Calling a function with a dynamically allocated array pointer

Question

I'm trying to call the following function which should fill an array with sensors' data and return the number of detected sensors:

uint8_t ds18b20_read_all(uint8_t pin, ds_sensor_t *result)

How am I supposed to call it without knowing the number of sensors? I thought about:

ds_sensor_t *result = (ds_sensor_t *) malloc(NUM_OF_SENSORS * sizeof(ds_sensor_t));
uint8_t count = ds18b20_read_all(pin, result);

But again I don't know NUM_OF_SENSORS. Should I just choose a large enough number?

EDIT: The source code of the function: https://github.com/SuperHouse/esp-open-rtos/blob/master/extras/ds18b20/ds18b20.c

I think you can use realloc function every time you read data of new sensor. en.cppreference.com/w/c/memory/realloc It means that you can addyour sensors one by one to result — Gor Asatryan
– Gor Asatryan, Commented Oct 14, 2017 at 16:45
I think there should be a constant that says you max available mumber of sensors (something like MAX_NUM_OF_SENSORS). Or something like this... Do you have source code of function ds18b20_read_all ? — Sandro
– Sandro, Commented Oct 14, 2017 at 16:48
The source code is available at github.com/SuperHouse/esp-open-rtos/blob/master/extras/ds18b20/… The documentation says nothing about how to call // Scan all ds18b20 sensors on bus and return its amount. // Result are saved in array of ds_sensor_t structure. — Mouk
– Mouk, Commented Oct 14, 2017 at 16:56

Ahmed Masud · Accepted Answer · 2017-10-14 17:13:10Z

3

Max sensors on 1-wire is 75. So you can have up to 75 ds18b20s attached. You can also query the wire to find out how many sensors are attached using int ds18b20_scan_devices(int pin, ds18b20_addr_t *addr_list, int addr_count); set addr_count = 75.

answered Oct 14, 2017 at 17:13

Ahmed Masud

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isp-zax · Accepted Answer · 2017-10-14 17:14:57Z

2

The header says, that the function you are asking about is an obsolete API. That usually means you should not be using it if replacement is provided:

// The following are obsolete/deprecated APIs ... uint8_t ds18b20_read_all(uint8_t pin, ds_sensor_t *result);

I suspect it's deprecated for the very reason you have questions about:. It's unclear how to use it.

New APIs (below is one of them) has addr_count as a parameter:

bool ds18b20_read_temp_multi(int pin, ds18b20_addr_t *addr_list, int addr_count, float *result_list);

answered Oct 14, 2017 at 17:14

isp-zax

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1 Comment

Mouk Over a year ago

Ok, I missed that somehow. It makes sense. I thought I missed something in the C language.

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