File Descriptor Management
File descriptors underlie all input/output mechanisms offered by
the system. They are used to implementation the
*-based functions found in
<stdio.h>, and all the file and network
communication facilities provided by the Python and Java
environments are eventually implemented in them.
File descriptors are small, non-negative integers in userspace, and are backed on the kernel side with complicated data structures which can sometimes grow very large.
If a descriptor is no longer used by a program and is not closed
explicitly, its number cannot be reused (which is problematic in
itself, see Dealing with the
select Limit), and
the kernel resources are not freed. Therefore, it is important
to close all descriptors at the earliest point in time
possible, but not earlier.
Error Handling during Descriptor Close
close system call is always
successful in the sense that the passed file descriptor is
never valid after the function has been called. However,
close still can return an error, for
example if there was a file system failure. But this error is
not very useful because the absence of an error does not mean
that all caches have been emptied and previous writes have
been made durable. Programs which need such guarantees must
open files with
O_SYNC or use
may also have to
fsync the directory
containing the file.
Closing Descriptors and Race Conditions
Unlike process IDs, which are recycle only gradually, the kernel always allocates the lowest unused file descriptor when a new descriptor is created. This means that in a multi-threaded program which constantly opens and closes file descriptors, descriptors are reused very quickly. Unless descriptor closing and other operations on the same file descriptor are synchronized (typically, using a mutex), there will be race conditions and I/O operations will be applied to the wrong file descriptor.
Sometimes, it is necessary to close a file descriptor
concurrently, while another thread might be about to use it in
a system call. In order to support this, a program needs to
create a single special file descriptor, one on which all I/O
operations fail. One way to achieve this is to use
socketpair, close one of the descriptors,
shutdown(fd, SHUTRDWR) on the
When a descriptor is closed concurrently, the program does not
close on the descriptor. Instead it
dup2 to replace the
descriptor to be closed with the dummy descriptor created
earlier. This way, the kernel will not reuse the descriptor,
but it will carry out all other steps associated with calling
a descriptor (for instance, if the descriptor refers to a
stream socket, the peer will be notified).
This is just a sketch, and many details are missing. Additional data structures are needed to determine when it is safe to really close the descriptor, and proper locking is required for that.
Lingering State after Close
By default, closing a stream socket returns immediately, and the kernel will try to send the data in the background. This means that it is impossible to implement accurate accounting of network-related resource utilization from userspace.
SO_LINGER socket option alters the
close, so that it will return
only after the lingering data has been processed, either by
sending it to the peer successfully, or by discarding it after
the configured timeout. However, there is no interface which
could perform this operation in the background, so a separate
userspace thread is needed for each
call, causing scalability issues.
Currently, there is no application-level countermeasure which
applies universally. Mitigation is possible with
connlimit match type in particular) and
specialized filtering devices for denial-of-service network
These problems are not related to the
TIME_WAIT state commonly seen in
netstat output. The kernel
automatically expires such sockets if necessary.
Preventing File Descriptor Leaks to Child Processes
Child processes created with
the initial set of file descriptors with their parent
process. By default, file descriptors are also preserved if
a new process image is created with
(or any of the other functions such as
Usually, this behavior is not desirable. There are two ways to turn it off, that is, to prevent new process images from inheriting the file descriptors in the parent process:
Set the close-on-exec flag on all newly created file descriptors. Traditionally, this flag is controlled by the
F_SETFDoperations of the
However, in a multi-threaded process, there is a race condition: a subprocess could have been created between the time the descriptor was created and the
FD_CLOEXECwas set. Therefore, many system calls which create descriptors (such as
openat) now accept the
socketpair), which cause the
FD_CLOEXECflag to be set for the file descriptor in an atomic fashion. In addition, a few new systems calls were introduced, such as
The downside of this approach is that every descriptor needs to receive special treatment at the time of creation, otherwise it is not completely effective.
fork, but before creating a new process image with
execve, all file descriptors which the child process will not need are closed.
Traditionally, this was implemented as a loop over file descriptors ranging from
1023. But this is only an approximation because it is possible to create file descriptors outside this range easily (see Dealing with the
selectLimit). Another approach reads
/proc/self/fdand closes the unexpected descriptors listed there, but this approach is much slower.
At present, environments which care about file descriptor leakage implement the second approach. OpenJDK 6 and 7 are among them.
Dealing with the
By default, a user is allowed to open only 1024 files in a single process, but the system administrator can easily change this limit (which is necessary for busy network servers). However, there is another restriction which is more difficult to overcome.
select function only supports a
FD_SETSIZE file descriptors
(that is, the maximum permitted value for a file descriptor
FD_SETSIZE - 1, usually 1023.) If a
process opens many files, descriptors may exceed such
limits. It is impossible to query such descriptors using
If a library which creates many file descriptors is used in
the same process as a library which uses
select, at least one of them needs to
select can be replaced with
poll or another event handling
mechanism. Replacing the
is the recommended approach.
Alternatively, the library with high descriptor usage can
relocate descriptors above the
limit using the following procedure.
Create the file descriptor
fdas usual, preferably with the
Before doing anything else with the descriptor
int newfd = fcntl(fd, F_DUPFD_CLOEXEC, (long)FD_SETSIZE);
newfdresult is non-negative, otherwise close
fdand report an error, and return.
fdand continue to use
The new descriptor has been allocated above the
FD_SETSIZE. Even though this algorithm
is racy in the sense that the
first descriptors could fill up, a very high degree of
physical parallelism is required before this becomes a problem.
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