query_exists

Utility function to check if a particular file exists.

This is implemented using query_info and as such does blocking I/O.

Note that in many cases it is racy to first check for file existence and then execute something based on the outcome of that, because the file might have been created or removed in between the operations. The general approach to handling that is to not check, but just do the operation and handle the errors as they come.

As an example of race-free checking, take the case of reading a file, and if it doesn't exist, creating it. There are two racy versions: read it, and on error create it; and: check if it exists, if not create it. These can both result in two processes creating the file (with perhaps a partially written file as the result). The correct approach is to always try to create the file with create which will either atomically create the file or fail with a g_io_error_exists error.

However, in many cases an existence check is useful in a user interface, for instance to make a menu item sensitive/insensitive, so that you don't have to fool users that something is possible and then just show an error dialog. If you do this, you should make sure to also handle the errors that can happen due to races when you execute the operation.

Example: Check if a file exists:

public static int main (string[] args) {
	File file = File.new_for_path ("my-test.txt");

	// Check if a particular file exists:
	bool tmp = file.query_exists ();
	if (tmp == true) {
		print ("File exists\n");
	} else {
		print ("File does not exist\n");
	}

	return 0;
}

valac --pkg gio-2.0 GLib.File.query_exists.vala

Parameters:

this	input File
cancellable	optional Cancellable object, null to ignore

Returns:

true if the file exists (and can be detected without error), false otherwise (or if cancelled).