从'const char *'分配给'char'使得integer从指针没有强制转换

Question

I am very new to C and am encountering an issue while trying to store my next_frame in a variable. Any help would be great as I think this is probably something simple I'm just missing.

If I just change the following it works fine, only when I try to store the next_frame in a variable does it not compile.

// Doesn't compile
oled_write_raw_P(next_frame, FRAME_SIZE);

// Compiles
oled_write_raw_P(frames[abs((FRAME_COUNT - 1) - current_frame)];, FRAME_SIZE);

Full Code

#define FRAME_COUNT 5 // Animation Frames
#define FRAME_SIZE 256
#define FRAME_DURATION 200 // MS duration of each frame

// Variables
uint32_t timer = 0;
uint8_t current_frame = 0;
char next_frame;

static void render_animation(void) {
    static const char PROGMEM frames[FRAME_COUNT][FRAME_SIZE] = {
        // Images here, removed for example
    };

    // If timer is more than 200ms, animate
    if (timer_elapsed32(timer) > FRAME_DURATION) {
        timer = timer_read32();
        current_frame = (current_frame + 1) % FRAME_COUNT;
        next_frame = frames[abs((FRAME_COUNT - 1) - current_frame)];

        // Set cursor position
        oled_set_cursor(128, 0);

        // Write next frame
        oled_write_raw_P(next_frame, FRAME_SIZE);
        
    }
}

These are the errors:

error: assignment to 'char' from 'const char *' makes integer from pointer without a cast [-Werror=int-conversion] next_frame = frames[abs((FRAME_COUNT - 1) - current_frame)];

error: passing argument 1 of 'oled_write_raw_P' makes pointer from integer without a cast [-Werror=int-conversion] oled_write_raw_P(next_frame, FRAME_SIZE);

Answer 1

The line

next_frame = frames[abs((FRAME_COUNT - 1) - current_frame)]

does not make sense.

The variable next_frame , to which you are assigning a value, has the type char . However, you are assigning it the expression

frames[abs((FRAME_COUNT - 1) - current_frame)]

which decays to a pointer to the first element of the sub-array, so the expression evaluates to a value of type const char * .

I'm not exactly sure what you want to accomplish, but I guess the solution to your problem is to change the type of next_frame to const char * , so that the types match. In order to do this, you can change the line

char next_frame;

to:

const char *next_frame;