cast increases required alignment of target type with pointer

Question

I am working on an ARM project (SAMD21J18A) which reads a Bitmap file. Per the Bitmap specs, a bitmaps height will be a 4 byte long signed integer. In my struct, I am declaring the height as int32_t. Per the BMP specs, the height can be negative, but I also need to determine the actual height because the height's negative value can be expressed in "Two's complement" (correct me if I am wrong). To do this, I am reading the buffer like so (index 22-25 are the 4 bytes for the height)

bmp.height = *(int32_t*)&header_buff[22];

However, I receive the following compiler warning:

cast increases required alignment of target type [-Wcast-align]

I'm using the arm-none-eabi-gcc compiler.

Here is snippet of my code:

struct bitmap_t  {  
  int32_t width;
  int32_t height;       
  /* other properties left out for simplicity */
};

I am loading the first 54 bytes into a buffer to read the header:

struct bitmap_t bmp;
int8_t header_buff[54];
/* code to read the bmp here */
bmp.height = *(int32_t*)&header_buff[22];

@user3386109 - Thanks you, but that poses another issue (perhaps needed in another post). If I do `bmp.height = (header_buff[25] << 24) | (header_buff[24] << 16) | (header_buff[23] << 8) | header_buff[22];` the value is represented in `Two's complaint notation`, which is not the correct value. — PhillyNJ, Dec 02 '17 at 21:59
Make your comment an answer, which I believe answers by question - I will open another post, to address my `Two's complaint notation` issues. Thx — PhillyNJ, Dec 02 '17 at 22:04
Is autocorrect changing "Two's complement" to "Two's complaint"? — pat, Dec 02 '17 at 22:10

score 2 · Accepted Answer · answered Dec 02 '17 at 22:05

2

If header_buff is aligned on a 4-byte boundary, then header_buff[22] is not on a 4-byte boundary. So you can't read the height like that. You need to read the bytes one at time and use shift and OR operations to recreate the height.

answered Dec 02 '17 at 22:05

user3386109

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score 2 · Answer 2 · answered Dec 02 '17 at 22:06

2

You might just use memcpy to extract the value from header_buff while preserving its byte ordering:

memcpy(&bmp.height, &header_buff[22], sizeof(bmp.height));

answered Dec 02 '17 at 22:06

Grzegorz Szpetkowski

36,988
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score 1 · Answer 3 · answered Sep 25 '18 at 22:20

Using structures:

#pragma pack(push, 1)
struct S {
    int32_t d;
};
#pragma pack(pop)

int32_t get_doubleword_packet_with_struct(uint8_t *data) {
    S *s = (S *)data;
    return s->d;
}

Using memcpy:

int32_t get_d_packet_with_memcpy(uint8_t *data) {
    int32_t dst;
    memcpy(&dst, data, sizeof(dst));
    return dst;
}

My preferred way:

Define complete header into packed structure then retype bitmap into this structure, or put this structure into union with byte array and to this array copy header from bitmap.

#pragma pack(push, 1)
struct BitmapHeader {
    // ... here will be all fields from bitmap header
    int32_t width;
    int32_t height;
    // ....
};
#pragma pack(pop)

union Bmp {
    BitmapHeader header;
    uint8_t raw_data[sizeof(BitmapHeader)];
} bmp;

// copy data into bmp.raw_data array
// access fields from bmp.header.height ..

If you use cortex M3, M4, M7 or M33, it is possible to disable the warning, it will work. But on Cortex M0, M0+ and M23 it will not work, because unaligned access is not supported on these cores. But all my examples on M0 and M0+ will work, and compiler will use memcpy or just make 4 x read byte of the 32 bit number. This only if the struct is packed.

cast increases required alignment of target type with pointer

3 Answers3