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DATE 2016-12-01

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Key: archive_learn Value: 2016-12-01

Key: id Value: 543549

MESSAGE
DATE 2016-12-06
FROM Christopher League
SUBJECT Re: [Learn] png data format
From learn-bounces-at-nylxs.com Tue Dec 6 17:14:21 2016
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Tue, 6 Dec 2016 17:14:15 -0500 (EST)
From: Christopher League
To: Ruben Safir , Hangout ,
learn-at-nylxs.com
In-Reply-To:
References: <66105244-4afa-4330-b0c2-0661bde965fd-at-mrbrklyn.com>
<87bmwpf857.fsf-at-contrapunctus.net> <878trtf7qw.fsf-at-contrapunctus.net>

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Date: Tue, 06 Dec 2016 17:14:12 -0500
Message-ID: <87h96gemdn.fsf-at-contrapunctus.net>
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Subject: Re: [Learn] png data format
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Ruben Safir writes:

> There is an aspect of this I just don't understand. Why does the order
> remain intact without ntohl when you look byte by byte through the
> allocated memory, but the order is reversed when you evaluate it as a
> continuous 32 bit unsigned int variable

Memory is essentially just an array of *bytes*, because memory is
byte-addressable. The addressable unit is never smaller than a byte --
you can't have a pointer just to a bit or three.

And if you THINK have a pointer to something bigger than a byte, you
REALLY DON'T. You STILL just have a pointer to a byte, but when you read
from it we're going to also read the next several bytes.

So when you do that, the way that larger values (such as 32-bit
integers) are stored as bytes MATTERS. And different machines do it
differently. (BTW, historically there isn't just big- and little-endian,
some machines used other alternatives that are often called
"middle-endian"! Thank your $deity (if any) that those architectures are
now extinct.)

I think this page helps:


CL

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1.0, user-scalable=3Dyes">




Ruben Safir ruben-at-mrbrklyn.com=
writes:



There is an aspect of this I just don=E2=80=99t understand. Why does the=
order remain intact without ntohl when you look byte by byte through the a=
llocated memory, but the order is reversed when you evaluate it as a contin=
uous 32 bit unsigned int variable



Memory is essentially just an array of bytes, because memory is=
byte-addressable. The addressable unit is never smaller than a byte =E2=80=
=93 you can=E2=80=99t have a pointer just to a bit or three.


And if you THINK have a pointer to something bigger than a byte, you REA=
LLY DON=E2=80=99T. You STILL just have a pointer to a byte, but when you re=
ad from it we=E2=80=99re going to also read the next several bytes.


So when you do that, the way that larger values (such as 32-bit integers=
) are stored as bytes MATTERS. And different machines do it differently. (B=
TW, historically there isn=E2=80=99t just big- and little-endian, some mach=
ines used other alternatives that are often called =E2=80=9Cmiddle-endian=
=E2=80=9D! Thank your $deity (if any) that those architectures are now exti=
nct.)


I think this page helps: /cmsc311/Notes/Data/endian.html" class=3D"uri">https://www.cs.umd.edu/class=
/sum2003/cmsc311/Notes/Data/endian.html


CL





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--===============2087657846==--

  1. 2016-12-01 Ruben Safir <ruben.safir-at-my.liu.edu> Subject: [Learn] Fwd: Patrolling the Dark Net,
  2. 2016-12-02 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Tutorial on threaded binary tree part 1: simple
  3. 2016-12-03 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Witner Labs
  4. 2016-12-04 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Nice lecture on Quantum Mechanics
  5. 2016-12-05 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] information access, copyright wars and DRM
  6. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  7. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  8. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  9. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] png data format
  10. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  11. 2016-12-06 Ruben Safir <ruben.safir-at-my.liu.edu> Re: [Learn] png data format
  12. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] png data format
  13. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  14. 2016-12-06 Ruben Safir <mrbrklyn-at-panix.com> Re: [Learn] png data format
  15. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  16. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] [Hangout-NYLXS] png data format
  17. 2016-12-06 John Bowler <john.cunningham.bowler-at-gmail.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  18. 2016-12-06 John Bowler <john.cunningham.bowler-at-gmail.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  19. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  20. 2016-12-06 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Ocaml
  21. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: [luny-talk] Humble Bundle O'Reilly UNIX books
  22. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] JT's words
  23. 2016-12-06 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] png data format
  24. 2016-12-07 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  25. 2016-12-08 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: png data format
  26. 2016-12-08 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  27. 2016-12-08 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  28. 2016-12-10 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] references to pointers
  29. 2016-12-10 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] C++ returning lvalue references and pointers and refs
  30. 2016-12-10 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] const puzzle and literal type arguments
  31. 2016-12-11 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  32. 2016-12-12 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Fwd: Re: png data format
  33. 2016-12-12 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] gene phylogienics of homonids
  34. 2016-12-13 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Bit Depth
  35. 2016-12-13 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] C++ Threads Workshop
  36. 2016-12-13 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Summer Jobs
  37. 2016-12-14 From: "Mancini, Sabin (DFS)" <Sabin.Mancini-at-dfs.ny.gov> Subject: [Learn] For Ruben ( + those in NYC Metro ) : Holiday Social Event
  38. 2016-12-14 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: [dinosaur] Ceratopsid (Centrosaurinae:
  39. 2016-12-15 Ruben Safir <ruben.safir-at-my.liu.edu> Re: [Learn] Happy Holidays
  40. 2016-12-15 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] zlib demo with vector::resize
  41. 2016-12-15 John Bowler <john.cunningham.bowler-at-gmail.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  42. 2016-12-15 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: [dinosaur] Elephant bird nuclear genome fragments
  43. 2016-12-15 Christopher League <league-at-contrapunctus.net> Subject: [Learn] zlib demo with vector::resize
  44. 2016-12-16 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] zlib demo with vector::resize
  45. 2016-12-16 Christopher League <league-at-contrapunctus.net> Re: [Learn] zlib demo with vector::resize
  46. 2016-12-16 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] PNG Parallel Programming problem
  47. 2016-12-17 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] [Hangout-NYLXS] I'm sure it's a coincidence, part n+1
  48. 2016-12-17 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] irc thread on the use of object methods in threads
  49. 2016-12-17 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] PNG threaded program
  50. 2016-12-18 ruben safir <ruben-at-mrbrklyn.com> Re: [Learn] Threads and Object Methods
  51. 2016-12-18 ruben safir <ruben-at-mrbrklyn.com> Re: [Learn] Threads and Object Methods
  52. 2016-12-18 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Look C++ is a functional programming language
  53. 2016-12-19 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] C++ Threading
  54. 2016-12-19 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] C++ Threading
  55. 2016-12-19 ISOC-NY announcements <announce-at-lists.isoc-ny.org> Subject: [Learn] [isoc-ny] JOB: Telecommunications Policy Specialist -at- NTIA
  56. 2016-12-20 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] F'ing Mouse Pad
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  58. 2016-12-21 Samir Iabbassen <Samir.Iabbassen-at-liu.edu> Re: [Learn] Noobdy is home
  59. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [Learn] Noobdy is home
  60. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Noobdy is home
  61. 2016-12-21 Samir Iabbassen <Samir.Iabbassen-at-liu.edu> Re: [Learn] thread concurancy
  62. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [Learn] thread concurancy
  63. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: lamda's in classes
  64. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: lamda's in classes
  65. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: thread concurancy
  66. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  67. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  68. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  69. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  70. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  71. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  72. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  73. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  74. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  75. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) thread concurancy
  76. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Threads and Object Methods
  77. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] I need help
  78. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Noobdy is home
  79. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] thread concurancy
  80. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] [Hangout-NYLXS] Marnchester by the Sea
  81. 2016-12-22 Ruben Safir <mrbrklyn-at-panix.com> Re: [Learn] [Hangout-NYLXS] And be aware you were an unexcused
  82. 2016-12-22 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: thread concurancy
  83. 2016-12-23 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] HOPL (History of Programming Languages)
  84. 2016-12-23 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Noobdy is home
  85. 2016-12-23 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Tiny Compiler in many languages at Rosettacode.org
  86. 2016-12-25 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Google and C++
  87. 2016-12-25 Ruben Safir <ruben.safir-at-my.liu.edu> Subject: [Learn] phylogentics
  88. 2016-12-27 Samir Iabbassen <Samir.Iabbassen-at-liu.edu> Re: [Learn] thread concurancy

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