Posts Tagged ‘programming’
SSH, cygwin and domain users
Yes you can log in to your local computer via ssh with a domain account.
If it seems you can’t (i.e., your password is rejected) then you most likely need to export your user accounts and groups so cygwin can see them.
Another clue that you need to export is if you get a message like:
Your group is currently "mkpasswd". This indicates that the /etc/passwd (and possibly /etc/group) files should be rebuilt. See the man pages for mkpasswd and mkgroup then, for example, run mkpasswd -l [-d] > /etc/passwd mkgroup -l [-d] > /etc/group Note that the -d switch is necessary for domain users.
To export domain users:
$ mkpasswd -d >> /etc/passwd
To export groups:
$ mkgroup > /etc/group
Troubleshooting
Errors logging in as domain user
2 [main] -bash 31884 C:\cygwin\bin\bash.exe: *** fatal error - couldn't dynamically determine load address for 'WSAGetLastError' (handle 0xFFFFFFFF), Win32 error 126Connection to localhost closed.
This is because the cygwin sshd service must also run as domain account. I solved this by changing the user to my domain account.
Closure
Closures have been floating around lately, cropping up in Ruby as blocks and procs, as well as pure functional languages.
[A closure] is a first-class function with free variables. Such a function is said to be “closed over” its free variables. A closure is defined within the scope of its free variables, and the extent of those variables is at least as long as the lifetime of the closure itself. The explicit use of closures is associated with functional programming and with languages such as ML, Lisp and Perl. Closures are used to implement continuation passing style, and in this manner, hide state. Constructs such as objects and monads can thus be implemented with closures.
Free variables
A free variable specifies a place holder in an expression. Whether it is bound or free depends on where it is declared with respect to the expression.
It is approximately correct to say:
- A variable is free if you can substitute a value for it and the resulting expression is meaningful.
- A variable is bound if the expression is a statement about all the possible values of the variable all at once. A bound variable is bound by an operator such as the integral sign, a quantifier, or a summation sign.
Or:
- [A free variable is] An occurrence of a variable in a logic formula which is not inside the scope of a quantifier.
- [A bound variable] In logic, [is] a variable that occurs within the scope of a quantifier, and cannot be replaced by a constant.
Example:
This expression is considered bound in x, and free in y. This expression holds for all values of x between the limits, but y can take only one value. The variable y stands for a fixed value, not specified inside the expression, while x is bound by the expression definition.
In mathematical examples, it is often mentioned that bound variables cannot be replaced with a constant, otherwise a meaningless expression would result — try replacing x with 1 in the integral above. Clearly d1 doesn’t make any sense.
Interestingly, as shown in the example, this can be applied to language:
Satomi found her book.
In this expression, the pronoun her is ambiguous. It may refer to Satomi, or any female declared outside the current context, or scope. The variable her is free.
Free variables in computer programming
In computer programming, a free variable is a variable referred to in a function that is not local (not declared within the scope of the function), or an argument of that function. An upvalue is a free variable that has been bound (closed over) with a closure.
So, in terms of a closure, a free variable is any variable in scope that is declared outside the closure itself, and is not supplied as an argument. By contrast, arguments and locals are always bound.
Closed over free variables
A closure is said to be closed over its free variables, what does that mean? This means completed by. A closure expression is completed by specifying values for its free variables.
[TBD: hoisting -- what a complier emits for free variables.
Closures in ruby
Though blocks are like closures in that they're closed over their free variables, they're not closures because they're not really first class functions -- a block cannot be passed around like an object.
A block can be converted to a proc, though. Capture a block as a proc using ampersand:
class Simple
attr_reader :saved_block
def initialize()
yield self if block_given?
end
def save_block_for_later(&proc)
@saved_block = proc
end
end
And the proc can be assigned like:
var_x= 'x'
simple = Simple.new
simple.save_block_for_later { puts "The current value for var_x = '#{var_x}'."}
This closure can then be invoked at a later time -- still bound to its free variables -- using call:
simple.saved_block.call
Which prints the text:
"This one has var_x defined as 'x'."
And the same as usual to supply arguments:
simple.save_block_for_later do |an_argument|
puts "The current value for var_x = '#{var_x}', " +
"and an_argument has been supplied as '#{an_argument}'."
end
simple.saved_block.call 'xxx'
Funargs
The funarg problem -- how to manage variable scoping when dealing with first-class functions.
Stack frames and locals
Traditionally, local variable scope is managed using stack frames.
The idea behind a stack frame is that each subroutine can act independently of its location on the stack, and each subroutine can act as if it is the top of the stack.
When a function is called, a new stack frame is created at the current esp location. A stack frame acts like a partition on the stack. All items from previous functions are higher up on the stack, and should not be modified. Each current function has access to the remainder of the stack, from the stack frame until the end of the stack page. The current function always has access to the "top" of the stack, and so functions do not need to take account of the memory usage of other functions or programs.
In short, functions are allocated temporary storage in a stack frame. This frame stores arguments and local variables. The frame is allocated before the function call, and cleaned up at function exit. The problem arises when a function returns another function.
Normally all local variables are removed with the stack frame, however if a function is returned that references locals, i.e., a closure, then these variables have to be kept alive.
CPU registers
[In computer architecture], a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere.
- ESP: stack pointer for top address of the stack
- EBP: stack base pointer for holding the address of the current stack frame
In terms of functions, this article describes the roles of the EBP and ESP registers. The ESP register marks the top of the stack
References
IDisposable and unmanaged memory
My pair and I had to implement IDisposable the other day, and I had almost forgotten how and why it is done the way it is, so I thought I’d make some notes. An exceptionally clear summary can be found in section 9.3 of Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries, which I have used as the basis.
Objects that:
- Contain references to unmanaged resources, i.e., objects that don’t have finalizers. These types of objects should also define a finalizer.
- – or– contain references to disposable objects.
should always implement IDisposable. Disposable objects offer clients a way to free resources deterministically, rather than whenever the CLR deems it necessary.
Here is a class that contains a simple implementation. It includes a finalizer because it contains a reference to an unmanaged object that doesn’t have its own.
public class UnmanagedResourceHolder : IDisposable {
IntPtr buffer; // An unmanaged resource
SafeHandle managedResource;
public UnmanagedResourceHolder () {
this.buffer = ... // init buffer
this.managedResource = ...
}
public void Dispose() {
Dispose(true);
// Only suppress if Dispose(true) has completed successfully
// to ensure finalizer gets a chance
GC.SuppressFinalize(this);
}
public ~UnmanagedResourceHolder() {
Dispose(false);
}
protected virtual void Dispose(Boolean disposing) {
// Can't find reference for the following, assume it's self-explanatory...
ReleaseBuffer(buffer);
if (disposing) {
// Run deterministic cleanup
if (managedResource != null) {
managedResource.Dispose();
}
}
}
}
Points to note:
- Unmanaged resources released on both paths. This ensures deterministic cleanup is available as well as finalizer cleanup.
- Managed resources are not released during finalizer. This is because managedResource is managed — it will handle its own finalization, plus the next reason.
- During finalization, (normally valid) assumptions about the internal state of an object are no longer reliable. Finalization occurs in an unpredictable order — for example, the managedResource field may have already been finalized.
- Provided Dispose() is called, finalization is skipped (though there is still overhead, see below).
- It is a good idea to provide a protected virtual Dispose to allow derived types to perform their own cleanup.
- Always invoke super type’s Dispose (if there is one) — for obvious reasons — when overriding in derived type.
A connection pool example
Why is it important to close database connections? Here’s what happens when connection is not explicitly closed:
[Trace] Audit Login -- network protocol: TCP/IP SQL:BatchStarting SELECT count(1) from User SQL:BatchCompleted SELECT count(1) from User Audit Logout
Here’s what happens when a connection is closed (or finalized):
[Trace] Audit Login -- network protocol: TCP/IP... SQL:BatchStarting SELECT count(1) from User SQL:BatchCompleted SELECT count(1) from User Audit Logout RPC:Completed exec sp_reset_connection
Identical, except that sp_reset_connection is invoked at the end.
In both cases, the connection remains sleeping (process is waiting for a lock or user input):
login_time last_batch hostname cmd status
2009-06-15 09:17:29.590 BENB AWAITING COMMAND sleeping
This behaviour is part of ADO.NET connection pooling. Connections remain ready like this until they are considered surplus (and removed from the pool), or the application exits. You can prove this easily enough yourself, quit your test fixture and then requery your connection state.
It is, therefore, important to close connections from an ADO.NET pooling standpoint. In order to make the in-memory connection available again.
If Open is invoked on a database connection, and there are no free connections available, an InvalidOperationException results with an error message like:
Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.
Querying connection states
Examine connections in SqlServer using master.db.sysprocesses:
select login_time, last_batch, hostname, cmd, status
from master.dbo.sysprocesses with(nolock)
where dbid = DB_ID('PersonalWind')
Finalizers
Finalizers are only for unmanaged resources. A finalizer provides a mechanism for releasing unmanaged resources when clients omit explicit disposal. Finalization occurs before the garbage collector reclaims managed memory, and is the last chance for objects to release unmanaged resources.
[MSDN, Object Lifetime: How Objects Are Created and Destroyed] The garbage collector in the CLR does not (and cannot) dispose of unmanaged objects, objects that the operating system executes directly, outside the CLR environment. This is because different unmanaged objects must be disposed of in different ways. That information is not directly associated with the unmanaged object; it must be found in the documentation for the object. A class that uses unmanaged objects must dispose of them in its Finalize method.
Though useful in certain circumstances, finalizers are notoriously difficult to implement, and incur real overhead:
- [MSDN] When allocated, finalizable objects are added to a finalization list. When these instances are no longer reachable and the GC runs, they’re moved to the “FReachable” queue, which is processed by the finalizer thread. Suppressing finalization with GC.SuppressFinalize sets a “do not run my finalizer” flag in the object’s header, such that the object will not get moved to the FReachable queue by the GC. As a result, while minimal, there is still overhead to giving an object a finalizer even if the finalizer does nothing or is suppressed.
- When the CLR needs to call a finalizer, it postpones reclamation of managed memory until the next round. This means finalizable objects are longer-lived — they use memory for longer.
Non-determinism
There is no way to predict when a finalizer will be called, because CLR decides when to reclaim memory based dynamically at runtime. Garbage collection is an expensive exercise, and is minimized by design, so memory can persist long after the variables that reference it have dropped out of scope. This may be unacceptable for some systems. Database connection pooling is a prime example of this. Failure to release connections by closing them when they’re no longer required quickly cripples a system.
References
- IDisposable, Finalizers: Section 9.3 of Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries.
- Examining Sql Server
- What is a sleeping/awaiting command session?
- Connection pooling in ADO.NET
- Connection pool overflows
- Database server connection pooling
