How do we determine where the mistake is in our code that causes a segmentation fault?
After writing some code, to determine where I have a segmentation fault, can gcc show me my have a mistake (or typing error) in a particular line number -- because that line caused the segmentation fault?
Gcc can't do that but gdb sure can. Compile you program using the -g switch, like this:
gcc -g -Wall -o program program.c
Then use gdb:
gdb ./program
run
[..segfault happens here..]
where
[offending code is shown here]
Here is a nice tutorial to get you started with gdb.
This is a bit of a brute force method, that you would want to use preprocessor debug flags to control, but you could put a flushed cout after each line and match that up. It's simple, and reusable.
You could also use a core dumb and then examine it with gdb. To get useful information you also need to compile with the -g flag.
Whenever you get the message
Segmantation fault (core dumped)
a core file is written into your current directory. And you can examine it with the command
gdb your_program core_file
The file contains the state of the memory when the program crashed. A core dump can be useful during the deployment of your software.
Make sure your system doesn't set the core dump file size to zero. You can change the size with ulimit -c. Be aware that core dumps can become huge.
Lucas's answer about core dumps is good. In my .cshrc I have:
alias core 'ls -lt core; echo where | gdb -core=core -silent; echo "\n"'
to display the backtrace by entering 'core'. And the date stamp, to ensure I am looking at the right file :(.
Added: If there is a stack corruption bug, then the backtrace applied to the core dump is often garbage. In this case, running the program within gdb can give better results, as per the accepted answer (assuming the fault is easily reproducible). And also beware of multiple processes dumping core simultaneously; some OS's add the PID to the name of the core file.
Also, you can give Valgrind a try: if you install Valgrind and run valgrind --leak-check=full , then it will run your program and display stack traces for any segfaults, as well as any invalid memory reads or writes and memory leaks. It's really quite useful.