Retrieve filename from file descriptor in C

Question

Is it possible to get the filename of a file descriptor  Linux  in C

User · Answer

Before writing this off as impossible I suggest you look at the source code of the lsof command.

There may be restrictions but lsof seems capable of determining the file descriptor and file name. This information exists in the /proc filesystem so it should be possible to get at from your program.

User · Answer

You can use fstat   to get the file s inode by struct stat  Then  using readdir   you can compare the inode you found with those that exist  struct dirent  in a directory  assuming that you know the directory  otherwise you ll have to search the whole filesystem  and find the corresponding file name   Nasty

User · Answer

In Windows  with GetFileInformationByHandleEx  passing FileNameInfo  you can retrieve the file name

User · Answer

Impossible  A file descriptor may have multiple names in the filesystem  or it may have no name at all   Edit  Assuming you are talking about a plain old POSIX system  without any OS-specific APIs  since you didn t specify an OS

User · Answer

As Tyler points out  there s no way to do what you require  directly and reliably   since a given FD may correspond to 0 filenames  in various cases  or   1  multiple  hard links  is how the latter situation is generally described   If you do still need the functionality with all the limitations  on speed AND on the possibility of getting 0  2      results rather than 1   here s how you can do it  first  fstat the FD -- this tells you  in the resulting struct stat  what device the file lives on  how many hard links it has  whether it s a special file  etc  This may already answer your question -- e g  if 0 hard links you will KNOW there is in fact no corresponding filename on disk   If the stats give you hope  then you have to  walk the tree  of directories on the relevant device until you find all the hard links  or just the first one  if you don t need more than one and any one will do   For that purpose  you use readdir  and opendir  amp c of course  recursively opening subdirectories until you find in a struct dirent thus received the same inode number you had in the original struct stat  at which time if you want the whole path  rather than just the name  you ll need to walk the chain of directories backwards to reconstruct it    If this general approach is acceptable  but you need more detailed C code  let us know  it won t be hard to write  though I d rather not write it if it s useless  i e  you cannot withstand the inevitably slow performance or the possibility of getting    1 result for the purposes of your application -

User · Answer

I had this problem on Mac OS X  We don t have a  proc virtual file system  so the accepted solution cannot work   We do  instead  have a F GETPATH command for fcntl    F GETPATH          Get the path of the file descriptor Fildes   The argu-                     ment must be a buffer of size MAXPATHLEN or greater    So to get the file associated to a file descriptor  you can use this snippet    include  lt sys syslimits h gt   include  lt fcntl h gt   char filePath PATH MAX   if  fcntl fd  F GETPATH  filePath     -1           do something with the file path     Since I never remember where MAXPATHLEN is defined  I thought PATH MAX from syslimits would be fine

User · Answer

You can use readlink on  proc self fd NNN where NNN is the file descriptor  This will give you the name of the file as it was when it was opened     however  if the file was moved or deleted since then  it may no longer be accurate  although Linux can track renames in some cases   To verify  stat the filename given and fstat the fd you have  and make sure st dev and st ino are the same   Of course  not all file descriptors refer to files  and for those you ll see some odd text strings  such as pipe  1538488   Since all of the real filenames will be absolute paths  you can determine which these are easily enough  Further  as others have noted  files can have multiple hardlinks pointing to them - this will only report the one it was opened with  If you want to find all names for a given file  you ll just have to traverse the entire filesystem

[c] Retrieve filename from file descriptor in C

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