Presentation to the class on how to use and create core dump files. also superficially covers the differences.

1. Core dump
How to: Generate and Use a Core
File
Linux vs Solaris Example
Stacy Watts
10/24/2012
CS 201 Fall 2012
Portland State University

2. About these slides
These slides are geared towards the students in
CS 201 at Portland State University.
For folks outside our class I am assuming you:
Have some programming experience
Haven't yet explored the idea of a core dump in
detail.
My habit for filenames is camelCase as you will
see in these slides. Your habit
might_be_different.

3. coredump vs segfault
Programming we get to see both of these
messages.
segfault is an access violation. You went
somewhere in memory you didn't have
permission to go!
coredump refers both to the action of taking a
complete snapshot of the memory and registers,
and the file generated by that action.

4. What is a core dump?
“A core dump consists of the recorded state of the
working memory of a computer program at a specific
time, generally when the program has terminated
abnormally (crashed).[1] In practice, other key pieces of
program state are usually dumped at the same time,
including the processor registers, which may include
the program counter and stack pointer, memory
management information, and other processor and
operating system flags and information. The name
comes from the once-standard core memory technology.
Core dumps are often used to diagnose or debug errors
in computer programs.”
http://wiki.answers.com/Q/What_is_a_core_dump

5. How can you get one?
(…and why would I want one?)
You can get a core dump fairly easily by doing
something programmatically you aren't allowed to
do.
All you need are one of the following conditions:
Is the memory you're trying to change yours?
Am I expecting an explicit type conversion?
Did I do something horribly wrong?
You also need the command-line value of “ulimit –c”
to not be 0
A value greater than 0 will generate a corefile

6. On multi-user systems like the linux or unix systems
on campus you are not guaranteed a file is
generated. ulimit –c is often set to 0.
To remove the corefile size limit:
ulimit –c unlimited
On Ubuntu, even with this set, if you are not root, you
may not get a core file. It is easiest to generate it
through gdb.
You are now ready to generate a core file. That file
can be read by gdb provided you compiled it on the
same machine.
to read in an existing file use:
gdb program corefile
where program is your compiled program and corefile
is the file that was dumped.
We can get all sorts of information back out.

7. When a program dumps core, the entire imprint of
what was happening at that point in program
execution is sent to a file. This includes:
recorded memory state
processor registers
program counter
stack pointer
memory management information
any other flags and information that system
includes

8. Identical source program code: linux left, solaris right
Generated using gcc –s fileName.c

9. Linux vs Solaris example
Not apples to apples – different machines means
different arch, also different gcc versions.
Commands we will use:
gdb executableName coreFile
Starts gdb with the corefile loaded
info locals
print
info args
list
backtrace

10. Show Linux example using
gcc –g file
gdb file
gdb commands
backtrace, etc
Show Solaris example
gcc –g file
a.out
gdb commands
backtrace, etc.
Show differences in even the machine code.

11. Running process example:
Core File Creation
gdb has a “generate-core-file” command to
create a core file(this example is off a running
process):
gdb -q – 1597
Generates a core file of the currently running
program.
This is great for infinite loops
Also great for hung programs (due to infinite loops
or not)

12. Using a core file
To use a core file once generated, these slides
will use gdb
To start gdb with a core file, issue the system
specific command from the shell:
gdb eg1 core
gdb eg1 -c coreFile
Gdb will load up the core file, eg1's program
listing, show how the program terminated and
present a message very much like we've just run
the program under gdb:

13. ...
Core was generated by `./eg1'.
Program terminated with signal 8, Floating point
exception.
...
#0 0x80483ea in wib (no1=8, no2=8) at eg1.c:7
7 result = no1 / diff;

14. While we see there are differences linux to unix
on the details of the corefile generated, once we
are using gdb as our interface to that file, we
won’t notice the differences in our basic usage of
the corefile.
Gdb does the symbol parsing and interpretation
for us, making it a very handy tool.
The file on its own passed to a system it wasn’t
compiled on won’t do you much good. You won’t
have the references to the symbols used in the
program.
Thus, a corefile on its own is interesting, but not
as useful as a corefile passed in with the
program you compiled.

15. Questions?

16. References list:
http://www.ibm.com/developerworks/library/l-gdb/
http://prefetch.net/blog/index.php/2006/12/21/generati
ng-core-files-from-gdb/
http://docs.oracle.com/cd/E23823_01/html/816-
5174/core-4.html
http://stackoverflow.com/questions/17965/generate-a-
core-dump-in-linux
http://docs.oracle.com/cd/E23823_01/html/816-
5165/gcore-1.html#REFMAN1gcore-1
http://wiki.answers.com/Q/What_is_a_core_dump