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I got this C language code from the client,which is about message encapsulation,but I don't have a good understanding of this code.
Can someone please explain what it's doing?
(*(pucBuf)++) = (unsigned char) (usValue >> 8);
can be read as:
get the value of usValue and right-shift it 8 bits;
cast that to an unsigned char type;
store it into the memory location pointed to by the pubBuf pointer;
advance pucBuf to point to the next sequential item of its type.
Most likely it's taking the high-order eight bits of a sixteen(-or-more)-bit value and storing that into a memory buffer, within some sort of loop.
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In a recent interview, I was asked the following coding problem. Could someone please help me understand how to address this problem in terms of C coding?
Q.
A processor has a 16*16 multiplier to represent 32-bit value.
Take two 32 bit value, multiply them and return in 64 bit format.
Let's call the 32 bit numbers A and B. Now, let's split them into 16 bit numbers, a0/b0 and a1/b1 (a0 is the bigger part).
Now, A*B == (a0<<16+a1) * (b0<<16+b1) == (a0 * b0) << 32 + (a1 * b0 + a0 * b1) << 16 + a1 * b1.
note: All the multiplications here are for 16 bit numbers (a0,a1,b0,b1) with the result as 32 bit number casted into 64 bit (the bigger 32 bits of the number are 0).
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this is a part of a C program that i didn't understand :
unsigned short twittlen;
int x;
x = atoi(argv[1]);
twittlen = x;
if(twittlen >= 64) {
printf("Nope , You don't know about Integer");
return -1;
}
if (x >= 64 )
printf("you got it ");
My problem is how to find an int that is greater than 64 but when converting it to unsigned short it will be less than 64 !
I looked alot about limit of those types of integers even on stackoverflow but i didn't find the answer about this !
Thanks in advance :)
When you assign an int value to an unsigned short, truncation will occur.
Assuming int is 4 bytes and short is 2, you simply need to provide the program with a value greater than 64 whose lower two bytes are less than 64.
The maximum unsigned 16-bit value is 65535:
0x0000FFFF
So if you enter the number 65536:
0x00010000
When truncated to just two bytes:
0x0000
Is zero.
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Does anybody know how I can do a correct conversion from a uint16 to a uint8 in a C program for my STM32F091?
I thought that it was like this:
uint16_t Test = 0x565;
uint8_t Test2 = (uint8_t)Test;
But that doesn't work very well, so does anyone has a suggestion?
[update from comment:]
I use a potentiometer that has a minimum value of 0x0 and a maximum value of 0xFFF. Test must have the value of ADC1->DR (this is a value from the ADC in the STM32F0)
Edit:
You cannot compress the data without information lost in this case. Since your data range from ADC is 0x000 to 0xFFF - there is no way for you to represent the data perfectly without information/precision lost in uint8 byte whose range is only from 0x00 to 0xFF
But you still can map the data with precision lost. For example:
uint8_t Test2 = (unsigned)(Test >> 4);
Will cause you to map every 16 range to a single value:
0-15 -> 0
16-31 -> 1
32-47 -> 2
//and so on
Since you only have 8 LEDs anyway, you could use uint8 byte to control the LED - though you will lose precision of the real value, but you are still able to represent the range of the value (per unit of 16) correctly with your 8 LEDs (since you only have 8 LEDs anyway)
Original:
The case is very likely cause by overflow when you cast bigger-sized uint16 data type to smaller-sized uint8:
uint16_t Test = 0x565;
uint8_t Test2 = (uint8_t)Test; //Test2 value is only 0x65, 0x500 is missing
The casting causes the most significant byte value (0x500) to be truncated.
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I tried to experiment with bit manipulations on a byte.
At first I tried to say that I have 1111 1111 (256) and 1000 0000(128).
So I do this:
printf("%u\n", 256 & 128);
I expect to get 128 but I get 0.
So I tried:
printf("%u\n", ((unsigned char) 256) & ((unsigned char) 128));
But that gives me the same result.
What is wrong with that?
1111 1111 is 255
So try
printf("%u\n", 255 & 128);
^^^
Take into account that the type of the integer constants 255 and 128 is int.
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I'm playing around attempting to check specific bytes within a packet's payload and test if they are >,<,!, = to a specified value. The boundary points are dynamic, what is the best way to evaluate the bytes between them?
For example, I have a packet with a payload (following the headers of course) and I want to evaluate between byte 5 and byte 10 to see if it is greater than some specified value.
The return value of memcmp() does a nice unsigned little endian compare.
return memcmp(&packet[IndexLo], &Reference, IndexHi - IndexLo + 1);
A portable method would simple compare 1 byte at a time.
Quick method but has a number of assumptions:
. Packet data and platform same endian and Little.
. Boundary_width <= sizeof inttype.
. unsigned arithmetic.
. Optimized for Packet_CompareMask().
. Accessing a width integer on any byte boundary OK.
. OK to access memory just past end of packet.
typedef uint64_t inttype;
int Packet_CompareMask(const char *packet, size_t IndexLo, unint64_t Mask, inttype Reference) {
// This fails on machine with alignment restricts on wide integers.
inttype x = *((inttype *) &packet[IndexLo]);
x &= Mask;
if (x < Reference) return -1;
return x > 0;
}
int Packet_CompareRange(const void *packet, size_t IndexLo, size_t IndexHi, inttype Reference) {
inttype Mask = 0;
ssize_t Diff = IndexHi - IndexLo;
if ((Diff <= 0) || (Diff > (sizeof(Mask) - 1))) {
; // handle error
}
// This only works on little endian machines. A variant would work with Big endian.
while (--Diff >= 0) {
Mask <<= 8;
Mask |= 0xFF;
}
return Packet_CompareRange(packet, IndexLo, Mask, Reference);
}