Re: how to the filename

[kaushal <kaushal@rocsys.com>]

Hi Steve,
	Thanks for the response.But how will lsof work for a particular PID.If
lsof can print all the files by their names then why can't any c
program?The idea was to prove that the file /dev/pts/9 or some no. is
opened by the bash and to it are the stdin,stdout and stderr
associated.For that ,the fds 0,1,and 2 are supposed to point to the same
file /dev/pts/9 or say /dev/tty3 ....
This can be proved using lsof -p <PID OF THE BASH> .But how to prove
that from within a c program?This lead to the sol if we can get the
filename from the file descriptor and print it on the screen.

regards-
kaushal.
On Thu, 2005-04-14 at 16:07, Steve Graegert wrote:
> On 4/14/05, kaushal <kaushal@rocsys.com> wrote:
> > Hello all,
> >        How can I get the filename/pathname given the open file descriptor?Does
> > fstat provide this feature internally?Can somebody give the code snippet
> > for this.
> 
> There is no such thing.  It is not possible to obtain a FD's filename
> reliably.  Unless you are absolutely sure that this particular FD
> points to a file (or directory)  and not to a socket, pipe or
> something similar, you will not be able to use fstat reliably.  Which
> of stat's fields are suggesting to be helpful reagarding to your
> problem?  st_ino?  How would you locate a file based on its file ID? 
> This would require scanning the complete file system (and probably
> more than one).  Another problem is, that an FD might be associated
> with other files at the same time or files can be stored inside a
> directory that you can't read due to lack of sufficient permissions. 
> What you are looking for is some kind of reverse lookup to unwind the
> many-to-one relationship of files and inodes.
> 
> A couple of years ago Floyd Davidson suggested some code that may
> point you to the right direction (not tested):
> 
> /* A demo program to locate file names related to an inode number */
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <string.h>
> #include <sys/stat.h>
> #include <dirent.h>
> #include <limits.h>
> 
> void scan_list(char *curdir, struct dirent **ptr_nl, int dirs);
> int file_select(const struct dirent *nl);
> 
> ino_t inode;
> char curdir[PATH_MAX] = ".";    /* default search directory */
> 
> int
> main(int argc, char **argv)
> {
>   struct dirent **namelist;
>   struct stat st;
> 
>   if (argc < 2 || argc > 3) {
>     fprintf(stderr,"usage:  %s inode [directory]\n", argv[0]);
>     exit(EXIT_FAILURE);
>   }
>   inode = strtoul(argv[1], NULL, 10);
>   if (!inode) {
>     fprintf(stderr,"Error:  invalid inode\n");
>     exit(EXIT_FAILURE);
>   }
>   if (argc == 3 && !lstat(argv[2], &st) && S_ISDIR(st.st_mode)) {
>       strcpy(curdir, argv[2]);
>   }
> 
>   scan_list(curdir, namelist,
>       scandir(curdir, &namelist, file_select, alphasort));
>   return EXIT_SUCCESS;
> }
> 
> /*
>  * returns 1 for directories, otherwise 0
>  *     and displays any filename which matches inode.
>  */
> int
> file_select(const struct dirent *nl)
> {
>   struct stat st;
>   char curfile[PATH_MAX];
>   
>   sprintf(curfile, "%s/%s", curdir, nl->d_name);
>   if (0 == lstat(curfile, &st)) {
>     /* report a matching inode number */
>     if (st.st_ino == inode) {
>       printf("  %6lu %-20s \n", (unsigned long) st.st_ino, curfile);
>     }
>     /* skip these directories */
>     if (!strcmp(nl->d_name, ".") || !strcmp(nl->d_name, "..")) {
>       return 0;
>     }
>     /* otherwise list all directories */
>     if (S_ISDIR(st.st_mode)) {
>       return 1;
>     }
>   } return 0;
> }
> 
> /* descend through all directories */
> void
> scan_list(char *olddir, struct dirent **ptr_nl, int dirs)
> {
>   char   savedir[PATH_MAX];
>   int    i;
>   struct dirent **namelist;
>   
>   if (dirs > 0) {
>     for (i = 0; i < dirs; ++i) {
>       strcpy(savedir, curdir);
>       sprintf(curdir,"%s/%s", olddir, ptr_nl[i]->d_name);
>       scan_list(curdir, namelist,
>          scandir(curdir, &namelist, file_select, alphasort));
>       strcpy(curdir, savedir);
>     }
>   }
> }
> 
> /* End of demo program */
> 
> Kind Regards
> 
>     \Steve
> 
> --
> 
> Steve Graegert <graegerts@gmail.com>
> Independent Software Consultant {C/C++ && Java && .NET}
> Mobile: +49 (176)  21 24 88 69
> Office: +49 (9131) 71 26 40 9