.tv_sec = clear_timeout ? 0 : SOCK_SND_TIMEOUT;

if (setsockopt (sock, SOL_SOCKET, SO_SNDTIMEO, &rw_timeout, sizeof (struct timeval))) {

fprintf (stderr, "setsockopt SO_SNDTIMEO failed\n");

}

}

md5sum a directory

Generate:

find (your-dir) -type f -print0 | xargs -0 md5sum >> (output-md5sume-file)

Verify:

md5sum -c (output-md5sum-file)

my example:

robert@robert-desktop ~/work/nbd-work/ttt $ ls

first second test

robert@robert-desktop ~/work/nbd-work/ttt $ find -type f -print0 | xargs -0 md5sum >> ~/Test/mydir.md5

robert@robert-desktop ~/work/nbd-work/ttt $ cat ~/Test/mydir.md5

d41d8cd98f00b204e9800998ecf8427e ./second

d41d8cd98f00b204e9800998ecf8427e ./first

7423d8fc12da81d779d0485b3d3fb67d ./test

robert@robert-desktop ~/work/nbd-work/ttt $ md5sum -c ~/Test/mydir.md5

./second: OK

./first: OK

./test: OK

robert@robert-desktop ~/work/nbd-work/ttt $ echo $?

0

Ref: How to create md5sum for a directory?

Use djgpp to build a DOS program in dosemu (under Linux)

Topic: How to use djgpp to build a DOS program in DOSEmu (running in Linux) ?

1. Install dosemu

DOSEmu (dosemu HOWTO)

2. Download djgpp files for dosemu

DJGPP (this name comes from ... DJ's GNU Programming Platform)

Use DJGPP Zip File Picker to download what you need (select dosemu for Operating System)

3. Install djgpp into dosemu

Refer to "installation instructions for dosemu" shown in DJGPP Zip File Picker Results. Key points:

* unzip those *.zip files to ~/djgpp/ (~ is your home in Linux)

* edit ~/.dosemu/drvie_c/autoexec.bat :

set PATH=d:\djgpp\BIN;%PATH%

set DJGPP=d:\djgpp\DJGPP.ENV

(dosemu defaults D:\ to ~/)

* reboot dosemu

4. Compile a helloworld in dosemu

* Edit main.c and its Makefile in ~/helloworld/ under Linux

* Then in dosbox, cd D:\helloworld\ and try to make it !

(gcc -o helloworld.exe main.c)

stop on first error (gcc, make)

Just want to see the first compilation error ? Try this:

-> gcc main.c -Wfatal-errors

-Wfatal-errors

This option causes the compiler to abort compilation on the first

error occurred rather than trying to keep going and printing

further error messages.

Ref:

man gcc

grub rescue after re-partition by Windows

Situation:

I install Ubuntu9.10 (with grub loader) in /dev/sda10.

(My MBR bootloader is spfdisk)

Someday, I boot the system to Windows (/dev/sda2) and re-partition /dev/sda3 to NTFS.

After rebooting to Ubuntu9.10, a problem comes: some Windows program has changed the disk MBR and my Ubuntu9.10 is no more in /dev/sda10. It's in /dev/sda9 in the new MBR.

So when boot to /dev/sda9, grub can't find linux in /dev/sda10.

It is in /dev/sda9 now !

--------- in grub rescure, manually boot to /dev/sda9 -------

grub rescue>

set prefix=(hd0,9)/boot/grub

insmod (hd0,9)/boot/grub/linux.mod

----- now you can use [tab] auto-completion ---

set root=(hd0,9) linux /boot/vmlinuz26 root=/dev/sda9 initrd /boot/kernel26.img boot

----- this will boot to Ubuntu 9.10 ------

sudo update-grub

sudo grub-install /dev/sda9

(this will update your grub settings, esp. your boot menu,

so that you can boot normally into Ubuntu9.10 next time)

Reference:

1. GRUB2 rescue guide

2. Linux Commands for GRUB2

generate a (pseudo) random number in sh

Print a pseudo-random byte (hex) in sh:

cat /dev/urandom | od -t x1 -A n -N 1

->89

-----------

Create a pseudo-random file of size 1 MB:

dd if=/dev/urandom of=a.log bs=1M count=1

Ref:

Re: generating random number in sh

http://linuxcommando.blogspot.com/2008/06/create-file-of-given-size-with-random.html

Install a New font in Ubuntu

For instance, I want to install a new font "Crisp" to my Ubuntu 9.10:

1. Create a new directory for your new font:

sudo mkdir /usr/share/fonts/truetype/crisp/

sudo cp ~/download/Crisp.ttf /usr/share/fonts/truetype/crisp/

2. Rebuild the font information files:

sudo fc-cache -f -v | grep -i crisp

-> /usr/share/fonts/truetype/crisp: caching, new cache contents: 1 fonts, 0 dirs

3. Restart the programs in which you want to use the new font.

--------

Ref:

Fonts - Ubuntu Wiki

Crisp font (proggy programming)