I would like to know the details about the third parameter of the hci_write_local_name function. The first one is thesocket descriptor. Second the local name. And third one is a number which i thought the length of the second parameter. But if i use strlen of the second parameter the function fails. In hciconfig.c the value of third parameter for the same function is 2000. I tested with 100 as value. That worked. Can anybody say what exactly the third parameter implies?
As ukBaz mentioned, the third parameter is the timeout as can be seen in the source code:-
int hci_write_local_name(int dd, const char *name, int to)
{
change_local_name_cp cp;
struct hci_request rq;
memset(&cp, 0, sizeof(cp));
strncpy((char *) cp.name, name, sizeof(cp.name) - 1);
memset(&rq, 0, sizeof(rq));
rq.ogf = OGF_HOST_CTL;
rq.ocf = OCF_CHANGE_LOCAL_NAME;
rq.cparam = &cp;
rq.clen = CHANGE_LOCAL_NAME_CP_SIZE;
if (hci_send_req(dd, &rq, to) < 0)
return -1;
return 0;
}
Most of BlueZ's HCI functions require a timeout parameter as the underlying hci_send_req() requires that.
More information can be found here and the rest of the file.
Related
Can't get how a method with this head: char * strcpy (char *cad1, const char *cad2), works in C in this sample:
'char * strcpy (char *cad1, const char *cad2){
char *aux = cad1;
for( ; *cad1++ = *cad2++; );
return cad1;
}'
Starting from the method signature or prototype, that tells a lot about the how it works: we have two parameters together with their respective types and a return type. All parameters in this case are pointers to char, more known as char pointers. Those char pointers are what is used in "C" as strings of characters. One parameter is a const, because that value must not be changed in the function, it MUST keep, the original value.
Strings in "C" have some peculiarities, once the pointer is created to a string it always points to the first characters in the string or index 0, the same as char *v = var[0], and can be incremented passing to the next char in the string such as v++. Other peculiarity in "C" is that all strings represented by char arrays end with a 0 character (ASCII null = 0).
The strcpy version account on that concepts and makes a for loop to copy each element in the char *cad2 to *cad1, that variables MUST be allocated statically or dynamically (malloc) before calling the function, and the return of the function in the code above is a pointer to the original variable (in that case *cad1, normally they return the copied one). In your function it was changed, I mean it is returning the original instead of the copied what looks wrong since you catch in the aux the pointer to the first element of the copied variable and you did not use it.
One good point to observe is the for loop:
for( ; *cad1++ = *cad2++; );
How it works is tricky, the first interesting point is that the for loop has tree parameters, and in "C" all are optional. The first is to initialize, the second is a boolean condition to continuing iterating, and the last one is to increment or decrement.
Next, tricky is is *cad1++ = *cad2++ a boolean expression? The answer is yes, it is. Since in "C" the value 0 (zero) is false, and anything else is true. Remember that I have said strings in "C" finishes always with a 0 (zero), so when evaluating and assigning to the copy the value of a pointer (using *cad1 will return the value pointed by a pointer variable, the star in the begin makes that magic) and reaches the end of the string that will return false and finish the iteration loop.
One point is interesting here, first the evaluation has less priority than the assignment in this case, what makes first the value being copied to the copy variable, then evaluating the boolean expression.
"C" is like this you writes a small code that have large meaning behind it. I hope you have understood the explanation. For further information have a look in "C" pointers at : https://www.tutorialspoint.com/cprogramming/c_pointers.htm.
char * strcpy (char *cad1, const char *cad2){
for( ; *cad1++ = *cad2++;);
return cad1;
}
the way this works, at the calling side, it can be used in two ways, but always requires a buffer to write to so the use is simmilar.
char arr[255];
memset(arr,0,sizeof(char) * 255); // clear the garbage initialized array;
strcpy(arr, "this is the text to copy that is 254 characters long or shorter.");
puts(arr);
or
char arr[255];
memset(arr,0,sizeof(char) * 255);
puts(strcpy(arr,"hello C!"));
sense the function returns the pointer to the buffer this works as well.
I am a bit confused about passing by reference and value in Go.
I've seen this explained of the * in front of a type.
in front of a type name, means that the declared variable will store an address of another variable of that type (not a value of that
type).
This just doesn't make sense to me.
In Java if I was passing a Database instance into a function I would do
databaseFunction(DatabaseType db) {
// do something
}
However in the go example I have it's passed like so.
func PutTasks(db *sql.DB) echo.HandlerFunc {
}
Why do we need to have the asterisk in front of the type?
According to this cheat sheet, I found.
func PrintPerson(p *Person) ONLY receives the pointer address
(reference)
I don't understand why I would only want to send a pointer address as a parameter.
First, Go technically has only pass-by-value. When passing a pointer to an object, you're passing a pointer by value, not passing an object by reference. The difference is subtle but occasionally relevant. For example, you can overwrite the pointer value which has no impact on the caller, as opposed to dereferencing it and overwriting the memory it points to.
// *int means you *must* pass a *int (pointer to int), NOT just an int!
func someFunc(x *int) {
*x = 2 // Whatever variable caller passed in will now be 2
y := 7
x = &y // has no impact on the caller because we overwrote the pointer value!
}
As to your question "Why do we need to have the asterisk in front of the type?": The asterisk indicates that the value is of type pointer to sql.DB, rather than a value of type sql.DB. These are not interchangeable!
Why would you want to send a pointer address? So that you can share the value between the caller of a function and the function body, with changes made inside the function reflected in the caller (for example, a pointer is the only way that a "setter" method can work on an object). While Java passes objects by reference always, Go passes by value always (i.e. it creates a copy of the value in the function); if you pass something to a function, and that function modifies that value, the caller won't see those changes. If you want changes to propogate outside the function, you must pass a pointer.
See also: the Go tour section on Pointers, the Go spec section on pointers, the Go spec section on the address operators
The purpose of reference semantics is to allow a function to manipulate data outside its own scope. Compare:
func BrokenSwap(a int, b int) {
a, b = b, a
}
func RealSwap(a *int, b *int) {
*a, *b = *b, *a
}
When you call BrokenSwap(x, y), there is no effect, because the function receives and manipulates a private copy of the data. By contrast, when you call RealSwap(&x, &y), you actually exchange the values of the caller's x and y. Taking the address of the variables explicitly at the call site informs the reader that those variables may be mutated.
Pass by Reference :-
When you pass a same variable into a function by different name.
Below example from C++ (as Go doesnt have this concept), where a and a1 are same variable.
void swap(int& a1, int& b1)
{
int tmp = a1;
a1 = b1;
b1 = tmp;
}
int main()
{
int a = 10, b = 20;
swap(a, b);
cout << "a " << a << " b " << b ;
}
Go passes everything as data( means it copies the data from current active frame to new active frame of new function). So if you pass values it copies the value and advantage is safety from accidental modification. And when it passes address of variable its copied also into the new pointer variables but has advantage of efficiency since size of pointer is smaller.
I want to use user assertion of value analysis plugin of Frama-C (Neon version), however I have some problem to come up with the suitable model of assume statement, which is very useful for me to apply particular constraints, for example, here is my test code:
#include "/usr/local/share/frama-c/builtin.h"
int main(void)
{
int selection = Frama_C_interval(0,10);
int a;
assume(selection > 5);
if (selection > 5)
{
a = 2;
}
else
{
a = 1;
}
//# assert a == 2;
return 0;
}
I want that the value of selection will be greater than 5 after this assume statement so that the assertion will be valid.
My initial attempt was to write this function
void assume(int a){ while(!a); return;}
, but it was unsuccessful.
Please help me, thanks.
The easiest way to constrain selection would be to use an assert (which of course won't be proved by Value). If you want to distinguish between the assert that are in fact hypotheses you make from the assert that you want to verify, you can use ACSL's naming mechanism, such as
//# assert assumption: selection > 5;
and verify that the only assert that are unknown are the ones named assumption.
Using an assume function cannot work as such, because it will only reduce the possible value of the a parameter to be non-zero. Value is not able to infer the relation between the value of a in assume and the value of selection in main. However, it is possible to help it a little bit. First, -slevel allows to propagate several abstract state in parallel. Second, an assert given in an disjunctive will force Value to split its state (if the -slevel is big enough to do so). Thus, with the following code
#include "builtin.h"
void assume(int a) { while(!a); return; }
int main(void)
{
int selection = Frama_C_interval(0,10);
int a;
/*# assert selection > 5 || selection <= 5; */
assume(selection > 5);
if (selection > 5)
{
a = 2;
}
else
{
a = 1;
}
//# assert a == 2;
return 0;
}
and the following command line:
frama-c -cpp-extra-args="-I$(frama-c -print-share-path)" -val -slevel 2
After the first assert (which obviously valid), Frama-C will propagate separately two states: one in which selection > 5 and one in which selection <= 5. In the first case, assume is called with 1 as argument, thus returns immediately, and the then branch of the if is taken, so that the second assert is valid. In the second state, assume is called with 0, and never returns. Thus for all cases where control reaches the second assert, it is valid.
Note that you really need to add the first assert inside the body of main, and to copy in ACSL the argument you pass to assume. Otherwise, the state split won't occur (more precisely, you will split on a, not on selection).
I am doing one project in which I define a data types like below
typedef QVector<double> QFilterDataMap1D;
typedef QMap<double, QFilterDataMap1D> QFilterDataMap2D;
Then there is one class with the name of mono_data in which i have define this variable
QFilterMap2D valid_filters;
mono_data Scan_data // Class
Now i am reading one variable from a .mat file and trying to save it in to above "valid_filters" QMap.
Qt Code: Switch view
for(int i=0;i<1;i++)
{
for(int j=0;j<1;j++)
{
Scan_Data.valid_filters[i][j]=valid_filters[i][j];
printf("\nValid_filters=%f",Scan_Data.valid_filters[i][j]);
}
}
The transferring is done successfully but then it gives run-time error
Windows has triggered a breakpoint in SpectralDataCollector.exe.
This may be due to a corruption of the heap, and indicates a bug in
SpectralDataCollector.exe or any of the DLLs it has loaded.
The output window may have more diagnostic information
Can anyone help in solving this problem. It will be of great help to me.
Thanks
Different issues here:
1. Using double as key type for a QMap
Using a QMap<double, Foo> is a very bad idea. the reason is that this is a container that let you access a Foo given a double. For instance:
map[0.45] = foo1;
map[15.74] = foo2;
This is problematic, because then, to retrieve the data contained in map[key], you have to test if key is either equal, smaller or greater than other keys in the maps. In your case, the key is a double, and testing if two doubles are equals is not a "safe" operation.
2. Using an int as key while you defined it was double
Here:
Scan_Data.valid_filters[i][j]=valid_filters[i][j];
i is an integer, and you said it should be a double.
3. Your loop only test for (i,j) = (0,0)
Are you aware that
for(int i=0;i<1;i++)
{
for(int j=0;j<1;j++)
{
Scan_Data.valid_filters[i][j]=valid_filters[i][j];
printf("\nValid_filters=%f",Scan_Data.valid_filters[i][j]);
}
}
is equivalent to:
Scan_Data.valid_filters[0][0]=valid_filters[0][0];
printf("\nValid_filters=%f",Scan_Data.valid_filters[0][0]);
?
4. Accessing a vector with operator[] is not safe
When you do:
Scan_Data.valid_filters[i][j]
You in fact do:
QFilterDataMap1D & v = Scan_Data.valid_filters[i]; // call QMap::operator[](double)
double d = v[j]; // call QVector::operator[](int)
The first one is safe, and create the entry if it doesn't exist. The second one is not safe, the jth element in you vector must already exist otherwise it would crash.
Solution
It seems you in fact want a 2D array of double (i.e., a matrix). To do this, use:
typedef QVector<double> QFilterDataMap1D;
typedef QVector<QFilterDataMap1D> QFilterDataMap2D;
Then, when you want to transfer one in another, simply use:
Scan_Data.valid_filters = valid_filters;
Or if you want to do it yourself:
Scan_Data.valid_filters.clear();
for(int i=0;i<n;i++)
{
Scan_Data.valid_filters << QFilterDataMap1D();
for(int j=0;j<m;j++)
{
Scan_Data.valid_filters[i] << valid_filters[i][j];
printf("\nValid_filters=%f",Scan_Data.valid_filters[i][j]);
}
}
If you want a 3D matrix, you would use:
typedef QVector<QFilterDataMap2D> QFilterDataMap3D;
I'm using Qt Creator 4.5 with GCC 4.3 and I'm having the following problem that I am not sure is Qt or C++ related: I call a function with a char * as an input parameter. Inside that function I make a dynamic allocation and I assign the address to the char *. The problem is when the function returns it does not point to this address anymore.
bool FPSengine::putData (char CommandByte , int Index)
{
char *msgByte;
structSize=putDatagrams(CommandByte, Index, msgByte);
}
int FPSengine::putDatagrams (char CommandByte, int Index, char *msgByte)
{
int theSize;
switch ( CommandByte ) {
case (CHANGE_CONFIGURATION): {
theSize=sizeof(MsnConfigType);
msgByte=new char[theSize];
union MConfigUnion {
char cByte[sizeof(MsnConfigType)];
MsnConfigType m;
};
MConfigUnion * msnConfig=(MConfigUnion*)msgByte;
...Do some assignments. I verify and everything is OK.
}
}
return theSize;
}
When I return the pointer it contains a completely different address than the one assigned in putDatagrams(). Why?
...
Ok thx I understand my mistake(rookie mistake :( ). When sending a pointer as an input parameter to the function you send the address of your data but not the address of your pointer so you cant make the pointer point somewhere else...it is actually a local copy like Index. The only case the data would of been returned succesfully with the use of a char * is by allocating the memory before the function call:
bool FPSengine::putData (char CommandByte , int Index)
{
char *msgByte;
msgByte=new char[sizeof(MsnConfigType)];
structSize=putDatagrams(CommandByte, Index, msgByte);
}
int FPSengine::putDatagrams (char CommandByte, int Index, char *msgByte)
{
int theSize;
switch ( CommandByte ) {
case (CHANGE_CONFIGURATION): {
theSize=sizeof(MsnConfigType);
union MConfigUnion {
char cByte[sizeof(MsnConfigType)];
MsnConfigType m;
};
MConfigUnion * msnConfig=(MConfigUnion*)msgByte;
...Do some assignments. I verify and everything is OK.
}
}
return theSize;
}
There are two ways. The pass-by-value way (C style):
int FPSengine::putDatagrams (char CommandByte, int Index, char **msgByte)
Note the second * for msgByte. Then inside of putDatagrams(), do:
*msgByte = new char[theSize];
In fact, anywhere in that function where you currently have msgByte, use *msgByte. When calling putDatagrams(), do:
structSize=putDatagrams(CommandByte, Index, &msgByte);
And the second way, since you're in C++, you could use pass-by-reference. Just change the signature of putDatagrams() to:
int FPSengine::putDatagrams (char CommandByte, int Index, char * &msgByte)
And you should be good. In this case, you shouldn't need to modify the caller or anything inside of your putDatagrams() routine.
Well, yes. Everything in C++ is, by default, passed by value. Parameters in the call putDatagrams(a, b, c) are sent by value - you wouldn't expect assigning to index in the code to change the value of b at the call site. Your msgByte=new char[theSize]; is just assigning to the local variable msgByte, overwriting the value passed in.
If you want to change a passed parameter such that the call site variable changes, you'll need to either pass by reference, or (in this case) pass a "pointer to a pointer` (and deference away the first pointer, assigning to the actual pointer).