Implementing ls...

[Proto]

Ok, as my next assignment our teacher told us to implement a subversion of ls... so far I've had no problems in implementing the basic ls functionality along with some basic flags like the -a one, and some custom ones out teacher is asking for.

For reading the directories I'm using the dirent.h library functions, however when analysing each file these functions will return to me a dirent structure that will give me the name of the file at max. This becomes a problem when I try to implement the 'ls -l' functionality, since I will be missing a lot of data. My question is that if there is a set of C function that will analize a file and return me the owner, creator, permissions, and those kind of things that the -l flag include...

Found the answer myself: stat fstat and lstat are your friends ... now the question is how the heck do you parse the st_mode field (i found an example here but i don't know what the sperm function is or what is it supposed to do.... (well it does some parsing but...)

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Last edited by Proto; April 7th, 2006 at 07:47..

[Ramsus K]

Check "man 2 stat" for information specific to your system.

     The sb argument is a pointer to a stat() structure as defined by
     <sys/stat.h> (shown below) and into which information is placed concern-
     ing the file.

     struct stat {
         dev_t    st_dev;    /* device inode resides on */
         ino_t    st_ino;    /* inode's number */
         mode_t   st_mode;   /* inode protection mode */
         nlink_t  st_nlink;  /* number or hard links to the file */
         uid_t    st_uid;    /* user-id of owner */
         gid_t    st_gid;    /* group-id of owner */
         dev_t    st_rdev;   /* device type, for special file inode */
         struct timespec st_atimespec;  /* time of last access */
         struct timespec st_mtimespec;  /* time of last data modification */
         struct timespec st_ctimespec;  /* time of last file status change */
         off_t    st_size;   /* file size, in bytes */
         quad_t   st_blocks; /* blocks allocated for file */
         u_long   st_blksize;/* optimal file sys I/O ops blocksize */
         u_long   st_flags;  /* user defined flags for file */
         u_long   st_gen;    /* file generation number */
     };

Then...

     The status information word st_mode has the following bits:

     #define S_IFMT 0170000           /* type of file */
     #define        S_IFIFO  0010000  /* named pipe (fifo) */
     #define        S_IFCHR  0020000  /* character special */
     #define        S_IFDIR  0040000  /* directory */
     #define        S_IFBLK  0060000  /* block special */
     #define        S_IFREG  0100000  /* regular */
     #define        S_IFLNK  0120000  /* symbolic link */
     #define        S_IFSOCK 0140000  /* socket */
     #define        S_IFWHT  0160000  /* whiteout */
     #define S_ISUID 0004000  /* set user id on execution */
     #define S_ISGID 0002000  /* set group id on execution */
     #define S_ISVTX 0001000  /* save swapped text even after use */
     #define S_IRUSR 0000400  /* read permission, owner */
     #define S_IWUSR 0000200  /* write permission, owner */
     #define S_IXUSR 0000100  /* execute/search permission, owner */

[Proto]

Oh yeah, indeed that helps. I had consulted my man previously but mine didn't have the implementation for the st_mode pre-defined bits so I though they were just some kind of setters, but if they are just the number that saves a lot of trouble

EDIT: Ok the ls is done, yay (though it would be better if readdir returned you pointers to alphabetically ordered dirents but aw wel... ill have to do it by hand i guess....)

Now I have to implement the ps... does c has functions to make this task easier or you have to do all the parsing by hand?

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Last edited by Proto; April 7th, 2006 at 18:57..

[Proto]

Ok well... I already found everything I needed... the /proc/<number>/stat file... however I have a problem here. When I want to implement the basic ps functionality, i only have to print the processes thay were initiated by the current terminal. That implies that I must compare the tty process id of the terminal my program is running on to the tty the process i am analysing on that moment. I know I could just do a call to ps | grep bash and from there get the pid, but i wanted to know if there is a less dirty solution to get the tty pid...

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Last edited by Proto; April 7th, 2006 at 20:20..

[Ramsus K]

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
        pid_t pid;
        pid = getpid(); /* current process id */
        printf("%d\n", pid);
        pid = getppid(); /* parent process id */
        printf("%d\n", pid);
        return 0;
}

[Proto]

Yeah, I realized the ppid thingie a little after I made the post... (but forgot to update) I must appear like an idiot now

Now my only remaining doubt is how to map the tty_nr that you obtain from the stat file to the actual terminal name... aw well, let's see of google can help me.

EDIT: On the quick an dirty side I guess I could stat() all the /dev/pts devices

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Last edited by Proto; April 8th, 2006 at 02:52..

[Kraelis]

cheater

__________________
--KrÆlis Cross


Mundus vult decipi, ergo decipiatur

===============
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[Proto]

How so? Isn't it one of the golden rules of programming: "If it works don't move it"

My new assignment now asks you to implement a distributed brute-force attack for obtaining the key a file is encrypted with... I just love this subject's projects

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Last edited by Proto; May 7th, 2006 at 03:45..

[Kraelis]

Fine.

Odd, though. I think implementing ls is easier than the other projects you've had. Whatever happened to increasing levels of difficulty?

Bah, your prof just decided to spice up a typical distributed job. As always, it's not the actual task that's hard, but the job management.

*Sour graping as to why MY profs never gave a a project like that*

__________________
--KrÆlis Cross


Mundus vult decipi, ergo decipiatur

===============
Athlon 64 x2 3800| Asrock ALiveNF6G-VSTA | Kingston DDR2 1 Gb RAM