Hi guys I'm trying to calculate the checksum of a struct that I've made.
I created a struct packet that contains multiple char and int variables.
Is there a way to calculate the checksum of such a struct?
My first guess was, calculating the packet size and then use a cycle to calculate for each position its value, sum those together and then return that value.
The problem is that I don't know how to calculate each position of this packet, since there are multiple type of variable cause it's a struct.
Do you have any suggestion?
What you need here is to access each byte of the struct. You can do that by taking its address, casting it to unsigned char *, assigning the address to a variable of that type, then using the variable to loop through the bytes:
unsigned int sum = 0;
unsigned char *p = (unsigned char *)&mystruct;
for (int i=0; i<sizeof(mystruct); i++) {
sum += p[i];
}
Note however that if your struct contains any padding that the values of the padding bytes are unspecified, so that can mess with your checksum. If that's the case, then you'll need to do the above for each field in the struct individually.
For example:
unsigned int sum = 0;
unsigned char *p = (unsigned char *)&mystruct.field1;
for (int i=0; i<sizeof(mystruct.field1); i++) {
sum += p[i];
}
p = (unsigned char *)&mystruct.field2;
for (int i=0; i<sizeof(mystruct.field2); i++) {
sum += p[i];
}
Related
I'm trying to iteratively copy an unsigned char array to a uint_32t variable (in 4 byte blocks), perform some operation on the uint_32t variable, and copy it back to the unsigned char array.
Here's my code:
unsigned char byteArray[len]
for (int i=0; i<len; i+=4) {
uint32_t tmpInt = 0;
memcpy(&tmpInt, byteArray+(i*4), sizeof(uint32_t));
// do some operation on tmpInt here
memcpy((void*)(byteArray+(i*4)), &tmpInt, sizeof(uint32_t));
}
It doesn't work though. What's wrong, and how can I achieve what I want to do?
The problem is that you are adding 4 to i with each iteration and multiplying by 4. You should be using byteArray + i.
Also, as #WeatherVane pointed out below, your loop would be more consistent with a sizeof():
for (int i = 0; i < len; i += sizeof(uint32_t)).
As others pointed out you are doing too much by incrementing i as well as multiplying it by the size of your target.
On top of this
the code shown might run into a buffer overflow issue reading beyond the source array.
the sizeof operator evaluates to size_t not int.
the code repeats defining the size of the target independently several times.
Fixing all, the result might look like this:
unsigned char byte_array[len];
typedef uint32_t target_type;
const size_t s = sizeof (target_type);
for (size_t i = 0; i < (len/s)*s; i += s) {
target_type target;
memcpy(&target, byte_array + i, s);
// do some operation on target here
memcpy(byte_array + i, &target, s);
}
To avoid the typedef just define the target outside of the for-loop:
unsigned char byte_array[len];
{
uint32_t target;
const size_t s = sizeof target;
for (size_t i = 0; i < (len/s)*s; i += s) {
memcpy(&target, byte_array + i, s);
// do some operation on target here
memcpy(byte_array + i, &target, s);
}
}
An equivalent to
byte_array + i
would be
&byte_array[i]
which might be more intuitively to read.
To avoid the "strange" (len/s)*s one could step away from using an index at all, but use a pointer instead:
for (unsigned char p = byte_array; p < byte_array + len; p += s) {
memcpy(&target, p, s);
// do some operation on target here
memcpy(p, &target, s);
}
In my opinion this is a more elegant solution.
I'm using a C program on Linux to read data from a serial port.
The data to read comes from Code Composer Studio from the line: UART_writePolling(uartHandle, (uint8_t*) &value, sizeof(float));
value is the float I want to read in C, where value = 1.5.
When I read in the data from the serial port, in C, into a buffer and print with printf("%u\n", (int)buffer[i]);
I get value to be:
0
0
4294967232
63
and when I insert buffer[i] into a.array and print with
printf("%d\n", a.array[i]);
I get value to be:
0
0
-64
63
I've also tried using unions:
unsigned int value = 0;
for (int j = 3; j >= 0; j--){
//value <<= 8;
value = value + (int)a.array[i+8+j];
}
printf("value: %u\n", value);
data.u = value;
printf("(float): %f\n", data.f);
which doesn't give the correct answer.
How can I use union to get the correct data as a float?
Do I need to use <<?
EDIT: better idea of the code
//headers
typedef struct {
int *array;
size_t used;
size_t size;
} Array;
void initArray(Array *a, size_t initialSize) {
a->array = (int *)malloc(initialSize * sizeof(int));
a->used = 0;
a->size = initialSize;
}
... //more functions/code to resize array and free the memory later
union Data {
float f;
unsigned int u;
};
int main(){
union Data data;
//open serial port code
char buffer[1]; /* Buffer to store the data received,
reading one at a time */
Array a;
initArray(&a, 5); /* initialise an array to store the read data
that is read into buffer*/
//while loop to read in data for some amount of time/data
int b_read = 0;
b_read = read(fd, &buffer, sizeof(buffer));
for (int i=0; i<b_read; i++){
printf("%u\n", (int)buffer[i]);
// how the first set of values above were printed
insertArray(&a, buffer[i]);
// also adding the values read to buffer into array a
}
//end while
// close the port
for(int i=0; i<no. of elements in array a; i++){
printf("%d\n", a.array[i]);
// how the second set of results were printed
}
//below is an attempt at using union and <<:
unsigned int value = 0;
for (int j = 3; j >= 0; j--){
//value <<= 8;
value = value + (int)a.array[i+8+j]; //index used is particular to my code, where value is in a specific place in the array
}
printf("value: %u\n", value);
data.u = value;
printf("(float): %f\n", data.f);
//these printfs don't give a reasonable answer
// free memory
return 0;
}
Once the bytes are in buffer starting at offset i, you can reinterpret the bytes as a float with:
float f;
memcpy(&f, buffer+i, sizeof f);
To use a union, you could use:
union { uint32_t u; float f; } x;
x.u = value;
float f = x.f;
However, this requires that value contain all 32 bits that represent the float. When you attempted to construct the value with:
//value <<= 8;
value = value + (int)a.array[i+8+j];
There are two issues. First, value <<= 8 is needed. I presume you tried it first and did not get a correct answer, so you commented it out. However, it is required. Second, this code to insert the bytes one-by-one into value is order-dependent. Once the shift is restored, it will insert greater-addressed bytes into less-significant bits of value. Systems generally arrange bytes in objects in one of two orders: More significant bytes in lower addresses or more significant bytes in greater addresses. We do not know which order your system uses, so we do not know whether your code to insert the greater-addressed bytes in less significant bytes is correct.
Note: The above assumes that the bytes are read and written in the same order, or that issues of endianness have already been handled in other code.
You use printf with %u but cast into a int. So maybe it's not surprising to have this behavior since 2^32 = 4294967296, and 4294967296 - 64 (your second printf result) = 4294967232 (your first printf result).
Just cast into "unsigned" if you use "%u" or cast into "int" if you use "%d".
I have a char * who points to the structure. Here is my structure:
struct prot
{
int size;
unsigned short codeAction;
void *data;
};
I recovered size and codeAction, but now I want to recover data.
And when I cast my last 8 bytes I have nothing in it.
The following code is just a test, it's a bad code:
char lol[4];
for (int i = 0; i < 4; i++)
lol[i] = test[i];
int size = *(int*)lol;
char loli[2];
int index = 0;
for (int i = 4; i < 6; i++)
{
loli[index] = test[i];
index++;
}
int code = *(short*)loli;
char lolo[8];
index = 0;
for (int i = 6; i < size; ++i)
{
lolo[index] = test[i];
index++;
}
void *newData = (char *)lolo; // how can I cast it?
How I can display the content of newData?
Your problem is that when casting lolo you actually cast a pointer to the char array you defined. So the result of the cast would be a char pointer to the first cell of the array.
Why don't you just use this as a struct and access the fields regularly?
Anyway, you want to use lolo as a 64 bit type pointer and the access what's in it.
void* newData = *((uint64_t*)lolo)
Besides, don't loop until size in the last for loop, loop only 8 times, until lolo is full. The number of bytes in newData itself (not what it points to) is constant, and is 4 bytes on 32bit machines, 8 bytes on 64bit ones.
Last thing - index++, not o++. o isn't defined, as much as I can see.
What's the best way to concatenate unsigned char arrays in C? Furthermore, is there a way to concatenate unsigned char arrays with char arrays? 2 of these unsigned char arrays are really just strings, but for simplicity, I'm treating them as unsigned char arrays.
The requirement is complex: there is a function that will take 1 (one) unsigned char array. That one unsigned char array is really 4 variables concatenated to make up that 1 unsigned char array. To add to the complexity, the first unsigned char array is really just a string of variable length, but its max length is 60 (i.e. sometimes it would have length = 15, other times = 60).
someFunctionAssignsFirst(unsigned char *first)
{
//it could be 15 or 60 chars long.
...
}
unsigned char first[60] = //someFunctionAssignsFirst() //This is a string i.e. "variable size string max size 60"
unsigned char second[8] = "always8."; //This is a string i.e. "01234567"
unsigned char third[32] = "always32"; //This is a cryptographic key
unsigned char fourth[32] = "always32"; //This is a cryptographic key
How would I go about getting:
unsigned char allstrings[sizeof(first)+sizeof(second)+sizeof(third)+sizeof(fourth)] = //all strings combined
?
I attempted some for loops, but the variable length first is disrupting the concatenation, and I'm sure there has to be a better way.
Full Disclosure: I'm not an expert, and I don't necessarily love C. Also for the requirement, not allowed C++ or any other language.
This is what I was trying to do, and (for clarification) I don't get a null character at the end so it's not really a string.
unsigned char *first = "this is a sample string, human readable";
unsigned char *second = "12345678" //always a number
//unsigned char third -> I have the value from before and it's a key
//unsigned char fourth -> I have the value from before and it's a key
unsigned char allstrings[sizeof(first) + sizeof(second) + sizeof(third) + sizeof(fourth)];
int counter = 0;
for (int i = 0; i <= sizeof(first); i++)
{
allstrings[counter] = first[i];
counter++;
}
for (int i = 0; i <= sizeof(second); i++)
{
allstrings[counter] = second[i];
counter++;
}
for (int i = 0; i <= sizeof(third); i++)
{
allstrings[counter] = third[i];
counter++;
}
for (int i = 0; i <= sizeof(fourth); i++)
{
allstrings[counter] = fourth[i];
counter++;
}
The allstrings variable, doesn't get anything beyond "readable" in my example above.
You need to use strcpy to copy over the first part, which is a string, then use memcpy to copy over the other 3, which are not strings but char arrays.
Note that the result is not a string but a char array, i.e. it is not null terminated.
unsigned char allstrings[strlen(first)+sizeof(second)+sizeof(third)+sizeof(fourth)];
strcpy(allstrings,first);
memcpy(allstrings+strlen(first),second,sizeof(second));
memcpy(allstrings+strlen(first)+sizeof(second),third,sizeof(third));
memcpy(allstrings+strlen(first)+sizeof(second)+sizeof(third),fourth,sizeof(fourth));
I guess you want to treat the array as buffer.
So it's fine to have the declarations,
but you don't need to define the content for this moment:
unsigned char first[60];
unsigned char second[8];
unsigned char third[32];
unsigned char fourth[32];
#define ALLSTRLEN sizeof(first) + sizeof(second) + sizeof(third) + sizeof(fourth)
unsigned char allstrings[ALLSTRLEN];
The code will keep the fixed size of arrays. and please notice that the arrays should be global or static for safety reasons.
Then you can copy the contents to arrays. I just put your code under main() to concatenate these arrays:
int main()
{
strcpy((char *)first, "this is a sample string, human readable");
// do something for second, third, fourth....
//
int counter = 0;
// first array is a normal string, we have to copy null character for it
for (int i = 0; i <= strlen((char *)first)+1; i++)
{
allstrings[counter] = first[i];
counter++;
}
for (int i = 0; i <= sizeof(second); i++)
{
allstrings[counter] = second[i];
counter++;
}
for (int i = 0; i <= sizeof(third); i++)
{
allstrings[counter] = third[i];
counter++;
}
for (int i = 0; i <= sizeof(fourth); i++)
{
allstrings[counter] = fourth[i];
counter++;
}
// allstrings is finished
}
Please notice this example just works in main() function; if you call a function to concatenate four arrays, the compiler has to pass the arrays as pointers, and the sizeof() will be wrong (equal to the pointer's size).
You can test the size by doing this:
printf("sizeof(second)=%d\n", sizeof(second));
I've tried many different variants of both of the ways, and cannot get it to sum the array. It's passed into the function as a pointer and need to calc the mean and return.
unsigned char calcMean(unsigned char *buffer, int height, int width)
{
unsigned char mean, sum=0;
counter i, k;
int size;
size = width*height;
mean = 0;
for (i = 0; i < size; i++) {
sum += buffer[i];
}
/*
for(k=0;k<(width*height);k++)
{
mean = mean + *Buffer;
frameBuffer++;
printf("%d\n", mean);
}
*/
return sum;
}
mean can be a char, since you would divide by size to calculate it. But the sum itself can be as high as size * 255 (8bits of unsigned char).
width * height can overflow too.
To be at the safe side (on 32 or 64bit machines), consider this:
16bit * 16bit integer would require a 32bit integer (for the dimension).
An unsigned char array with a size up to 4294967295 would then require 64bits for the sum.
So, if possible, use explicit integer sizes (C99):
uint8_t calcMean(uint8_t *buffer, uint16_t height, uint16_t width)
{
uint64_t sum=0;
size_t i;
size_t size;//size_t is large enough to store a pointer,
//so it would have 32 or 64bits on corresponding platforms
//(see uintptr_t, etc.)
size = ((size_t)width)*((size_t)height);
for (i = 0; i < size; i++) {
sum += buffer[i];
}
return sum / size;
}
To guard yourself from buffer overflow for the buffer[] array, assuming you have array defined and initialized with values, pass size of array along with pointer to array's first element rather than the method you have currently to calcMean().
Call to calcMean() shall be:
uint8_t array_name[3]={1,2,3}; /* Array size and values assumed */
...
calcMean(array_name, sizeof(array_name))
calcMean() definition shall be :
uint8_t calcMean(uint8_t *buffer, size_t buffer_size)
{
uint64_t sum=0;
size_t i;
for(i = 0; i < buffer_size; i++)
{
sum += buffer[i];
}
..../*Do whatever you want if at all needed */
return (sum / size);
}