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  1. #include<stdio.h>
  2. #include <string.h>
  3.  
  4. /** copy the bytes at data directly into a double. 
  5.     and return that.
  6. */
  7. double directValue(const char *data)
  8. {
  9. 	double result;
  10. 	char *dest = (char *)&result;
  11.  
  12. 	memcpy(dest, data, sizeof(double));
  13.  
  14. 	return result;
  15. }
  16.  
  17. /** copy the bytes at data into a double, reversing the
  18.     byte order, and return that.
  19. */
  20. double reverseValue(const char *data)
  21. {
  22. 	double result;
  23.  
  24. 	char *dest = (char *)&result;
  25.  
  26. 	for(int i=0; i<sizeof(double); i++) 
  27. 	{
  28. 		dest[i] = data[sizeof(double)-i-1];
  29. 	}
  30. 	return result;
  31. }
  32.  
  33. /** Adjust the byte order from network to host.
  34.     On a big endian machine this is a NOP.
  35. 	There is no error handling 
  36. */
  37. double ntohd(double src)
  38. {
  39. #	if		!defined(__FLOAT_WORD_ORDER__) \
  40. 		||	!defined(__ORDER_LITTLE_ENDIAN__)
  41. #		error "oops: unknown byte order"
  42. #	endif
  43.  
  44. #	if __FLOAT_WORD_ORDER__ == __ORDER_LITTLE_ENDIAN__
  45. 		return reverseValue((char *)&src);
  46. #	else
  47. 		return src;
  48. #	endif
  49. }
  50.  
  51. int main() 
  52. {
  53. 	// big endian, i.e. network byte order
  54. 	const char onedotsomethingbytes[]
  55. 				= {0x3F, 0xF3, 0xC0, 0xCA, 0x42, 0x83, 0xDE, 0x1B};
  56.  
  57. 	double directDbl = directValue(onedotsomethingbytes);			
  58. 	// on a big endian machine this would print 0.12345...
  59. 	printf("direct:   %le\n", directDbl);
  60.  
  61. 	// on a little endian machine *this* would print 0.12345...
  62. 	printf("reverse:  %le\n", reverseValue(onedotsomethingbytes));
  63.  
  64. 	// Use the double's memory as data. This is equivalent to using 
  65. 	// the char array. It's also legal.
  66. 	printf("reverse:  %le\n", reverseValue((char *)&directDbl));
  67.  
  68. 	// Try the ntohd with compile time decision.
  69. 	printf("ntohd:    %le\n", ntohd(directDbl));
  70.  
  71.     return 0;
  72. }
Success #stdin #stdout 0s 2156KB
comments (?)
stdin 
Standard input is empty
stdout 
direct:   1.927630e-174
reverse:  1.234568e+00
reverse:  1.234568e+00
ntohd:    1.234568e+00
https://ideone.com/aV9mj4
language:
C (gcc 8.3)
created:
8 years ago
visibility:
public

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