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213 lines
8.4 KiB
Markdown
213 lines
8.4 KiB
Markdown
# Reading Directories
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In this project you will write your first assembly language program from scratch. The program will use library calls to open the given Linux directory (or the current directory if no command line argument is given) for reading. It will read every directory entry, printing out each file's inode number, type and name.
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## Samples
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### Output when no command line argument is given
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```text
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perryk@ROCI pk_dirent % ./a.out
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40905818 0x04 .
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7943328 0x04 ..
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40905889 0x08 README.md
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40908053 0x08 a.out
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40906048 0x08 main.c
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40905888 0x08 .gitignore
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40905819 0x04 .git
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40907182 0x04 .vscode
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perryk@ROCI pk_dirent %
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```
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The first column is the named file's inode number. Think of an inode number as a file's unique (per file system) serial number. You must print it left justified in a field of 20 digits using `printf` - since it is possible you've never used `printf` before, I will supply the correct (for `C`) statement:
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```c
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printf("%-20llu 0x%02x %s\n", de->d_ino, de->d_type, de->d_name);
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```
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In my code, `de` is defined:
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```c
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struct dirent * de;
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```
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The second column is the file's type printed as a single byte's worth of hex.
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The third column is the file's name.
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### Output when a command line argument is given
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```text
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perryk@ROCI pk_dirent % ./a.out /
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2 0x04 .
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1 0x04 ..
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1152921504606781440 0x0a home
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1152921500312809862 0x04 usr
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1152921500312809678 0x04 bin
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1152921500312809756 0x04 sbin
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1152921500311879697 0x08 .file
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1152921500312809753 0x0a etc
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1152921500312845202 0x0a var
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1152921500311879700 0x04 Library
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1152921500311879701 0x04 System
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1152921500311879696 0x0a .VolumeIcon.icns
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1152921500312809755 0x04 private
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1152921500311879698 0x04 .vol
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1152921500312809676 0x04 Users
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1152921500311879699 0x04 Applications
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1152921500312809754 0x04 opt
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1152921500312809752 0x04 dev
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1152921500312809677 0x04 Volumes
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1152921500312809861 0x0a tmp
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1152921500312809751 0x04 cores
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perryk@ROCI pk_dirent %
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```
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### Output when a bad command line argument is given
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```text
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perryk@ROCI pk_dirent % ./a.out fooble
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fooble: No such file or directory
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perryk@ROCI pk_dirent %
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```
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The error string is produced by `perror()`.
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## man
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Here is where you will get documentation for `perror()`, `opendir()`, `closedir()`, and `readdir()`. The man page for `readdir()` also describes `struct dirent`.
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```text
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man perror
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man opendir
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man closedir
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man readdir
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```
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`man` is your friend, though of course in the 21st century it should be called `person`. To learn more about `man`, do the obvious thing:
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```text
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man man
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```
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**DON'T DO THIS FROM A MAC TERMINAL -- WHY? STEVE JOBS THAT'S WHY.**
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It will be equally pointless to try the above Linux shell commands from a Windows command prompt but hey - give it a try. So where should you read these `man` pages? In your ARM Linux VM, of course.
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The reason to not read the `man` pages on the Mac is that everything beyond the name of the functions will be different. You know, "Think Different."
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## `opendir()`
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This function takes a NULL terminated `C-string` and attempts to open it as a directory. Get the details from the `man` page. If you get an error return, pass the attempted directory name to `perror()` to get the right error message.
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## `closedir()`
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Call this function to close a successfully opened directory. Get the details from the `man` page.
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## `readdir()`
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Call this function to be given a pointer to the next `dirent` or `NULL` if there are no more (or there is an error). Pay attention to the `man` page to distinguish between no more `dirent` structures and an error. In short, `errno` should be initialized to 0 then checked once you've gotten a `NULL` back from `readdir()`.
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## Source code to a `C` version
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At the beginning of this document I said:
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```text
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In this project you will write your first assembly language program from scratch.
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```
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but here's the source code to my `C` version because you may be just getting started with `C` and Linux programming.
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```c
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#include <stdio.h> /* 1 */
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#include <errno.h> /* 2 */
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#include <dirent.h> /* 3 */
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/* 4 */
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int main(int argc, char ** argv) { /* 5 */
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int retval = 1; /* 6 */
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char * dirname = "."; /* 7 */
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/* 8 */
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if (argc > 1) /* 9 */
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dirname = argv[1]; /* 10 */
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/* 11 */
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DIR * dir = opendir(dirname); /* 12 */
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if (dir) { /* 13 */
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struct dirent * de; /* 14 */
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errno = 0; /* 15 */
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while ((de = readdir(dir)) != NULL) /* 16 */
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printf("%-20llu 0x%02x %s\n", de->d_ino, de->d_type, de->d_name); /* 17 */
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if (errno != 0) /* 18 */
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perror("readdir() failed"); /* 19 */
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closedir(dir); /* 20 */
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retval = (errno != 0); // force error return to be 1 /* 21 */
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} /* 22 */
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else /* 23 */
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perror(dirname); /* 24 */
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return retval; /* 25 */
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} /* 26 */
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```
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### Line 6 and Line 21
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Command line programs return 0 to who called them when all is well. A non-zero return value signifies and error.
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### Lines 7, 9 and 10
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Notice how the program is made to default to the current directory (`"."`) which can be overridden if a command line argument is supplied.
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### Line 15
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`errno` is initialized to 0 and then quizzed to see if it turned non-zero when `readdir()` finally returns `NULL`.
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### Line 17
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Implementing this line is where you will need to calculate the correct offsets to each data member.
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See the book chapter on `struct`.
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### Line 18
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The error condition is distinguished from the end of the directory by looking at `errno`.
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## Getting the address of `errno`
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`errno` is an `extern`. To store anything into it (or query its contents), you must have its address. For reasons which will be explained, getting its address is accomplished by calling a [library function](http://refspecs.linux-foundation.org/LSB_4.0.0/LSB-Core-generic/LSB-Core-generic/baselib---errno-location.html).
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## Remember to properly set the return value of `main()`
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If all ends well, zero should be returned from `main()`. If any error is found, a value of 1 should be returned.
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Check in this way:
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```text
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pk_dirent > ./a.out
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4327 0x04 .
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4328 0x04 ..
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4329 0x08 main.s
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4330 0x08 README.md
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4331 0x08 a.out
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4332 0x08 main.c
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4333 0x08 .gitignore
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4334 0x08 project.s
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4335 0x04 .git
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4336 0x04 .vscode
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pk_dirent > echo $?
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0
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pk_dirent > ./a.out main.s
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main.s: Not a directory
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pk_dirent > echo $?
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1
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pk_dirent >
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```
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`$?` is a shell variable that contains the value returned from the last program run by the shell.
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## Likely source of error
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If you're printing garbage, double check your calculations of offsets within the `dirent`. While this isn't the only explanation, it is a likely explanation.
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## Setting expectations
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I provide the following not as a challenge, but to set your expectations.
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My assembly language solution is about 60 lines plus comments. If you find yourself writing much more than this, you're doing it wrong.
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