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The ns-3 tracing system is built on the concepts of independent tracing sources and tracing sinks; along with a uniform mechanism for connecting sources to sinks.
Trace sources are entities that can signal events that happen in a simulation and provide access to interesting underlying data. For example, a trace source could indicate when a packet is received by a net device and provide access to the packet contents for interested trace sinks. A trace source might also indicate when an iteresting state change happens in a model. For example, the congestion window of a TCP model is a prime candidate for a trace source.
Trace sources are not useful by themselves; they must be connected to other pieces of code that actually do something useful with the information provided by the source. The entities that consume trace information are called trace sinks. Trace sources are generators of events and trace sinks are consumers. This explicit division allows for large numbers of trace sources to be scattered around the system in places which model authors believe might be useful.
There can be zero or more consumers of trace events generated by a trace source. One can think of a trace source as a kind of point-to-multipoint information link. Your code looking for trace events from a particular piece of core code could happily coexist with other code doing something entirely different from the same information.
Unless a user connects a trace sink to one of these sources, nothing is output. By
using the tracing system, both you and other people at the same trace source are
getting exactly what they want and only what they want out of the system. Neither
of you are impacting any other user by changing what information is output by the
system. If you happen to add a trace source, your work as a good open-source
citizen may allow other users to provide new utilities that are perhaps very useful
overall, without making any changes to the ns-3 core.
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Let’s take a few minutes and walk through a simple tracing example. We are going to need a little background on Callbacks to understand what is happening in the example, so we have to take a small detour right away.
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The goal of the Callback system in ns-3 is to allow one piece of code to
call a function (or method in C++) without any specific inter-module dependency.
This ultimately means you need some kind of indirection – you treat the address
of the called function as a variable. This variable is called a pointer-to-function
variable. The relationship between function and pointer-to-function pointer is
really no different that that of object and pointer-to-object.
In C the canonical example of a pointer-to-function is a pointer-to-function-returning-integer (PFI). For a PFI taking one int parameter, this could be declared like,
int (*pfi)(int arg) = 0;
What you get from this is a variable named simply “pfi” that is initialized to the value 0. If you want to initialize this pointer to something meaningful, you have to have a function with a matching signature. In this case, you could provide a function that looks like,
int MyFunction (int arg) {}
If you have this target, you can initialize the variable to point to your function:
pfi = MyFunction;
You can then call MyFunction indirectly using the more suggestive form of the call,
int result = (*pfi) (1234);
This is suggestive since it looks like you are dereferencing the function pointer just like you would dereference any pointer. Typically, however, people take advantage of the fact that the compiler knows what is going on and will just use a shorter form,
int result = pfi (1234);
This looks like you are calling a function named “pfi,” but the compiler is
smart enough to know to call through the variable pfi indirectly to
the function MyFunction.
Conceptually, this is almost exactly how the tracing system will work.
Basically, a trace source is a callback. When a trace sink expresses
interest in receiving trace events, it adds a Callback to a list of Callbacks
internally held by the trace source. When an interesting event happens, the
trace source invokes its operator() providing zero or more parameters.
The operator() eventually wanders down into the system and does something
remarkably like the indirect call you just saw. It provides zero or more
parameters (the call to “pfi” above passed one parameter to the target function
MyFunction.
The important difference that the tracing system adds is that for each trace source there is an internal list of Callbacks. Instead of just making one indirect call, a trace source may invoke any number of Callbacks. When a trace sink expresses interest in notifications from a trace source, it basically just arranges to add its own function to the callback list.
If you are interested in more details about how this is actually arranged in
ns-3, feel free to peruse the Callback section of the manual.
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We have provided some code to implement what is really the simplest example
of tracing that can be assembled. You can find this code in the tutorial
directory as fourth.cc. Let’s walk through it.
/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */ /* * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation; * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include "ns3/object.h" #include "ns3/uinteger.h" #include "ns3/traced-value.h" #include "ns3/trace-source-accessor.h" #include <iostream> using namespace ns3;
Most of this code should be quite familiar to you. As mentioned above, the trace system makes heavy use of the Object and Attribute systems, so you will need to include them. The first two includes above bring in the declarations for those systems explicitly. You could use the core module header, but this illustrates how simple this all really is.
The file, traced-value.h brings in the required declarations for tracing
of data that obeys value semantics. In general, value semantics just means that
you can pass the object around, not an address. In order to use value semantics
at all you have to have an object with an associated copy constructor and
assignment operator available. We extend the requirements to talk about the set
of operators that are pre-defined for plain-old-data (POD) types. Operator=,
operator++, operator—, operator+, operator==, etc.
What this all really means is that you will be able to trace changes to a C++ object made using those operators.
Since the tracing system is integrated with Attributes, and Attributes work
with Objects, there must be an ns-3 Object for the trace source
to live in. The next code snippet declares and defines a simple Object we can
work with.
class MyObject : public Object
{
public:
static TypeId GetTypeId (void)
{
static TypeId tid = TypeId ("MyObject")
.SetParent (Object::GetTypeId ())
.AddConstructor<MyObject> ()
.AddTraceSource ("MyInteger",
"An integer value to trace.",
MakeTraceSourceAccessor (&MyObject::m_myInt))
;
return tid;
}
MyObject () {}
TracedValue<int32_t> m_myInt;
};
The two important lines of code, above, with respect to tracing are the
.AddTraceSource and the TracedValue declaration of m_myInt.
The .AddTraceSource provides the “hooks” used for connecting the trace
source to the outside world through the config system. The TracedValue
declaration provides the infrastructure that overloads the operators mentioned
above and drives the callback process.
void
IntTrace (int32_t oldValue, int32_t newValue)
{
std::cout << "Traced " << oldValue << " to " << newValue << std::endl;
}
This is the definition of the trace sink. It corresponds directly to a callback
function. Once it is connected, this function will be called whenever one of the
overloaded operators of the TracedValue is executed.
We have now seen the trace source and the trace sink. What remains is code to connect the source to the sink.
int
main (int argc, char *argv[])
{
Ptr<MyObject> myObject = CreateObject<MyObject> ();
myObject->TraceConnectWithoutContext ("MyInteger", MakeCallback(&IntTrace));
myObject->m_myInt = 1234;
}
Here we first create the Object in which the trace source lives.
The next step, the TraceConnectWithoutContext, forms the connection
between the trace source and the trace sink. Notice the MakeCallback
template function. This function does the magic required to create the
underlying ns-3 Callback object and associate it with the function
IntTrace. TraceConnect makes the association between your provided
function and the overloaded operator() in the traced variable referred
to by the “MyInteger” Attribute. After this association is made, the trace
source will “fire” your provided callback function.
The code to make all of this happen is, of course, non-trivial, but the essence
is that you are arranging for something that looks just like the pfi()
example above to be called by the trace source. The declaration of the
TracedValue<int32_t> m_myInt; in the Object itself performs the magic
needed to provide the overloaded operators (++, —, etc.) that will use the
operator() to actually invoke the Callback with the desired parameters.
The .AddTraceSource performs the magic to connect the Callback to the
Config system, and TraceConnectWithoutContext performs the magic to
connect your function to the trace source, which is specified by Attribute
name.
Let’s ignore the bit about context for now.
Finally, the line,
myObject->m_myInt = 1234;
should be interpreted as an invocation of operator= on the member
variable m_myInt with the integer 1234 passed as a parameter.
It turns out that this operator is defined (by TracedValue) to execute
a callback that returns void and takes two integer values as parameters —
an old value and a new value for the integer in question. That is exactly
the function signature for the callback function we provided — IntTrace.
To summarize, a trace source is, in essence, a variable that holds a list of callbacks. A trace sink is a function used as the target of a callback. The Attribute and object type information systems are used to provide a way to connect trace sources to trace sinks. The act of “hitting” a trace source is executing an operator on the trace source which fires callbacks. This results in the trace sink callbacks registering interest in the source being called with the parameters provided by the source.
If you now build and run this example,
./waf --run fourth
you will see the output from the IntTrace function execute as soon as the
trace source is hit:
Traced 0 to 1234
When we executed the code, myObject->m_myInt = 1234;, the trace source
fired and automatically provided the before and after values to the trace sink.
The function IntTrace then printed this to the standard output. No
problem.
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The TraceConnectWithoutContext call shown above in the simple example is
actually very rarely used in the system. More typically, the Config
subsystem is used to allow selecting a trace source in the system using what is
called a config path. We saw an example of this in the previous section
where we hooked the “CourseChange” event when we were playing with
third.cc.
Recall that we defined a trace sink to print course change information from the mobility models of our simulation. It should now be a lot more clear to you what this function is doing.
void
CourseChange (std::string context, Ptr<const MobilityModel> model)
{
Vector position = model->GetPosition ();
NS_LOG_UNCOND (context <<
" x = " << position.x << ", y = " << position.y);
}
When we connected the “CourseChange” trace source to the above trace sink, we used what is called a “Config Path” to specify the source when we arranged a connection between the pre-defined trace source and the new trace sink:
std::ostringstream oss;
oss <<
"/NodeList/" << wifiStaNodes.Get (nWifi - 1)->GetId () <<
"/$ns3::MobilityModel/CourseChange";
Config::Connect (oss.str (), MakeCallback (&CourseChange));
Let’s try and make some sense of what is sometimes considered relatively
mysterious code. For the purposes of discussion, assume that the node
number returned by the GetId() is “7”. In this case, the path
above turns out to be,
"/NodeList/7/$ns3::MobilityModel/CourseChange"
The last segment of a config path must be an Attribute of an
Object. In fact, if you had a pointer to the Object that has the
“CourseChange” Attribute handy, you could write this just like we did
in the previous example. You know by now that we typically store pointers to
our nodes in a NodeContainer. In the third.cc example, the Nodes of
interest are stored in the wifiStaNodes NodeContainer. In fact, while
putting the path together, we used this container to get a Ptr<Node> which we
used to call GetId() on. We could have used this Ptr<Node> directly to call
a connect method directly:
Ptr<Object> theObject = wifiStaNodes.Get (nWifi - 1);
theObject->TraceConnectWithoutContext ("CourseChange", MakeCallback (&CourseChange));
In the third.cc example, we actually want an additional “context” to
be delivered along with the Callback parameters (which will be explained below) so we
could actually use the following equivalent code,
Ptr<Object> theObject = wifiStaNodes.Get (nWifi - 1);
theObject->TraceConnect ("CourseChange", MakeCallback (&CourseChange));
It turns out that the internal code for Config::ConnectWithoutContext and
Config::Connect actually do find a Ptr<Object> and call the appropriate
TraceConnect method at the lowest level.
The Config functions take a path that represents a chain of Object
pointers. Each segment of a path corresponds to an Object Attribute. The last
segment is the Attribute of interest, and prior segments must be typed to contain
or find Objects. The Config code parses and “walks” this path until it
gets to the final segment of the path. It then interprets the last segment as
an Attribute on the last Object it found while walking the path. The
Config functions then call the appropriate TraceConnect or
TraceConnectWithoutContext method on the final Object. Let’s see what
happens in a bit more detail when the above path is walked.
The leading “/” character in the path refers to a so-called namespace. One
of the predefined namespaces in the config system is “NodeList” which is a
list of all of the nodes in the simulation. Items in the list are referred to
by indices into the list, so “/NodeList/7” refers to the eighth node in the
list of nodes created during the simulation. This reference is actually a
Ptr<Node> and so is a subclass of an ns3::Object.
As described in the Object Model section of the ns-3 manual, we support
Object Aggregation. This allows us to form an association between different
Objects without any programming. Each Object in an Aggregation can be reached
from the other Objects.
The next path segment being walked begins with the “$” character. This
indicates to the config system that a GetObject call should be made
looking for the type that follows. It turns out that the MobilityHelper used in
third.cc arranges to Aggregate, or associate, a mobility model to each of
the wireless Nodes. When you add the “$” you are asking for another Object that
has presumably been previously aggregated. You can think of this as switching
pointers from the original Ptr<Node> as specified by “/NodeList/7” to its
associated mobility model — which is of type “$ns3::MobilityModel”. If you
are familiar with GetObject, we have asked the system to do the following:
Ptr<MobilityModel> mobilityModel = node->GetObject<MobilityModel> ()
We are now at the last Object in the path, so we turn our attention to the
Attributes of that Object. The MobilityModel class defines an Attribute
called “CourseChange.” You can see this by looking at the source code in
src/mobility/mobility-model.cc and searching for “CourseChange” in your
favorite editor. You should find,
.AddTraceSource (``CourseChange'',
``The value of the position and/or velocity vector changed'',
MakeTraceSourceAccessor (&MobilityModel::m_courseChangeTrace))
which should look very familiar at this point.
If you look for the corresponding declaration of the underlying traced variable
in mobility-model.h you will find
TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;
The type declaration TracedCallback identifies m_courseChangeTrace
as a special list of Callbacks that can be hooked using the Config functions
described above.
The MobilityModel class is designed to be a base class providing a common
interface for all of the specific subclasses. If you search down to the end of
the file, you will see a method defined called NotifyCourseChange():
void
MobilityModel::NotifyCourseChange (void) const
{
m_courseChangeTrace(this);
}
Derived classes will call into this method whenever they do a course change to
support tracing. This method invokes operator() on the underlying
m_courseChangeTrace, which will, in turn, invoke all of the registered
Callbacks, calling all of the trace sinks that have registered interest in the
trace source by calling a Config function.
So, in the third.cc example we looked at, whenever a course change is
made in one of the RandomWalk2dMobilityModel instances installed, there
will be a NotifyCourseChange() call which calls up into the
MobilityModel base class. As seen above, this invokes operator()
on m_courseChangeTrace, which in turn, calls any registered trace sinks.
In the example, the only code registering an interest was the code that provided
the config path. Therefore, the CourseChange function that was hooked
from Node number seven will be the only Callback called.
The final piece of the puzzle is the “context.” Recall that we saw an output
looking something like the following from third.cc:
/NodeList/7/$ns3::MobilityModel/CourseChange x = 7.27897, y = 2.22677
The first part of the output is the context. It is simply the path through
which the config code located the trace source. In the case we have been looking at
there can be any number of trace sources in the system corresponding to any number
of nodes with mobility models. There needs to be some way to identify which trace
source is actually the one that fired the Callback. An easy way is to request a
trace context when you Config::Connect.
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The first question that inevitably comes up for new users of the Tracing system is, “okay, I know that there must be trace sources in the simulation core, but how do I find out what trace sources are available to me”?
The second question is, “okay, I found a trace source, how do I figure out the config path to use when I connect to it”?
The third question is, “okay, I found a trace source, how do I figure out what the return type and formal arguments of my callback function need to be”?
The fourth question is, “okay, I typed that all in and got this incredibly bizarre error message, what in the world does it mean”?
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The answer to this question is found in the ns-3 Doxygen. Go to the
ns-3 web site “here”
and select the “Doxygen (stable)” link “Documentation” on the navigation
bar to the left side of the page. Expand the “Modules” book in the NS-3
documentation tree a the upper left by clicking the “+” box. Now, expand
the “Core” book in the tree by clicking its “+” box. You should now
see three extremely useful links:
The list of interest to us here is “the list of all trace sources.” Go
ahead and select that link. You will see, perhaps not too surprisingly, a
list of all of the trace sources available in the ns-3 core.
As an example, scroll down to ns3::MobilityModel. You will find
an entry for
CourseChange: The value of the position and/or velocity vector changed
You should recognize this as the trace source we used in the third.cc
example. Perusing this list will be helpful.
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The easiest way to do this is to grep around in the ns-3 codebase for someone
who has already figured it out, You should always try to copy someone else’s
working code before you start to write your own. Try something like:
find . -name '*.cc' | xargs grep CourseChange | grep Connect
and you may find your answer along with working code. For example, in this
case, ./ns-3-dev/examples/wireless/mixed-wireless.cc has something
just waiting for you to use:
Config::Connect (``/NodeList/*/$ns3::MobilityModel/CourseChange'',
MakeCallback (&CourseChangeCallback));
If you cannot find any examples in the distribution, you can find this out
from the ns-3 Doxygen. It will probably be simplest just to walk
through the “CourseChanged” example.
Let’s assume that you have just found the “CourseChanged” trace source in
“The list of all trace sources” and you want to figure out how to connect to
it. You know that you are using (again, from the third.cc example) an
ns3::RandomWalk2dMobilityModel. So open the “Class List” book in
the NS-3 documentation tree by clicking its “+” box. You will now see a
list of all of the classes in ns-3. Scroll down until you see the
entry for ns3::RandomWalk2dMobilityModel and follow that link.
You should now be looking at the “ns3::RandomWalk2dMobilityModel Class
Reference.”
If you now scroll down to the “Member Function Documentation” section, you
will see documentation for the GetTypeId function. You constructed one
of these in the simple tracing example above:
static TypeId GetTypeId (void)
{
static TypeId tid = TypeId ("MyObject")
.SetParent (Object::GetTypeId ())
.AddConstructor<MyObject> ()
.AddTraceSource ("MyInteger",
"An integer value to trace.",
MakeTraceSourceAccessor (&MyObject::m_myInt))
;
return tid;
}
As mentioned above, this is the bit of code that connected the Config and Attribute systems to the underlying trace source. This is also the place where you should start looking for information about the way to connect.
You are looking at the same information for the RandomWalk2dMobilityModel; and the information you want is now right there in front of you in the Doxygen:
This object is accessible through the following paths with Config::Set and Config::Connect: /NodeList/[i]/$ns3::MobilityModel/$ns3::RandomWalk2dMobilityModel
The documentation tells you how to get to the RandomWalk2dMobilityModel
Object. Compare the string above with the string we actually used in the
example code:
"/NodeList/7/$ns3::MobilityModel"
The difference is due to the fact that two GetObject calls are implied
in the string found in the documentation. The first, for $ns3::MobilityModel
will query the aggregation for the base class. The second implied
GetObject call, for $ns3::RandomWalk2dMobilityModel, is used to “cast”
the base class to the concrete imlementation class. The documentation shows
both of these operations for you. It turns out that the actual Attribute you are
going to be looking for is found in the base class as we have seen.
Look further down in the GetTypeId doxygen. You will find,
No TraceSources defined for this type. TraceSources defined in parent class ns3::MobilityModel: CourseChange: The value of the position and/or velocity vector changed Reimplemented from ns3::MobilityModel
This is exactly what you need to know. The trace source of interest is found in
ns3::MobilityModel (which you knew anyway). The interesting thing this
bit of Doxygen tells you is that you don’t need that extra cast in the config
path above to get to the concrete class, since the trace source is actually in
the base class. Therefore the additional GetObject is not required and
you simply use the path:
/NodeList/[i]/$ns3::MobilityModel
which perfectly matches the example path:
/NodeList/7/$ns3::MobilityModel
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The easiest way to do this is to grep around in the ns-3 codebase for someone
who has already figured it out, You should always try to copy someone else’s
working code. Try something like:
find . -name '*.cc' | xargs grep CourseChange | grep Connect
and you may find your answer along with working code. For example, in this
case, ./ns-3-dev/examples/wireless/mixed-wireless.cc has something
just waiting for you to use. You will find
Config::Connect (``/NodeList/*/$ns3::MobilityModel/CourseChange'',
MakeCallback (&CourseChangeCallback));
as a result of your grep. The MakeCallback should indicate to you that
there is a callback function there which you can use. Sure enough, there is:
static void
CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
{
...
}
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If there are no examples to work from, this can be, well, challenging to actually figure out from the source code.
Before embarking on a walkthrough of the code, I’ll be kind and just tell you
a simple way to figure this out: The return value of your callback will always
be void. The formal parameter list for a TracedCallback can be found
from the template parameter list in the declaration. Recall that for our
current example, this is in mobility-model.h, where we have previously
found:
TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;
There is a one-to-one correspondence between the template parameter list in
the declaration and the formal arguments of the callback function. Here,
there is one template parameter, which is a Ptr<const MobilityModel>.
This tells you that you need a function that returns void and takes a
a Ptr<const MobilityModel>. For example,
void
CourseChangeCallback (Ptr<const MobilityModel> model)
{
...
}
That’s all you need if you want to Config::ConnectWithoutContext. If
you want a context, you need to Config::Connect and use a Callback
function that takes a string context, then the required argument.
void
CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
{
...
}
If you want to ensure that your CourseChangeCallback is only visible
in your local file, you can add the keyword static and come up with:
static void
CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
{
...
}
which is exactly what we used in the third.cc example.
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This section is entirely optional. It is going to be a bumpy ride, especially
for those unfamiliar with the details of templates. However, if you get through
this, you will have a very good handle on a lot of the ns-3 low level
idioms.
So, again, let’s figure out what signature of callback function is required for
the “CourseChange” Attribute. This is going to be painful, but you only need
to do this once. After you get through this, you will be able to just look at
a TracedCallback and understand it.
The first thing we need to look at is the declaration of the trace source.
Recall that this is in mobility-model.h, where we have previously
found:
TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;
This declaration is for a template. The template parameter is inside the
angle-brackets, so we are really interested in finding out what that
TracedCallback<> is. If you have absolutely no idea where this might
be found, grep is your friend.
We are probably going to be interested in some kind of declaration in the
ns-3 source, so first change into the src directory. Then,
we know this declaration is going to have to be in some kind of header file,
so just grep for it using:
find . -name '*.h' | xargs grep TracedCallback
You’ll see 124 lines fly by (I piped this through wc to see how bad it was). Although that may seem like it, that’s not really a lot. Just pipe the output through more and start scanning through it. On the first page, you will see some very suspiciously template-looking stuff.
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::TracedCallback () TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::ConnectWithoutContext (c ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::Connect (const CallbackB ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::DisconnectWithoutContext ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::Disconnect (const Callba ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (void) const ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1) const ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ... TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::operator() (T1 a1, T2 a2 ...
It turns out that all of this comes from the header file
traced-callback.h which sounds very promising. You can then take a
look at mobility-model.h and see that there is a line which confirms
this hunch:
#include "ns3/traced-callback.h"
Of course, you could have gone at this from the other direction and started
by looking at the includes in mobility-model.h and noticing the
include of traced-callback.h and inferring that this must be the file
you want.
In either case, the next step is to take a look at src/core/traced-callback.h
in your favorite editor to see what is happening.
You will see a comment at the top of the file that should be comforting:
An ns3::TracedCallback has almost exactly the same API as a normal ns3::Callback but instead of forwarding calls to a single function (as an ns3::Callback normally does), it forwards calls to a chain of ns3::Callback.
This should sound very familiar and let you know you are on the right track.
Just after this comment, you will find,
template<typename T1 = empty, typename T2 = empty,
typename T3 = empty, typename T4 = empty,
typename T5 = empty, typename T6 = empty,
typename T7 = empty, typename T8 = empty>
class TracedCallback
{
...
This tells you that TracedCallback is a templated class. It has eight possible type parameters with default values. Go back and compare this with the declaration you are trying to understand:
TracedCallback<Ptr<const MobilityModel> > m_courseChangeTrace;
The typename T1 in the templated class declaration corresponds to the
Ptr<const MobilityModel> in the declaration above. All of the other
type parameters are left as defaults. Looking at the constructor really
doesn’t tell you much. The one place where you have seen a connection made
between your Callback function and the tracing system is in the Connect
and ConnectWithoutContext functions. If you scroll down, you will see
a ConnectWithoutContext method here:
template<typename T1, typename T2,
typename T3, typename T4,
typename T5, typename T6,
typename T7, typename T8>
void
TracedCallback<T1,T2,T3,T4,T5,T6,T7,T8>::ConnectWithoutContext ...
{
Callback<void,T1,T2,T3,T4,T5,T6,T7,T8> cb;
cb.Assign (callback);
m_callbackList.push_back (cb);
}
You are now in the belly of the beast. When the template is instantiated for
the declaration above, the compiler will replace T1 with
Ptr<const MobilityModel>.
void
TracedCallback<Ptr<const MobilityModel>::ConnectWithoutContext ... cb
{
Callback<void, Ptr<const MobilityModel> > cb;
cb.Assign (callback);
m_callbackList.push_back (cb);
}
You can now see the implementation of everything we’ve been talking about. The
code creates a Callback of the right type and assigns your function to it. This
is the equivalent of the pfi = MyFunction we discussed at the start of
this section. The code then adds the Callback to the list of Callbacks for
this source. The only thing left is to look at the definition of Callback.
Using the same grep trick as we used to find TracedCallback, you will be
able to find that the file ./core/callback.h is the one we need to look at.
If you look down through the file, you will see a lot of probably almost incomprehensible template code. You will eventually come to some Doxygen for the Callback template class, though. Fortunately, there is some English:
This class template implements the Functor Design Pattern.
It is used to declare the type of a Callback:
- the first non-optional template argument represents
the return type of the callback.
- the second optional template argument represents
the type of the first argument to the callback.
- the third optional template argument represents
the type of the second argument to the callback.
- the fourth optional template argument represents
the type of the third argument to the callback.
- the fifth optional template argument represents
the type of the fourth argument to the callback.
- the sixth optional template argument represents
the type of the fifth argument to the callback.
We are trying to figure out what the
Callback<void, Ptr<const MobilityModel> > cb;
declaration means. Now we are in a position to understand that the first
(non-optional) parameter, void, represents the return type of the
Callback. The second (non-optional) parameter, Ptr<const MobilityModel>
represents the first argument to the callback.
The Callback in question is your function to receive the trace events. From
this you can infer that you need a function that returns void and takes
a Ptr<const MobilityModel>. For example,
void
CourseChangeCallback (Ptr<const MobilityModel> model)
{
...
}
That’s all you need if you want to Config::ConnectWithoutContext. If
you want a context, you need to Config::Connect and use a Callback
function that takes a string context. This is because the Connect
function will provide the context for you. You’ll need:
void
CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
{
...
}
If you want to ensure that your CourseChangeCallback is only visible
in your local file, you can add the keyword static and come up with:
static void
CourseChangeCallback (std::string path, Ptr<const MobilityModel> model)
{
...
}
which is exactly what we used in the third.cc example. Perhaps you
should now go back and reread the previous section (Take My Word for It).
If you are interested in more details regarding the implementation of
Callbacks, feel free to take a look at the ns-3 manual. They are one
of the most frequently used constructs in the low-level parts of ns-3.
It is, in my opinion, a quite elegant thing.
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Earlier in this section, we presented a simple piece of code that used a
TracedValue<int32_t> to demonstrate the basics of the tracing code.
We just glossed over the way to find the return type and formal arguments
for the TracedValue. Rather than go through the whole exercise, we
will just point you at the correct file, src/core/traced-value.h and
to the important piece of code:
template <typename T>
class TracedValue
{
public:
...
void Set (const T &v) {
if (m_v != v)
{
m_cb (m_v, v);
m_v = v;
}
}
...
private:
T m_v;
TracedCallback<T,T> m_cb;
};
Here you see that the TracedValue is templated, of course. In the simple
example case at the start of the section, the typename is int32_t. This means
that the member variable being traced (m_v in the private section of the
class) will be an int32_t m_v. The Set method will take a
const int32_t &v as a parameter. You should now be able to understand
that the Set code will fire the m_cb callback with two parameters:
the first being the current value of the TracedValue; and the second
being the new value being set.
The callback, m_cb is declared as a TracedCallback<T, T> which
will correspond to a TracedCallback<int32_t, int32_t> when the class is
instantiated.
Recall that the callback target of a TracedCallback always returns void.
Further recall that there is a one-to-one correspondence between the template
parameter list in the declaration and the formal arguments of the callback
function. Therefore the callback will need to have a function signature that
looks like:
void
MyCallback (int32_t oldValue, int32_t newValue)
{
...
}
It probably won’t surprise you that this is exactly what we provided in that simple example we covered so long ago:
void
IntTrace (int32_t oldValue, int32_t newValue)
{
std::cout << "Traced " << oldValue << " to " << newValue << std::endl;
}
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