BMP File C implementation - c

Question

I am trying to implement creating an image file in C. The format of the image I want to be is RGB565. I have followed these two websites 1 2 to try and get a viewable image but I do not seem to get it working. Is the problem in the "fwrite" function because of the little endian thing?
For testing purposes I want an image that is 16*4 large and all pixels are of the same color. (The code is a mess as it is just for testing purposes before implementing it in a bigger project.
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
int main() {
uint16_t myData[64];
for(int i = 0; i<64; i++)
{
myData[i] = 0x241B; //Random Color
}
typedef struct /**** BMP file header structure ****/
{
uint16_t bfType; /* Magic number for file */
uint32_t bfSize; /* Size of file */
uint16_t bfReserved1; /* Reserved */
uint16_t bfReserved2; /* ... */
uint32_t bfOffBits; /* Offset to bitmap data */
} BITMAPFILEHEADER;
typedef struct /**** BMP file info structure ****/
{
uint32_t biSize; /* Size of info header */
uint32_t biWidth; /* Width of image */
uint32_t biHeight; /* Height of image */
uint16_t biPlanes; /* Number of color planes */
uint16_t biBitCount; /* Number of bits per pixel */
uint32_t biCompression; /* Type of compression to use */
uint32_t biSizeImage; /* Size of image data */
uint32_t biXPelsPerMeter; /* X pixels per meter */
uint32_t biYPelsPerMeter; /* Y pixels per meter */
uint32_t biClrUsed; /* Number of colors used */
uint32_t biClrImportant; /* Number of important colors */
} BITMAPINFOHEADER;
BITMAPFILEHEADER myFileHeader = {0x4D42, 198, 0, 0, 70};
BITMAPINFOHEADER myInfoHeader = {40, 16, 4, 1, 16, 0, 0, 1000, 1000, 0, 0};
uint32_t RGB565ColorTable[] = {0x7E00000, 0xF8000000, 0x001F0000, 0};
FILE *fptr; fptr = fopen("testImage.bmp","w+");
if(fptr == NULL) {
printf("Error!");
exit(1); }
fwrite(&myFileHeader, sizeof(BITMAPFILEHEADER), 1, fptr);
fwrite(&myInfoHeader, sizeof(BITMAPINFOHEADER), 1, fptr);
fwrite(&RGB565ColorTable, sizeof(RGB565ColorTable), 1, fptr);
fwrite(&myData, sizeof(myData), 1, fptr);
fclose(fptr);
return 0; }
Hex Output (after adding attribute((packed)) to structs ):

Answer 1

I did clean up the code. And it looks like it is working:
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
typedef struct /**** BMP file header structure ****/
{
uint16_t bfType; /* Magic number for file */
uint32_t bfSize; /* Size of file */
uint16_t bfReserved1; /* Reserved */
uint16_t bfReserved2; /* ... */
uint32_t bfOffBits; /* Offset to bitmap data */
} __attribute__((packed)) BITMAPFILEHEADER;
typedef struct /**** BMP file info structure ****/
{
uint32_t biSize; /* Size of info header */
uint32_t biWidth; /* Width of image */
uint32_t biHeight; /* Height of image */
uint16_t biPlanes; /* Number of color planes */
uint16_t biBitCount; /* Number of bits per pixel */
uint32_t biCompression; /* Type of compression to use */
uint32_t biSizeImage; /* Size of image data */
uint32_t biXPelsPerMeter; /* X pixels per meter */
uint32_t biYPelsPerMeter; /* Y pixels per meter */
uint32_t biClrUsed; /* Number of colors used */
uint32_t biClrImportant; /* Number of important colors */
} __attribute__((packed)) BITMAPINFOHEADER;
int main()
{
FILE *fptr;
BITMAPFILEHEADER myFileHeader = {
.bfType = 0x4D42,
.bfSize = 198,
.bfOffBits = 70
};
BITMAPINFOHEADER myInfoHeader = {
.biSize = 40,
.biWidth = 16,
.biHeight = 4,
.biPlanes = 1,
.biBitCount = 16,
.biCompression = 0,
.biSizeImage = 0,
.biXPelsPerMeter = 1000,
.biYPelsPerMeter = 1000,
.biClrUsed = 0,
.biClrImportant = 0
};
uint32_t RGB565ColorTable[] = {
0x7E00000, 0xF8000000, 0x001F0000, 0
};
uint16_t myData[64];
for(int i = 0; i<64; i++)
{
myData[i] = 0x241B; //Random Color
}
fptr = fopen("testImage.bmp","w+");
if(fptr == NULL) {
printf("Error!");
exit(1);
}
fwrite(&myFileHeader, sizeof(myFileHeader), 1, fptr);
fwrite(&myInfoHeader, sizeof(myInfoHeader), 1, fptr);
fwrite(&RGB565ColorTable, sizeof(RGB565ColorTable), 1, fptr);
fwrite(&myData, sizeof(myData), 1, fptr);
fclose(fptr);
return 0;
}
I do have the following output :
hexdump -C testImage.bmp
00000000 42 4d c6 00 00 00 00 00 00 00 46 00 00 00 28 00 |BM........F...(.|
00000010 00 00 10 00 00 00 04 00 00 00 01 00 10 00 00 00 |................|
00000020 00 00 00 00 00 00 e8 03 00 00 e8 03 00 00 00 00 |................|
00000030 00 00 00 00 00 00 00 00 e0 07 00 00 00 f8 00 00 |................|
00000040 1f 00 00 00 00 00 1b 24 1b 24 1b 24 1b 24 1b 24 |.......$.$.$.$.$|
00000050 1b 24 1b 24 1b 24 1b 24 1b 24 1b 24 1b 24 1b 24 |.$.$.$.$.$.$.$.$|
*
000000c0 1b 24 1b 24 1b 24 |.$.$.$|

Answer 2

You need to pack the file header struct so that the compiler won't pad the entries. You can use __attribute__((packed)) with gcc when defining the structs to accomplish this.

Answer 3

With MSVC you can enclose a struct with
#pragma pack(push, 1)
// struct
#pragma pack(pop)
to make it packed. I have found it is only needed for BITMAPFILEHEADER on a 32-bit compilation.