How to Map Data in embedded systems? - c

Question

I am asking the question for the second time because the first time I apparently did not manage to get my request across (insert a sad smiley here).The previous post has been deleted as I thought it could just add to the confusion if I just edit it.
Problem: I am getting a packet of data (payload) from a data source. This data is given to me in an array and I would like to split this data (like in a gateway) and send it to other peripherals.
The package data I received could look like this:
uint8_t received_payload[] = {0x11, 0x22, 0x33, 0x44, 0x55, 0x66};
It is noticeable that what is behind this data is not apparent. This is because it can be measurement data, which can sometimes come from several sensors and these measurement values ​​were thrown together in a data package.
I can assume that the measured values ​​come periodically and that I can recognize data packets based on an ID. (This does not matter here, however).
What is interesting for me now is that I want to pass on these measured values. Under certain circumstances split into two or more separate messages.
The order and position should be given somehow and somewhere.
In the following I have shown an example of how these can be listed. I would like to emphasize again that this is only an EXAMPLE and that I have freely come up with these values. I just want to show visually how the data could be swapped under certain circumstances.
uint8_t totransmit_payload_1[] = {0x11, 0x22, 0x33};
uint8_t totransmit_payload_2[] = {0x44, 0x66, 0x55};
So I would like to have something stored somewhere that corresponds to the following information content:
totransmit_payload_1[0] = received_payload[0];
totransmit_payload_1[1] = received_payload[1];
totransmit_payload_1[2] = received_payload[2];
totransmit_payload_2[0] = received_payload[3];
totransmit_payload_2[1] = received_payload[5];
totransmit_payload_2[2] = received_payload[4];
This process should work bidirectionally and have a kind of dynamic because it won't be possible for me to write "how the data is split up" in the code.(Before questions arise. When I boot my system via SPI, I get a configuration file from a MicroSD card which describes how my system should behave.)
Of course I have already tried to find solutions on the World Wide Web, but I find it difficult to find something for my low level application. I am on a microcontroller of the STM class and only program in C.
What I found that would fit my application would be a kind of mapping.
Here as references: What Is Data Mapping?
Do you guys or girls have experience with something like that? Or ideas on how something like this could work efficiently?
Thanks for your help :D
Edit: I just want to know the best way to save in C which position in an array references to another position in another array.
Without hardcoding it.

Answer 1

Seems like you you start asking about data copying, then change midway to data mapping? Anyway, these are small examples of both:
#include <string.h> // for memcpy
...
void CopyBytes(const* uint8_t received_payload, uint8_t* totransmit_payload_1, uint8_t* totransmit_payload_2)
{
// of course you'll need to make sure these buffers are appropriately sized
memcpy(totransmit_payload_1, received_payload, 3);
memcpy(totransmit_payload_2, received_payload + 3, 3);
}
Learn more about memcpy on the man page.
If you want some data mapping instead, I would create space for the actual data in one place (the receive buffer for instance), then you can have as many pointers as you want pointing to that data:
uint8_t received_payload[] = {0x11, 0x22, 0x33, 0x44, 0x55, 0x66};
uint8_t* totransmit_payload_1[3]; // creates an array of uint8_t _pointers_
uint8_t* totransmit_payload_2[3]; // does the same
// now you can do your mapping
totransmit_payload_1[0] = &(received_payload[0]); // save the _address_ of received_payload[0] to totransmit_payload_1[0]
totransmit_payload_1[1] = &(received_payload[1]); // (don't think the parentheses are necessary, but I like them to be perfectly clear the [] dereference happens first)
totransmit_payload_1[2] = &(received_payload[2]);
totransmit_payload_2[0] = &(received_payload[3]);
totransmit_payload_2[1] = &(received_payload[5]);
totransmit_payload_2[2] = &(received_payload[4]);
Now, any time data changes in received_payload, you can access it from the totransmit_payload structures since they point to that data. For example:
received_payload[0] = 5;
printf("received_payload[0] = %" PRIu8 "\n", *(totransmit_payload_1[0])); // prints 5;
// It's "bi-directional", you can dereference the pointer and assign a value to that memory location, then access it from received_payload
*(totransmit_payload_2[2]) = 17;
printf("received_payload[4] = %" PRIu8 "\n", received_payload[4]); // prints 17