Can someone trace the reason for segmentation fault? - linux

Question

public class Watcher: Object
{
private int _fd;
private uint _watch;
private IOChannel _channel;
private uint8[] _buffer;
private int BUFFER_LENGTH;
public Watcher(string path, Linux.InotifyMaskFlags mask){
_buffer = new uint8[BUFFER_LENGTH];
//➔ Initialize notify subsystem
_fd = Linux.inotify_init();
if(_fd < 0){
error(#"Failed to initialize the notify subsystem: $(strerror(errno))");
}
//➔ actually adding abstraction to linux file descriptor
_channel = new IOChannel.unix_new(_fd);
//➔ watch the channel for given condition
//➔ IOCondition.IN => When the channel is ready for reading , IOCondition.HUP=>Hangup(Error)
_watch = _channel.add_watch(IOCondition.IN | IOCondition.HUP, onNotified);
if(_watch < 0){
error(#"Failed to add watch to channel");
}
//➔ Tell linux kernel to watch for any mask(for ex; access, modify) on a given filepath
var ok = Linux.inotify_add_watch(_fd, path, mask);
if(ok < 0){
error(#"Failed to add watch to path -- $path : $(strerror(errno))");
}
print(#"Watching for $(mask) on $path");
}
protected bool onNotified(IOChannel src, IOCondition condition)
{
if( (condition & IOCondition.HUP) == IOCondition.HUP){
error(#"Received hang up from inotify, can't get update");
}
if( (condition & IOCondition.IN) == IOCondition.IN){
var bytesRead = Posix.read(_fd, _buffer, BUFFER_LENGTH);
Linux.InotifyEvent *pevent = (Linux.InotifyEvent*) _buffer;
handleEvent(*pevent);
}
return true;
}
protected void handleEvent(Linux.InotifyEvent ev){
print("Access Detected!\n");
Posix.exit(0);
}
~Watcher(){
if(_watch != 0){
Source.remove(_watch);
}
if(_fd != -1){
Posix.close(_fd);
}
}
}
int main(string[] args) requires (args.length > 1)
{
var watcher = new Watcher(args[1], Linux.InotifyMaskFlags.ACCESS);
var loop = new MainLoop();
loop.run();
return 0;
}
The above code can be found on "Introducing Vala Programming - Michael Lauer"
Proof of failure:
Image displaying failure on access to the file being watched for access
Terminal 1:
./inotifyWatcher
Terminal 2:
cat
As soon as I access the file, segmentation fault occurs.
I have also tried using gdb for the cause of failure, but it's mostly cryptic for me. I am using parrot(debian/64-bit) on my machine. Also, I am new to this(stackoverflow, linux kernel program).

Answer 1

Vala source line numbers can be included in the binary when compiling with the --debug switch. The line numbers appear in the .debug_line DWARF section of an ELF binary:
valac --debug --pkg linux inotifyWatcher.vala
Run the binary using gdb in the first terminal:
gdb --args ./inotifyWatcher .
(gdb) run
The dot specifies to watch the current directory. Then when the current directory is access with a command like ls the watching program segmentation faults. The output from GDB is:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000401a86 in watcher_onNotified (self=0x412830, src=0x40e6e0, condition=G_IO_IN) at inotifyWatcher.vala:51
51 handleEvent(*pevent);
GDB includes the line number, 51, from the source file and shows the line.
So the problem is to do with reading from the file descriptor then passing the buffer to handleEvent. You probably want to check bytesRead is greater than zero and I'm not sure about the use of pointers in this example. Explicit pointers like that should rarely need to be used in Vala, it may require a change to the binding, e.g. using ref to modify the way the argument is passed.