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haas:spring2015:cprog:projects:eocehints [2015/04/19 13:23] – [0x1: more octal] wedge | haas:spring2015:cprog:projects:eocehints [2015/04/19 20:44] (current) – [0x1: more octal] wedge | ||
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If you were to convert " | If you were to convert " | ||
- | If you pass that decimal 640 to the chmod() function, you'd end up with the sticky bit being set (T in other) along with user write, and NOTHING else. Not 640 as we desire. | + | If you pass that decimal 640 to the chmod() function, you'd end up with the sticky bit being set (T in other) along with user write, and NOTHING else. Not 0640 as we desire, but instead 01200. |
So, entering 640 on the command-line would not result in a direct conversion to octal 0640... some converting will be in order. | So, entering 640 on the command-line would not result in a direct conversion to octal 0640... some converting will be in order. | ||
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As you can see, even if you had a 0640, the leading zero would be dropped in the conversion, because **atoi(3)** is apparently only cognizant of decimal values (and good, because that would have taken the fun out of this particular problem... you stand to learn some important things by working through this process). | As you can see, even if you had a 0640, the leading zero would be dropped in the conversion, because **atoi(3)** is apparently only cognizant of decimal values (and good, because that would have taken the fun out of this particular problem... you stand to learn some important things by working through this process). | ||
- | And also, do you see that regardless of displaying it in octal, decimal, or hex, it is the same value? They' | + | And also, do you see that regardless of displaying it in octal, decimal, or hex, it is the same value? They' |
> Why doesn' | > Why doesn' | ||
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We would experience similar neatness with decimal if we started playing with base 10, base 100, and base 1000 values (in such a case, decimal would be to base 100 and 1000 what binary is to bases 8 and 16). | We would experience similar neatness with decimal if we started playing with base 10, base 100, and base 1000 values (in such a case, decimal would be to base 100 and 1000 what binary is to bases 8 and 16). | ||
+ | |||
+ | =====0x1: when a number isn't a number but a representation of a number we'd like it to be===== | ||
+ | > Does **chmod(2)** have to be in octal or are there other ways that it can work. | ||
+ | |||
+ | No, you can think of it as being in binary, octal, decimal, or hex... or any base, really, so long as that value, when converted to octal, matches the desired permissions. | ||
+ | |||
+ | After all: | ||
+ | * 0640 in binary is: 000110100000 | ||
+ | * 0640 in hex is: 1A0 | ||
+ | * 0640 in decimal is: 416 | ||
+ | |||
+ | Once the number is in the variable, it can instantly and effortlessly be represented in base 8, 10, or 16. It can be thought of as any one of those, and it really doesn' | ||
+ | |||
+ | The only difference is when we choose to visualize them... when you SEE a number, it has to take a form (and abide by a base)... when you input a number, we apply the same notions. But once stored in a single variable on the computer, its original form is unimportant. | ||
+ | |||
+ | The value provided on the command line has to conform with the octal permissions, | ||
+ | |||
+ | > Converting argv[1]' | ||
+ | |||
+ | The command-line " | ||
+ | |||
+ | So, if the first digit of argv[1] is a ' | ||
+ |