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--- haas:spring2017:sysprog:projects:sci0 [2017/01/26 16:32] – [Squaring values ending with 5] wedge
+++ haas:spring2017:sysprog:projects:sci0 [2017/02/14 17:00] (current) – [Submission] wedge
@@ Line 31: / Line 31: @@
 </code>
+====Setting permissions====
+Although we've largely operated without direct attention to it, our data and our access to it very much depends upon the permissions set on them.
+You cannot view any source code written without the read permission being set; you cannot save changes unless you have write permission enabled; and you cannot run your compiled programs if the execution permission was not applied.
+Different operating systems and different filesystems manifest file permissions differently; for this problem we will specifically explore the UNIX file permissions, by writing a program that accepts a value and a file from the command-line and does the pre-processing necessary to convert that value into a form compatible with the mechanism for actually changing the permission.
+UNIX file permissions are represented as a 3-digit octal (base 8!) value, and each octal value can have the following values:
+^  value  ^  description  |
+|  4  |  apply read permission to that particular mode  |
+|  2  |  apply write permission to that particular mode  |
+|  1  |  apply execute permission to that particular mode  |
+|  0  |  apply no permissions to that particular mode  |
+Being an octal value, we can express results ranging from 0-7, and that is precisely how many variations we need to specify all the possible combinations here.
+For example, if we wanted read (4) and write (2) permission, we'd add them together... 4+2 is 6; if we wanted read, write, AND execute: 4+2+1 = 7.
+There are 3 'tiers' of permissions to consider:
+^  tier  ^  description  |
+|  user  |  permissions applied to the assigned owner of the file  |
+|  group  |  permissions applied to the assigned group of the file  |
+|  other  |  permissions applied to anyone else (the world)  |
+More specifically:
+^  ^  user  ^  group  ^  other  |
+|  read  |  0400  |  0040  |  0004  |
+|  write  |  0200  |  0020  |  0002  |
+|  execute  |  0100  |  0010  |  0001  |
+|  none  |  0000  |  0000  |  0000  |
+To form the octal permission, we figure out what permissions to apply to the user (a combination of read, write, execute, or none), and then for the group, and finally for the world. That is the 3-digit octal value we need.
+The problem is that the mode as specified on the command-line will be available as a string (argv[1], an array of characters), so your main task will be to convert that character data into base 8 data (or binary, or hex, the same thing in the end).
+Your program should expect data to be provided as follows:
+  * argv[0]: program name
+  * argv[1]: 3-digit permission (as a string)
+  * argv[2-n]: file to which we want to apply permissions
+This can be done rather effectively using logic operations (almost crazy easily).
+To get a better handle on this, you may run the following commands when logged into lab46:
+<cli>
+lab46:~$ ls -l /etc/motd /etc/shadow /bin/ls
+-rwxr-xr-x 1 root root   118280 Mar 14 11:47 /bin/ls
+-rw-r--r-- 1 root root   859 Mar 14 12:16 /etc/motd
+-rw-r----- 1 root shadow 729 Oct 21 04:55 /etc/shadow
+lab46:~$
+</cli>
+Ignoring the leading '-' (that refers to file type), we see that **/bin/ls** has **rwx** (7) applied to the user root, **r-x** (5) applied to the group root, and **r-x** applied to everyone else. That means its octal permission is 0755.
+**/etc/motd** has permissions of **0644**, while **/etc/shadow** has permissions of **0640**.
 =====Program=====
-It is your task to write the program that will use the above method to compute the square of the input value ending with a 5 (you are to input the entire number, including the 5 at the end).
+It is your task to write the program that will use the **chmod(2)** system call to provide 3- or 4-digit octal values to appropriately change the permissions on the indicated file(s).
 Your program should:
-  * prompt the user for the number (input)
+  * accept as command-line arguments:
-    * input number as an unsigned short integer
+    * the 3- or 4-digit octal value
+    * the file(s)
   * perform the task (process)
-    * isolate one's digit mathematically, store in a variable (unsigned short int)
+    * as stated, you are not allowed to use any of the **strtol(3)** family of functions to do the base conversion for you.
-    * isolate remaining digits mathematically, store in another variable (unsigned short int)
+      * This isn't to say you cannot use them; you may, you just cannot use them to do any of the conversion work (beyond ASCII to base 10).
-    * perform the algorithm on the two pieces, storing their results in two separate variables (of type unsigned short int)
+  * display error or usage if applicable.
-  * display the final value (output)
+    * if error or usage, make sure you return a 1 instead of 0.
-    * display the beginning and ending parts together (but stored in separate variables)
-    * display the resulting number to STDOUT (right-justified in a space supporting the largest possible value -- see output example below)
-    * display any supporting text to STDERR (display of source values left-justified in a space supporting 3-digit values -- see output example below).
-  * because we have not officially learned how to do selection/have the computer react to conditions, please implement with the assumption that the user will ALWAYS input a correct value. Do not worry about having to check for invalid or illegal input values (I will not be checking for such when I evaluate your project).
-=====Execution=====
-<cli>
+=====FAQs/Hints=====
-lab46:~/src/cprog/sof0$ ./sof0
-Enter value: 75
-  x 75  =   5625
-lab46:~/src/cprog/sof0$
-</cli>
-The execution of the program is short and simple- obtain the input, do the processing, produce the output, and then terminate.
+====0x0: Octal====
-Note how the two "75" values are left-justified within a 3-space slot (with the multiplication 'x' and equal sign '=' being padded with a space on either side). This information should all be displayed to STDERR.
+> In argv[1] the number input is an array of characters but it needs conversion to a singular octal value. Why?
+>
-Similarly, here's an example of 105x105:
+As **argv[1]** is an array of characters... if you give it 640, it'll actually be "640\0", that is, ASCII character '6', followed by ASCII character '4', followed by ASCII character '0', followed by the NULL terminator.
-<cli>
+'6' has a numeric value of 54 (decimal).
-lab46:~/src/cprog/sof0$ ./sof0
-Enter value: 105
-x 105 =  11025
-lab46:~/src/cprog/sof0$
-</cli>
-The 'x' and '=' padding persists, but because we're squaring a 3-digit value vs. a 2-digit value, we occupy the entire allocated space on the screen.
+If you were to convert "640" to an integer value 640, that would be 640 in base 10; 640(10) to base 8 would be: 1200
-If you'd like to verify successful output to STDOUT/STDERR, you can perform the following tests. First, verify that the answer (and ONLY the answer), is being sent to STDOUT -- we do this by eliminating STDERR entirely:
+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.
-<cli>
+So, entering 640 on the command-line would not result in a direct conversion to octal 0640... some converting will be in order.
-lab46:~/src/cprog/sof0$ ./sof0 2> /dev/null <<< 105
-lab46:~/src/cprog/sof0$
-</cli>
-What we are doing here is two-fold:
+====0x1: more octal====
+> Since octal values start with a leading zero, if I insert an ASCII '0' at the start of the string and then convert it using atoi(3), wouldn't that work? It doesn't seem to be the case.
+>
-  * We are using command-line I/O redirection to redirect STDERR (which is bound to file descriptor #2) to the system bit-bucket.
+That leading zero is only a convenience, implemented on a case by case basis. It would appear **atoi(3)** does not implement it. Look at this:
-  * We are "redirecting" STDIN using a **here string**, providing the program's input of **105** right on the command-line at time of execution.
-Similarly, if we were to eliminate STDOUT entirely (for verifying STDERR output):
+<code c>
+#include <stdio.h>
+#include <stdlib.h>
+int main(int argc, char **argv)
+{
+        int result = atoi(argv[1]);  // take the first argument and convert it to an integer
+        printf("argv[1] is \"%s\"\n", argv[1]); // display the original string, for comparison
+        // display the result in octal, decimal, and hex
+        printf("result is %o in octal, %d in decimal, and %x in hexadecimal\n", result, result, result);
+        return(0);
+}
+</code>
+And look at the output:
 <cli>
-lab46:~/src/cprog/sof0$ ./sof0 1> /dev/null
+lab46:~$ ./testatoi 640
-Enter value: 75
+argv[1] is "640"
-  x 75  = lab46:~/src/cprog/sof0$
+result is 1200 in octal, 640 in decimal, and 280 in hexadecimal
+lab46:~$ ./testatoi 0640
+argv[1] is "0640"
+result is 1200 in octal, 640 in decimal, and 280 in hexadecimal
 </cli>
-What we are doing here:
+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).
-  * We are using command-line I/O redirection to redirect STDOUT (which is bound to file descriptor #1) to the system bit-bucket.
+And also, do you see that regardless of displaying it in octal, decimal, or hex, it is the same value? They're all being sourced from an integer variable called result... a regular old int... so it ultimately is up to how we instruct the computer to interpret it... after all, EVERYTHING is in binary, even if we are thinking through the problem exclusively in a different base.
-=====Verification=====
+> Why doesn't adding the leading zero make it octal?
-One of the tests I will perform for output compliance of your code will involve comparing your program's output against a range of input values, to see if they all output in conformance with project specifications.
+>
-I will make use of a checksum to verify exactness.
+The leading zero is a convenient way to identify an octal value. It is a means to mark one, but by no means a required form. That C and other facilities on the computer support a leading 0 for octal and a leading 0x for hex makes our lives easier, but only goes as far as support for such things has been implemented.
-You will need to run this from your sof0 project directory with a compiled and operational binary by the name of **sof0**.
+We do the same in language. If I were to say the following value is in base 8:    72033
-You can check your project by typing in the following at the prompt:
+You would understand because I identified it as such... note the lack of the leading zero. If I wanted to be more brief, instead of saying "the following value is in base 8" I could just prefix a 0 on, because that shortcut is generally understood (within the context of assignable values in C syntax). But it is by no means the only way to do it.
+===atoi===
+And note, there is nothing magical about **atoi(3)**... it is just a function. It takes an array currently filled with ASCII-equivalents of single digit decimal numbers and coalesces those separate digits into one. We've played with things like this in our early labs this semester (there are advantages to having a number broken up into separate digits, there are also advantages to having a number combined as a single value).
+The overall scope of this problem presents you with a desired-octal value currently represented as a string-- each 'digit' is a separate entity, and we want to combine them into a single value, only preserving the octal value (where many tools assume decimal).
+And as we know: 031 is not 31. 031 is 25.
+So, if we read in 031 as a decimal 31 yet desire to then represent it as octal, we'd instead have 037.
+===the neatness of binary and octal (and hex)===
+There are certain advantages when working in similar bases that are all powers of two. Quite advantageous things.
+That base 8 is one of those bases means this problem can take advantage of some very simple and very effective logic operations that would not be as simple or direct in decimal (10 is not a power of 2).
+That each octal digit represents three binary bits should be kept in mind. This problem entirely plays off how well binary values and octal values just sync up (because, well, they do).
+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).
+====0x2: 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't make a difference, because they're all the same (in that 0640 == 0x1A0 == 000110100000 == 416). That's just how numbers work (on or off the computer).
+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, just as the chmod command does.
+> Converting argv[1]'s "640" to 0640 seems confusing because they are two completely different values.
+The command-line "640" (the string) isn't a decimal; it isn't one number (and as such shouldn't be considered an automatically intelligible number, that's where our program comes in, to make sense of it)... it is an ASCII representation of a 3-digit number (or, a sequence of three two-digit decimal numbers that represent the ASCII character being displayed). Due to the context of how we're interpreting it, we desire that number to be ultimately represented as a single octal quantity (of 3-digits), because that is what the chmod(2) function requires.
+So, if the first digit of argv[1] is a '6' (that's what, a decimal 54?), we know for the three bits that correspond with that field (the user field), we want to apply read (4, or 100 binary) and write (2, or 010 binary) for a total of 6 (110 binary). For the user field, read is 0400 (XXX 000 000 in binary (marked with the X's)). For the group field, read is 0040. Look at where they end up lining up in binary.
+=====Execution=====
+If you run without proper arguments:
 <cli>
-lab46:~/src/cprog/sof0$ pchk cprog sof0
+lab46:~/src/sysprog/sci0$ ./mychmod
+./mychmod <MODE> <FILE> [FILE...]
+lab46:~/src/sysprog/sci0$
 </cli>
-If all aligns, you will see this:
+Run with invalid mode:
 <cli>
-==================================================
+lab46:~/src/sysprog/sci0$ ./mychmod 0987 file1
-=   CPROG sof0 project output validation tool    =
+ERROR: invalid mode
-==================================================
-sof0 checksum is: 822a47fb2a45845500b6c10878045bd5
-your checksum is: 822a47fb2a45845500b6c10878045bd5
-==================================================
-    verification: SUCCESS!
-==================================================
 </cli>
-If something is off, your checksum will not match the sof0 checksum, and verification will instead say "**MISMATCH**", like follows (note that a mismatched checksum can be anything, and likely not what is seen in this example):
+Run with valid mode (success means you just get your prompt back):
 <cli>
-==================================================
+lab46:~/src/sysprog/sci0$ ./mychmod 640 file1
-=   CPROG sof0 project output validation tool    =
+lab46:~/src/sysprog/sci0$
-==================================================
-sof0 checksum is: 822a47fb2a45845500b6c10878045bd5
-your checksum is: 92af264c86823a61529948caaeac53e0
-==================================================
-    verification: MISMATCH
-==================================================
 </cli>
-=====Questions / Food for Thought=====
-These are things I'd like you to contemplate, even use as potential material on your weekly journal entry. The more you think about and understand the problem, the better your ability to solve it and other problems.
-  * Why/how does this trick work for 1-digit numbers?
-  * Considering our 1-, 2-, and 3-digit domain restriction for this project, how many candidate values are there for input?
-  * What is the smallest input value?
-  * What is the largest input value?
-  * How many input values are there that end in **5**?
-  * What is the largest square that can be calculated given the project input restrictions?
-  * How many digits is the largest square?
-  * How can knowing how many digits the largest square is help you implement your solution?
-=====Submission=====
-To successfully complete this project, the following criteria must be met:
-  * Code must compile cleanly (no warnings or errors)
+=====Submission=====
-  * Executed program must display a total of 2 lines, one for input, one for output.
-    * The computed number must be output to STDOUT
-    * Any supporting output must be output to STDERR
-    * Output is formatted in the manner in the sample above
-  * Output must be correct, and match the form given in the sample output above.
-  * Code must be nicely and consistently indented
-  * Code must implement solution using the mental math technique described above
-  * Code must be commented
-    * have a properly filled-out comment banner at the top
-    * have at least 20% of your program consist of **<nowiki>//</nowiki>**-style descriptive comments
-  * Output Formatting (including spacing) of program must conform to the provided output (see above).
-  * Track/version the source code in a repository
-  * 25% late penalty per day after deadline
-  * Program is submit in a C source file called **sof0.c** and compiles without warning, note, nor error with gcc on lab46.
-  * Submit a copy of your source code to me using the **submit** tool.
 To submit this program to me using the **submit** tool, run the following command at your lab46 prompt:
 <cli>
-$ submit cprog sof0 sof0.c
+$ submit sysprog sci0 mychmod.c
-Submitting cprog project "sof0":
+Submitting sysprog project "sci0":
-    -> sof0.c(OK)
+    -> mychmod.c(OK)
 SUCCESSFULLY SUBMITTED
@@ Line 178: / Line 248: @@
 You should get some sort of confirmation indicating successful submission if all went according to plan. If not, check for typos and or locational mismatches.
-I'll be looking for the following:
-<code>
-:sof0:final tally of results (52/52)
-*:sof0:adequate indentation and comments in sof0.c [4/4]
-*:sof0:data stored and calculated in correct and separate variables [4/4]
-*:sof0:effective usage of fprintf() and fscanf() [4/4]
-*:sof0:program uses indicated algorithm [12/12]
-*:sof0:output conforms to project specifications [12/12]
-*:sof0:runtime tests of sof0.c succeed [12/12]
-*:sof0:no negative compiler messages for sof0.c [4/4]
-</code>

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