Read headers of BMP image file type.

Makefile

CC=gcc
CFLAGS=-Wfatal-errors -std=gnu11 -Wall

readbmp.elf: readbmp.c
	$(CC) $(CFLAGS) -o $@ $^

readbmp.c

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <arpa/inet.h>

typedef struct __attribute__((__packed__)) FileHeader
{
  unsigned short bfType;      //  2 bytes -- The characters "BM"
  unsigned int   bfSize;      //  4 bytes -- The size of the file in bytes
  unsigned short bfReserved1; //  2 bytes -- Unused - must be zero
  unsigned short bfReserved2; //  2 bytes -- Unused - must be zero
  unsigned int   bfOffBits;   //  4 bytes -- Offset to start of Pixel Data
} FileHeader;

typedef struct __attribute__((__packed__)) ImageHeader
{
  unsigned int   biSize;          // 4 -- Header Size - Must be at least 40
  unsigned int   biWidth;         // 4 -- Image width in pixels
  unsigned int   biHeight;        // 4 -- Image height in pixels
  unsigned short biPlanes;        // 2 -- Must be 1
  unsigned short biBitCount;      // 2 -- Bits per pixel - 1, 4, 8, 16, 24, or 32
  unsigned int   biCompression;   // 4 -- Compression type (0 = uncompressed)
  unsigned int   biSizeImage;     // 4 -- Image Size - may be zero for uncompressed images
  unsigned int   biXPelsPerMeter; // 4 -- Preferred resolution in pixels per meter
  unsigned int   biYPelsPerMeter; // 4 -- Preferred resolution in pixels per meter
  unsigned int   biClrUsed;       // 4 -- Number Color Map entries that are actually used
  unsigned int   biClrImportant;  // 4 -- Number of significant colors
} ImageHeader;

void usage(char const *programe_name)
{
  printf("%s -- Print all image headers\n", programe_name);
  printf("Usage:        %s <image_file>\n", programe_name);
  exit(1);
}

int main(int argc, char const *argv[])
{
  if (argc < 2)
  {
    usage(argv[0]);
  }

  FILE *imgfile = fopen(argv[1], "rb");
  if (imgfile == NULL)
  {
    perror("Cannot open image file.");
    exit(2);
  }

  FileHeader fileHeader;
  ImageHeader imgHeader;
  size_t nItem = 0;
  nItem = fread(&fileHeader, sizeof(FileHeader), 1, imgfile);
  if (nItem != 1)
  {
    puts("Corrupted file.");
    printf("Number:%lu\n", nItem);
    fclose(imgfile);
    exit(3);
  }
  nItem = fread(&imgHeader, sizeof(ImageHeader), 1, imgfile);
  if (nItem != 1)
  {
    puts("Corrupted file.");
    fclose(imgfile);
    exit(3);
  }

  if ((fileHeader.bfType != 0x4d42) || (fileHeader.bfReserved1 != 0)
    || (fileHeader.bfReserved2 != 0))
  {
    puts("Not BMP file type.");
    printf("bfType:%d, bfReserved1:%d, bfReserved2:%d\n",
      fileHeader.bfType, fileHeader.bfReserved1, fileHeader.bfReserved2);
    fclose(imgfile);
    exit(4);
  }
  puts("================================================");
  printf("File Name                                       : %s\n", argv[1]);
  printf("Offset to start of Pixel Data                   : %d\n", fileHeader.bfOffBits);
  printf("Header Size - Must be at least 40               : %d\n", imgHeader.biSize);
  printf("Image size (width x height in pixels)           : %dx%d\n", imgHeader.biWidth, imgHeader.biHeight);
  printf("Must be 1                                       : %d\n", imgHeader.biPlanes);
  printf("Bits per pixel - 1, 4, 8, 16, 24, or 32         : %d\n", imgHeader.biBitCount);
  printf("Compression type (0 = uncompressed)             : %d\n", imgHeader.biCompression);
  printf("Image Size - may be zero for uncompressed images: %d\n", imgHeader.biSizeImage);
  printf("Preferred resolution in pixels per meter        : %d\n", imgHeader.biXPelsPerMeter);
  printf("Preferred resolution in pixels per meter        : %d\n", imgHeader.biYPelsPerMeter);
  printf("Number Color Map entries that are actually used : %d\n", imgHeader.biClrUsed);
  printf("Number of significant colors                    : %d\n", imgHeader.biClrImportant);
  fclose(imgfile);
  return 0;
}