A core dump file is a snapshot of a program's memory at the time it crashed. It contains valuable information that can help in understanding the cause of the crash and potentially fixing the issue. However, core dump files can be overwhelming and difficult to decipher, especially for novice users. In this article, we will focus on one specific aspect of core dump files on Linux – how to identify opened files.
When a program crashes, it often leaves behind open files that were being accessed at the time of the crash. These files can be critical in understanding the state of the program and what actions were being performed before the crash. To extract this information from a core dump file, we will be using a tool called "gdb" – the GNU Project Debugger.
First, we need to make sure that our system is configured to create core dump files. This can be done by executing the following command in the terminal:
ulimit -c unlimited
This will set the maximum size of core dump files to unlimited. Next, we need to run the program that we want to analyze and let it crash. Once the program crashes, a core dump file will be created in the current directory. Now, we can open the core dump file using gdb by running the following command:
gdb <program> <core dump file>
Replace <program> with the name of the program that crashed and <core dump file> with the name of the core dump file that was generated.
Once gdb is launched, we can use the "bt" command to get a backtrace of the program's execution before the crash. This will give us a list of functions that were called and in what order. We can then use the "frame" command to select a specific frame and see the values of the variables at that point in the program's execution.
To identify opened files, we need to look for the "open" or "fopen" functions in the backtrace. These functions are used to open files and will give us important information such as the file path and the mode in which the file was opened (read, write, append, etc.).
For example, if we see an "open" function in the backtrace with the file path "/etc/passwd", we can conclude that the program was trying to access the password file at the time of the crash. This can be helpful in understanding what the program was trying to do and what could have potentially caused the crash.
Another useful command in gdb is "list". This will show us the source code of the program at the current frame. By looking at the source code, we can get a better understanding of what the program was trying to do with the opened file.
In addition to identifying opened files, gdb can also help us identify file descriptors that were in use at the time of the crash. File descriptors are small integers that represent open files in a program. By using the "info file" command in gdb, we can see a list of all the file descriptors and the corresponding files that were opened.
In conclusion, core dump files can be a valuable source of information when trying to debug a program crash. By using gdb and following the steps outlined in this article, we can easily identify opened files and gain insight into what the program was doing before it crashed. This information can be crucial in finding and fixing the root cause of the crash.
