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Re: [avr-libc-dev] const struct in program memory

From: avr-libc-devel
Subject: Re: [avr-libc-dev] const struct in program memory
Date: Mon, 17 Jun 2019 12:38:16 +0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1

You certainly can use PROGMEM (or the newer __flash) on structs in program memory.  But you need to use the right methods to access the data.  Can you post a short sample code that you tried but which did not work?

typedef struct {
        uint16_t int_part;
        uint8_t frac_part;
        float fl_part;
} __attribute__ ((packed)) bw_data;

const bw_data bw_tab[] PROGMEM;

const bw_data bw_tab[] PROGMEM = {
        [BW_7_8] = { //7.81
                .int_part = 7,
                .frac_part = 81,
                .fl_part = 7.81f,
        .... skip ....

float symbols_time_ms(const param_t *const par, const uint8_t symbols_num){
        float temp;
        temp = 1 << (par->sf + 6); //2^SF
temp = (float) temp * (float) symbols_num / bw_tab[par->bw].fl_part;
        return temp;


When I call the function symbols_time_ms (), the result is incorrect if the PROGMEM modifier is uncommented.
Thank you very much for your answer and attention to my question.

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