001: // structs and bitfield, the GIF image format example
002: #include<stdio.h>
003: #include<stdlib.h>
004: #include<math.h>
005: 
006: // this struct correspond to the exact data organization of the initial
007: // header of a GIF file (13 bytes)
008: struct gif_header
009: {
010:   // beginning of the file:
011:   unsigned long signature         :24; // it shouled contain 'G', 'I', 'F'
012:   unsigned long version           :16; // usually '8' '9' but also '8' '7' exists
013:   unsigned long subversion        :8;  // typically 'a'
014: 
015:   // logical Screen Width:
016:   unsigned long width             :16; // canvas width
017:   unsigned long height            :16; // canvas height
018: 
019:   // packed fields:
020:   unsigned long global_color      :1;  // true when the file features a global palette
021:   unsigned long color_res         :3;  // used to determine how many colors we have in the image
022:   unsigned long sort_flag         :1;  // true when colors in the palette are sorted according the frequency
023:   unsigned long color_table_size  :3;  // used to compute the size of the palette
024: 
025:   unsigned long bg_color_index    :8;  // backround color (if any)
026:   unsigned long pixel_aspect_ratio:8;  // when != 0 can be used to compute pixel aspect ratio
027: };
028: // 13 bytes (actually we should use __attribute__((packed)) to obtain this)
029: 
030: 
031: 
032: void print_gif_data(const char *);
033: 
034: int main(int argc, char **argv)
035: {
036: 
037:   printf("struct gif_header needs %ld bytes to be stored\n", sizeof(struct gif_header));
038: 
039:   print_gif_data("files/logical.gif");
040:   print_gif_data("files/test.gif");
041:   print_gif_data("files/test2.gif");
042:   print_gif_data("files/test3.gif");
043:   print_gif_data("files/animated.gif");
044:   print_gif_data("files/gif87a.gif");
045:   print_gif_data("files/test.jpg");
046: 
047:   return 0;
048: }
049: 
050: void print_gif_data(const char *s)
051: {
052: 
053:   FILE *bf;
054: 
055:   bf=fopen(s, "rb"); 
056:   if(!bf)
057:   {
058:     perror("\nError in opening the file");
059:     return;
060:   }
061: 
062:   struct gif_header a;
063: 
064:   if( fread(&a, sizeof(a), 1, bf) != 1 )
065:   {
066:     printf("\nUnable to read %ld bytes from the file\n", sizeof(a));
067:     fclose(bf);
068:     return;
069:   }
070:   fclose(bf);
071: 
072:   if(a.signature != 0x464947)
073:   {
074:     printf("\n%s is an invalid GIF file\n", s);
075:     return;
076:   }
077:   
078: 
079:   printf("\nHeader of the GIF image file '%s':\n", s);
080:     printf(" %d x %d width x height\n", a.width, a.height);
081:     if(a.global_color)
082:     {
083:       printf(" The file has a global color table\n");
084:       printf(" Background color index is %d\n", a.bg_color_index);
085:       printf(" Color table size is %d\n", a.color_table_size);
086:     }
087:     else
088:     {
089:       printf( " The file has not a global color table\n");
090:     }
091: 
092:     printf(" Color Resolution is %d\n", a.color_res);
093:     if(a.sort_flag)
094:       printf(" color values are sorted according to their frequency\n");
095:     else
096:       printf(" color values are unsorted\n");
097: 
098:     if(a.pixel_aspect_ratio)
099:       printf(" the aspect ratio of each pixel is %d\n", a.pixel_aspect_ratio);
100: 
101: 
102:   return;
103: }
104: 
105: