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  1. #include <stddef.h>
  2. #include <stdlib.h>
  3.  
  4. /*** Constants */
  5. enum e_rgb {
  6.     Red,
  7.     Green,
  8.     Blue,
  9.     RGBCount
  10.   };
  11.  
  12. /*** Object types */
  13. typedef unsigned char rgb[RGBCount];
  14.  
  15. /*** Function declarations */
  16. struct s_picture * alloc_picture(int w, int h);
  17. rgb * picture_row(struct s_picture * pic, int row);
  18.  
  19. /*** Struct/union definitions */
  20. struct s_picture {
  21.     int width;
  22.     int height;
  23.     rgb * data;
  24.   };
  25.  
  26. /*** Function definitions */
  27. int main(void) {
  28.     struct s_picture * pic;
  29.     rgb * row_ten;
  30.  
  31.     /* Allocate a 10 x 10 picture */
  32.     pic = alloc_picture(10, 10);
  33.     if (!pic)
  34.       return EXIT_FAILURE;
  35.  
  36.     /* Produce a pointer to the tenth row */
  37.     row_ten = picture_row(pic, 9);
  38.  
  39.     /* The next three sections all do the same thing */
  40.     Red[*row_ten] = 42;
  41.     Green[*row_ten] = 42;
  42.     Blue[*row_ten] = 42;
  43.  
  44.     row_ten[0][Red] = 42;
  45.     row_ten[0][Green] = 42;
  46.     row_ten[0][Blue] = 42;
  47.  
  48.     (*row_ten)[Red] = 42;
  49.     (*row_ten)[Green] = 42;
  50.     (*row_ten)[Blue] = 42;
  51.  
  52.     /* Clean-up and terminate */
  53.     free(pic);
  54.     return EXIT_SUCCESS;
  55.   }
  56.  
  57. /* Allocate storage for a picture */
  58. struct s_picture * alloc_picture(int w, int h) {
  59.     /* Wrap up a picture */
  60.     struct s_wrapper {
  61.         struct s_picture pic[1];
  62.         /* This member is here for alignment */
  63.         rgb data[1];
  64.       };
  65.     size_t size;
  66.     void * ptr;
  67.     struct s_wrapper * wrapper;
  68.     unsigned char * bytes;
  69.  
  70.     /* Compute the amount of storage to allocate */
  71.     size = (
  72.         /* Don't allocate for the alignment member */
  73.         offsetof(struct s_wrapper, data)
  74.         + sizeof wrapper->data * w * h
  75.       );
  76.     if (size < sizeof *wrapper)
  77.       size = sizeof *wrapper;
  78.  
  79.     /* Allocate the storage */
  80.     ptr = malloc(size);
  81.     /* Return a null pointer on failure */
  82.     if (!ptr)
  83.       return ptr;
  84.  
  85.     wrapper = ptr;
  86.     bytes = ptr;
  87.  
  88.     wrapper->pic->width = w;
  89.     wrapper->pic->height = h;
  90.  
  91.     /* There is a problem with the next line */
  92.     wrapper->pic->data = wrapper->data;
  93.     /*
  94.      * Since there is only one element for 'wrapper->data',
  95.      * that means that the pointer value yielded by that
  96.      * expression could have such bounds associated with it.
  97.      * Instead, we derive a pointer value from 'bytes', whose
  98.      * bounds would be those of the call to 'malloc', above
  99.      */
  100.     wrapper->pic->data = (rgb *) (bytes + offsetof(struct s_wrapper, data));
  101.     /* That was safer */
  102.  
  103.     return wrapper->pic;
  104.   }
  105.  
  106. /* Produce a pointer to the specified row in a picture */
  107. rgb * picture_row(struct s_picture * pic, int row) {
  108.     return pic->data + row * pic->width;
  109.   }
  110.  
Success #stdin #stdout 0.01s 1848KB
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https://ideone.com/JbdvX
language:
C (gcc 8.3)
created:
12 years ago
visibility:
secret

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