Retrieve filename from file descriptor in C

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.

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

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?

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 &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;-).

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.

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.

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 <sys/syslimits.h>
#include <fcntl.h>


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.

There is no official API to do this on OpenBSD, though with some very convoluted workarounds, it is still possible with the following code, note you need to link with -lkvm and -lc. The code using FTS to traverse the filesystem is from this answer.

#include <string>
#include <vector>


#include <cstdio>
#include <cstring>


#include <sys/stat.h>
#include <fts.h>


#include <sys/sysctl.h>
#include <kvm.h>


using std::string;
using std::vector;


string pidfd2path(int pid, int fd) {
string path; char errbuf[_POSIX2_LINE_MAX];
static kvm_t *kd = nullptr; kinfo_file *kif = nullptr; int cntp = 0;
kd = kvm_openfiles(nullptr, nullptr, nullptr, KVM_NO_FILES, errbuf); if (!kd) return "";
if ((kif = kvm_getfiles(kd, KERN_FILE_BYPID, pid, sizeof(struct kinfo_file), &cntp))) {
for (int i = 0; i < cntp; i++) {
if (kif[i].fd_fd == fd) {
FTS *file_system = nullptr; FTSENT *child = nullptr; FTSENT *parent = nullptr;
vector<char *> root; char buffer[2]; strcpy(buffer, "/"); root.push_back(buffer);
file_system = fts_open(&root[0], FTS_COMFOLLOW | FTS_NOCHDIR, nullptr);
if (file_system) {
while ((parent = fts_read(file_system))) {
child = fts_children(file_system, 0);
while (child && child->fts_link) {
child = child->fts_link;
if (!S_ISSOCK(child->fts_statp->st_mode)) {
if (child->fts_statp->st_dev == kif[i].va_fsid) {
if (child->fts_statp->st_ino == kif[i].va_fileid) {
path = child->fts_path + string(child->fts_name);
goto finish;
}
}
}
}
}
finish:
fts_close(file_system);
}
}
}
}
kvm_close(kd);
return path;
}


int main(int argc, char **argv) {
if (argc == 3) {
printf("%s\n", pidfd2path((int)strtoul(argv[1], nullptr, 10),
(int)strtoul(argv[2], nullptr, 10)).c_str());
} else {
printf("usage: \"%s\" <pid> <fd>\n", argv[0]);
}
return 0;
}

If the function fails to find the file, (for example, because it no longer exists), it will return an empty string. If the file was moved, in my experience when moving the file to the trash, the new location of the file is returned instead if that location wasn't already searched through by FTS. It'll be slower for filesystems that have more files.

The deeper the search goes in the directory tree of your entire filesystem without finding the file, the more likely you are to have a race condition, though still very unlikely due to how performant this is. I'm aware my OpenBSD solution is C++ and not C. Feel free to change it to C and most of the code logic will be the same. If I have time I'll try to rewrite this in C hopefully soon. Like macOS, this solution gets a hardlink at random (citation needed), for portability with Windows and other platforms which can only get one hard link. You could remove the break in the while loop and return a vector if you want don't care about being cross-platform and want to get all the hard links. DragonFly BSD and NetBSD have the same solution (the exact same code) as the macOS solution on the current question, which I verified manually. If a macOS user wishes to get a path from a file descriptor opened any process, by plugging in a process id, and not be limited to just the calling one, while also getting all hard links potentially, and not being limited to a random one, see this answer. It should be a lot more performant that traversing your entire filesystem, similar to how fast it is on Linux and other solutions that are more straight-forward and to-the-point. FreeBSD users can get what they are looking for in this question, because the OS-level bug mentioned in that question has since been resolved for newer OS versions.

Here's a more generic solution which can only retrieve the path of a file descriptor opened by the calling process, however it should work for most Unix-likes out-of-the-box, with all the same concerns as the former solution in regards to hard links and race conditions, although performs slightly faster due to less if-then, for-loops, etc:

#include <string>
#include <vector>


#include <cstring>


#include <sys/stat.h>
#include <fts.h>


using std::string;
using std::vector;


string fd2path(int fd) {
string path;
FTS *file_system = nullptr; FTSENT *child = nullptr; FTSENT *parent = nullptr;
vector<char *> root; char buffer[2]; strcpy(buffer, "/"); root.push_back(buffer);
file_system = fts_open(&root[0], FTS_COMFOLLOW | FTS_NOCHDIR, nullptr);
if (file_system) {
while ((parent = fts_read(file_system))) {
child = fts_children(file_system, 0);
while (child && child->fts_link) {
child = child->fts_link; struct stat info = { 0 };
if (!S_ISSOCK(child->fts_statp->st_mode)) {
if (!fstat(fd, &info) && !S_ISSOCK(info.st_mode)) {
if (child->fts_statp->st_dev == info.st_dev) {
if (child->fts_statp->st_ino == info.st_ino) {
path = child->fts_path + string(child->fts_name);
goto finish;
}
}
}
}
}
}
finish:
fts_close(file_system);
}
return path;
}

An even quicker solution which is also limited to the calling process, but should be somewhat more performant, you could wrap all your calls to fopen() and open() with a helper function which stores basically whatever C equivalent there is to an std::unordered_map, and pair up the file descriptor with the absolute path version of what is passed to your fopen()/open() wrappers (and the Windows-only equivalents which won't work on UWP like _wopen_s() and all that nonsense to support UTF-8), which can be done with realpath() on Unix-likes, or GetFullPathNameW() (*W for UTF-8 support) on Windows. realpath() will resolve symbolic links (which aren't near as commonly used on Windows), and realpath() / GetFullPathNameW() will convert your existing file you opened from a relative path, if it is one, to an absolute path. With the file descriptor and absolute path stored an a C equivalent to a std::unordered_map (which you likely will have to write yourself using malloc()'d and eventually free()'d int and c-string arrays), this will again, be faster than any other solution that does a dynamic search of your filesystem, but it has a different and unappealing limitation, which is it will not make note of files which were moved around on your filesystem, however at least you can check whether the file was deleted using your own code to test existence, it also won't make note of the file in whether it was replaced since the time you opened it and stored the path to the descriptor in memory, thus giving you outdated results potentially. Let me know if you would like to see a code example of this, though due to files changing location I do not recommend this solution.