How to generate core dump for an application in Linux

In order to troubleshoot application issue, a process content in memory can be dumped to a file and then the file can be analysed with debug tool, such as gdb in Linux. 
Another way of doing this is to send QUIT signal to the PID (kill –3  PID), but the thread dump will be directed to stdout, which can be viewed with “cat /proc/PID/fd/1 |tee /tmp/dump.log”, messages will be constantly directed to  /proc/PID/fd/1 until the process is stopped. So it is useful for real time debugging.
The following java application example use gcore command in gdb. (gcore or kill –3 will not stop the process )
Linux default core file size is 0, which means core dump is disabled, It needs to be changed to unlimited

#ulimit -a | grep core
core file size          (blocks, -c) 0
#ulimit –c unlimited
#ulimit -a | grep core
core file size          (blocks, -c) unlimited

Firstly, find the virtual memory size of the process, the PID is 10008 in following example.
# ps aux | egrep 'VSZ| 10008'

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root       10008  0.4  8.3 2231312 660100 ?      Sl   Jun03  43:57 /opt/sunjdk6/bin/java

The process VSZ=2.2GB, it will be the size of the core file. Go to a dump dir which has more than 2.2GB free space
$cd /var/tmp

Attach to running process PID.
$gdb --pid=10008

At the gdb prompt, enter gcore command
gdb>gcore

wait for few minutes for core file to be generated. type in quit to exit gdb, answer yes to detach the process.
gdb>quit

core file is generated
$ ls -lh /var/tmp/core.10008

-rw-r--r-- 1 root root 2.2G Jun 10 11:59 /var/tmp/core.10008

file command reveals the source program name

$ file /var/tmp/core.10008
/var/tmp/core.10008: ELF 64-bit LSB core file AMD x86-64, version 1 (SYSV), SVR4-style, from 'java'