return byte array with Arduino

Question

I have an array that contains bytes of information (a payload), but I want to use this array in two functions. How can I return it? I don't know if I have to declare it different or in other place.

My code:

void loop() {
    byte payload[] = find_fix();
    delay(5*60000);
}


byte find_fix(){
    byte payload[9];

    double lati = sodaq_gps.getLat();
    double longi = sodaq_gps.getLon();
    int nsatis = sodaq_gps.getNumberOfSatellites();
    long lax = lati * 10000000;
    long lox = longi * 10000000;

    payload[0] = (byte) ((lax & 0xFF000000) >> 24 );
    payload[1] = (byte) ((lax & 0x00FF0000) >> 16 );
    payload[2] = (byte) ((lax & 0x0000FF00) >> 8 );
    payload[3] = (byte) ((lax & 0X000000FF));
    payload[4] = (byte) ((lox & 0xFF000000) >> 24 );
    payload[5] = (byte) ((lox & 0x00FF0000) >> 16 );
    payload[6] = (byte) ((lox & 0x0000FF00) >> 8 );
    payload[7] = (byte) ((lox & 0X000000FF));
    payload[8] = (byte) (nsatis);
    SerialUSB.print(" MIGUEL. LATITUD: ");
    SerialUSB.print(payload[0], HEX);
    SerialUSB.print(payload[1], HEX);
    SerialUSB.print(payload[2], HEX);
    SerialUSB.println(payload[3], HEX);
    SerialUSB.print(" MIGUEL. LONGITUD: ");
    SerialUSB.print(payload[4], HEX);
    SerialUSB.print(payload[5], HEX);
    SerialUSB.print(payload[6], HEX);
    SerialUSB.println(payload[7], HEX);
    SerialUSB.print(" MIGUEL. Num Sats: ");
    SerialUSB.println(payload[8], HEX);

    return payload[];
}

I want to use 9 bytes of information, first I declare it byte payload[9]; and then I start writing on it. This works well, but now I want to return it for use it in another functions but I can't.

Stefan Bossbaly · Accepted Answer · 2019-10-23 19:21:09Z

4

You can't return stack allocated memory from a function. Once the function call ends that data will be reclaimed when the stack pointer is updated. You need to either dynamically allocate the memory on the heap like so ...

byte* find_fix(){
    byte* payload = new byte[9]; // Allocate memory on the heap

    return payload;
}

void loop() {
    byte* payload = find_fix();
    delete[] payload; // Free memory once it is no longer needed
}

Or if you know the max size of the array, you can pass in memory to the find_fix function like so ...

void find_fix(byte* payload) {
    ....
}

void loop() {
    byte payload[9];
    find_fix(payload);
}

edited Oct 23, 2019 at 19:21

answered Oct 23, 2019 at 19:03

Stefan Bossbaly

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4 Comments

Lleims Over a year ago

How I know the max size I'm using your second option, but appears this error invalid conversion from 'byte* {aka unsigned char*}' to 'uint32_t {aka long unsigned int}' [-fpermissive]

Stefan Bossbaly Over a year ago

How are you calling the function? The compiler is upset that the find_fix(byte* payload) is expecting a byte* {aka unsigned char*} parameter but you are providing a uint32_t

Lleims Over a year ago

find_fix(payload); the same as you put

Stefan Bossbaly Over a year ago

I just compiled a test program ...

#include <stdio.h>  void find_fix(unsigned char* payload) {     printf("%x\n", payload[0]); }  int main() {     unsigned char payload[9];     payload[0] = 0xFF;     find_fix(payload);     return 0; }

with $ gcc -Wall test.c -o test and it works just fine. Must be something with the code. What type is the payload variable. Maybe change it to unsigned char?

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