Dereferencing char into a struct

Question

I am sending some data over an NRF24 link, using the RF24Network library. So far I have only sent numbers, now I also want to send a string, so I have set up my payload struct as

struct payload_t {                  // Structure of our payload
   unsigned long data;
   unsigned long datatype;
   char sensorid[20];
   unsigned long counter;
};

As far as I do understand, I can not use string here, since I need to know the length of the data I am sending.

At the moment I am using sensors without an internal ID, so in loop(), I am calling:

 writeToNetwork(voltage,'v',"nosid",packets_sent++);

My writeToNetwork function is defined as

 bool writeToNetwork(unsigned long data,unsigned long type,char sid[20],unsigned long package){

and within that, I am setting up the payload with

 payload_t payload = { data,type,sid, package };

Which gives me an

 error: invalid conversion from ‘char*’ to ‘char’ [-fpermissive] 

Clearly, I have no interest of the address the pointer is referencing, so I need to somehow dereference the sid - pointer to get my array into the struct. How can I do that?

Or am I barking up the wrong tree? I tried to transfer the id as a string:

#include <string.h>
bool writeToNetwork(unsigned long data,unsigned long type,String sid,unsigned long package){
char sidchar[20];
sid.toCharArray(sidchar,20);
payload_t payload = { data,type,sidchar, package };

Which gives me the error

array must be initialized with a brace-enclosed initializer

Which I do not understand. Doing

char sidchar[20]={0};

gives me the same error.

Answer 1

In your situation, your best bet is probably to use the standard C function strcpy:

payload_t payload;
char* source = "abcdef";
strcpy(payload.sensorid, source);

Note that source string does not have to be declared as char source[20] and can be declared as the rather standard C string char*.

Also, since you are limiting the size of the string to be passed in payload, you may prefer to use strncpy which will ensure no more than 20 characters get copied:

payload_t payload;
char* source = "abcdef";
strncpy(payload.sensorid, source, 20);
// Explicitly add a string termination to the end of string
payload.sensorid[19] = 0;

In the snippet above, please notice the last line, it is necessary in the situation where source string length is 20 or more non-null characters, as in this case, strncpy will copy all 20 characters but will not add the terminating 0 at the end of the string.