After MPI_Recv, only the first index of the array is filled and the rest of th eelements are remaining '0'. When I print the Array before sending, the elements are non-zero. But after receiving, except for the first one, all others are zero.
I have checked the size of the array and the datatype.
Check if the 'count' argument in MPI_Recv is correctly indicating the total elements of the array being sent and received. I had the same problem which got rectified when I accurately specified the number of elements in the array. Here is the syntax:
int MPI_Recv(void *buf, int count, MPI_Datatype datatype, int source, int tag,
MPI_Comm comm, MPI_Status *status)
Related
So here is the thing, I'm receiving 1 byte from Bluetooth transmission. When using QDebug I get this message:
The array with error has "\x06"
The line that fails is this:
bool ok = true;
int v = value.toInt(&ok,0);
Because ok has false. But I'm trying to wrap my head around the fact that, How can the conversion fail in the first place if the data represented in that byte (as a sequence of zeros and ones) will always have a valid integer representation. (one byte can always be represented as a int between -127 and 128). So I'm left with the question, how can the conversion fail?
Reading the documentation does not provide many clues as it does not say how the byte array will be interpreted.
QByteArray::toInt converts a string representation in the default C locale to an integer. That means to successfully convert the value in your example, your byte array must contain the string "0x06", which consists of 4 bytes.
To convert a single byte to an int, just extract it:
int i = value[0];
Type promotion will widen the char to an int
I'm using google wrapper (sqlite3pp) to insert a char array that contain some zeros. The problem that is the SQLite is dropping the zero and the next elements after it.
char array[11] = {1,2,3,4,5,0,3,4,0,6,7};
sqlite3pp::command cmd(db, "INSERT INTO messages (id, payload) VALUES (?, ?)");
cmd.bind(1,index);
cmd.bind(2,&array[0],sizeof(array));
This code only insert: 1 2 3 4 5
The payload type is varchar.
Any idea?
sqlite3pp defines, among others, these two overloads for the bind() function:
int bind(int idx, char const* value, bool fstatic = true);
int bind(int idx, void const* value, int n, bool fstatic = true);
You want to use the second one with explicit length, but the first one is selected, while sizeof(array), evaluated to be 11, is truncated to bool value true and passed as fstatic instead of size. The wrapper then thinks the value is a plain NUL-terminated string and thus stores just the part till the first zero.
You can help the compiler to select the right version e.g. by providing the implicit parameter like so:
cmd.bind(2, &array[0], sizeof(array), true);
(Or false when the array will be deallocated before the query is done executing.)
Additionally, there can be problems with reading the rows as well - e.g. the default sqlite3pp getter for std::string won't work with binary zeroes and the content needs to be retrieved explicitly like this:
payload.assign(static_cast<const char*>(i->get<const void*>(2)), i->column_bytes(2));
I need to implement but I am not sure how can I as I am completely new into this. A function called get_values that has the prototype:
void get_values(unsigned int value, unsigned int *p_lsb, unsigned int *p_msb,
unsigned int *p_combined)
The function computes the least significant byte and the most significant byte of the value
parameter. In addition, both values are combined. For this problem:
a. You may not use any loop constructs.
b. You may not use the multiplication operator (* or *=).
c. Your code must work for unsigned integers of any size (4 bytes, 8 bytes, etc.).
d. To combine the values, append the least significant byte to the most significant one.
e. Your implementation should be efficient.
The following driver (and associated output) provides an example of using the function you are
expected to write. Notice that in this example an unsigned int is 4 bytes, but your function
needs to work with an unsigned int of any size.
Driver
int main() {
unsigned int value = 0xabcdfaec, lsb, msb, combined;
get_values(value, &lsb, &msb, &combined);
printf("Value: %x, lsb: %x, msb: %x, combined: %x\n", value, lsb, msb, combined);
return 0;
}
Output
Value: abcdfaec, lsb: ec, msb: ab, combined: abec
I think you want to look into bitwise and and bit shifting operators. The last piece of the puzzle might be the sizeof() operator if the question is asking that the code should work with platforms with different sized int types.
My input char array is 1024 and I have specified any local item size=32. My each workitem is working on 16 char, I have specified global item size=64. In my kernel I have calculated data_tid as follows.
int offset=16;
int gr_id=get_group_id(0);
int id = get_global_id(0);
int data_tid=(gr_id*32+id)*offset;
I am using NVIDIA Geforce 9800GT.
My code is throwing floating point exception. I already seen that this is caused by global_size not divisible by local size but in my case it is divisible.
Can anyone please tell me where is the problem?
You want to use get_local_id instead of get_global_id. The local id is the index of the work item in each group. The global id is the index of the work item in the entire range.
You could also directly use int data_tid = get_global_id(0) * offset;
I want to use MPI (MPICH2) on windows. I write this command:
MPI_Barrier(MPI_COMM_WORLD);
And I expect it blocks all Processors until all group members have called it. But it is not happen. I add a schematic of my code:
int a;
if(myrank == RootProc)
a = 4;
MPI_Barrier(MPI_COMM_WORLD);
cout << "My Rank = " << myrank << "\ta = " << a << endl;
(With 2 processor:) Root processor (0) acts correctly, but processor with rank 1 doesn't know the a variable, so it display -858993460 instead of 4.
Can any one help me?
Regards
You're only assigning a in process 0. MPI doesn't share memory, so if you want the a in process 1 to get the value of 4, you need to call MPI_Send from process 0 and MPI_Recv from process 1.
Variable a is not initialized - it is possible that is why it displays that number. In MPI, variable a is duplicated between the processes - so there are two values for a, one of which is uninitialized. You want to write:
int a = 4;
if (myrank == RootProc)
...
Or, alternatively, do an MPI_send in the Root (id 0), and an MPI_recv in the slave (id 1) so the value in the root is also set in the slave.
Note: that code triggers a small alarm in my head, so I need to check something and I'll edit this with more info. Until then though, the uninitialized value is most certainly a problem for you.
Ok I've checked the facts - your code was not properly indented and I missed the missing {}. The barrier looks fine now, although the snippet you posted does not do too much, and is not a very good example of a barrier because the slave enters it directly, whereas the root will set the value of the variable to 4 and then enter it. To test that it actually works, you probably want some sort of a sleep mechanism in one of the processes - that will yield (hope it's the correct term) the other process as well, preventing it from printing the cout until the sleep is over.
Blocking is not enough, you have to send data to other processes (memory in not shared between processes).
To share data across ALL processes use:
int MPI_Bcast(void* buffer, int count, MPI_Datatype datatype, int root, MPI_Comm comm )
so in your case:
MPI_Bcast(&a, 1, MPI_INT, 0, MPI_COMM_WORLD);
here you send one integer pointed by &a form process 0 to all other.
//MPI_Bcast is sender for root process and receiver for non-root processes
You can also send some data to specyfic process by:
int MPI_Send( void *buf, int count, MPI_Datatype datatype, int dest,
int tag, MPI_Comm comm )
and then receive by:
int MPI_Recv(void* buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status *status)