#include <unistd.h> #include <fcntl.h> int fcntl(int fd, int cmd, ... /* arg */ );
fcntl() can take an optional third argument. Whether or not this argument is required is determined by cmd. The required argument type is indicated in parentheses after each cmd name (in most cases, the required type is long, and we identify the argument using the name arg), or void is specified if the argument is not required.
The file status flags and their semantics are described in open(2).
struct flock {
...
short l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(F_GETLK only) */
...
};
The l_whence, l_start, and l_len fields of this structure specify the range of bytes we wish to lock. Bytes past the end of the file may be locked, but not bytes before the start of the file.
l_start is the starting offset for the lock, and is interpreted relative to either: the start of the file (if l_whence is SEEK_SET); the current file offset (if l_whence is SEEK_CUR); or the end of the file (if l_whence is SEEK_END). In the final two cases, l_start can be a negative number provided the offset does not lie before the start of the file.
l_len specifies the number of bytes to be locked. If l_len is positive, then the range to be locked covers bytes l_start up to and including l_start+l_len-1. Specifying 0 for l_len has the special meaning: lock all bytes starting at the location specified by l_whence and l_start through to the end of file, no matter how large the file grows.
POSIX.1-2001 allows (but does not require) an implementation to support a negative l_len value; if l_len is negative, the interval described by lock covers bytes l_start+l_len up to and including l_start-1. This is supported by Linux since kernel versions 2.4.21 and 2.5.49.
The l_type field can be used to place a read (F_RDLCK) or a write (F_WRLCK) lock on a file. Any number of processes may hold a read lock (shared lock) on a file region, but only one process may hold a write lock (exclusive lock). An exclusive lock excludes all other locks, both shared and exclusive. A single process can hold only one type of lock on a file region; if a new lock is applied to an already-locked region, then the existing lock is converted to the new lock type. (Such conversions may involve splitting, shrinking, or coalescing with an existing lock if the byte range specified by the new lock does not precisely coincide with the range of the existing lock.)
In order to place a read lock, fd must be open for reading. In order to place a write lock, fd must be open for writing. To place both types of lock, open a file read-write.
As well as being removed by an explicit F_UNLCK, record locks are automatically released when the process terminates or if it closes any file descriptor referring to a file on which locks are held. This is bad: it means that a process can lose the locks on a file like /etc/passwd or /etc/mtab when for some reason a library function decides to open, read and close it.
Record locks are not inherited by a child created via fork(2), but are preserved across an execve(2).
Because of the buffering performed by the stdio(3) library, the use of record locking with routines in that package should be avoided; use read(2) and write(2) instead.
Advisory locks are not enforced and are useful only between cooperating processes.
Mandatory locks are enforced for all processes. If a process tries to perform an incompatible access (e.g., read(2) or write(2)) on a file region that has an incompatible mandatory lock, then the result depends upon whether the O_NONBLOCK flag is enabled for its open file description. If the O_NONBLOCK flag is not enabled, then system call is blocked until the lock is removed or converted to a mode that is compatible with the access. If the O_NONBLOCK flag is enabled, then the system call fails with the error EAGAIN.
To make use of mandatory locks, mandatory locking must be enabled both on the file system that contains the file to be locked, and on the file itself. Mandatory locking is enabled on a file system using the "-o mand" option to mount(8), or the MS_MANDLOCK flag for mount(2). Mandatory locking is enabled on a file by disabling group execute permission on the file and enabling the set-group-ID permission bit (see chmod(1) and chmod(2)).
The Linux implementation of mandatory locking is unreliable. See BUGS below.
If you set the O_ASYNC status flag on a file descriptor by using the F_SETFL command of fcntl(), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor. F_SETSIG can be used to obtain delivery of a signal other than SIGIO. If this permission check fails, then the signal is silently discarded.
Sending a signal to the owner process (group) specified by F_SETOWN is subject to the same permissions checks as are described for kill(2), where the sending process is the one that employs F_SETOWN (but see BUGS below).
If the file descriptor fd refers to a socket, F_SETOWN also selects the recipient of SIGURG signals that are delivered when out-of-band data arrives on that socket. (SIGURG is sent in any situation where select(2) would report the socket as having an "exceptional condition".)
The following was true in 2.6.x kernels up to and including kernel 2.6.11:
struct f_owner_ex {
int type;
pid_t pid;
};
The
type
field will have one of the values
F_OWNER_TID,
F_OWNER_PID,
or
F_OWNER_PGRP.
The
pid
field is a positive integer representing a thread ID, process ID,
or process group ID.
See
F_SETOWN_EX
for more details.
By using F_SETSIG with a nonzero value, and setting SA_SIGINFO for the signal handler (see sigaction(2)), extra information about I/O events is passed to the handler in a siginfo_t structure. If the si_code field indicates the source is SI_SIGIO, the si_fd field gives the file descriptor associated with the event. Otherwise, there is no indication which file descriptors are pending, and you should use the usual mechanisms (select(2), poll(2), read(2) with O_NONBLOCK set etc.) to determine which file descriptors are available for I/O.
By selecting a real time signal (value >= SIGRTMIN), multiple I/O events may be queued using the same signal numbers. (Queuing is dependent on available memory). Extra information is available if SA_SIGINFO is set for the signal handler, as above.
Note that Linux imposes a limit on the number of real-time signals that may be queued to a process (see getrlimit(2) and signal(7)) and if this limit is reached, then the kernel reverts to delivering SIGIO, and this signal is delivered to the entire process rather than to a specific thread.
Using these mechanisms, a program can implement fully asynchronous I/O without using select(2) or poll(2) most of the time.
The use of O_ASYNC, F_GETOWN, F_SETOWN is specific to BSD and Linux. F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, and F_SETSIG are Linux-specific. POSIX has asynchronous I/O and the aio_sigevent structure to achieve similar things; these are also available in Linux as part of the GNU C Library (Glibc).
Leases are associated with an open file description (see open(2)). This means that duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the same lease, and this lease may be modified or released using any of these descriptors. Furthermore, the lease is released by either an explicit F_UNLCK operation on any of these duplicate descriptors, or when all such descriptors have been closed.
Leases may only be taken out on regular files. An unprivileged process may only take out a lease on a file whose UID (owner) matches the file system UID of the process. A process with the CAP_LEASE capability may take out leases on arbitrary files.
When a process (the "lease breaker") performs an open(2) or truncate(2) that conflicts with a lease established via F_SETLEASE, the system call is blocked by the kernel and the kernel notifies the lease holder by sending it a signal (SIGIO by default). The lease holder should respond to receipt of this signal by doing whatever cleanup is required in preparation for the file to be accessed by another process (e.g., flushing cached buffers) and then either remove or downgrade its lease. A lease is removed by performing an F_SETLEASE command specifying arg as F_UNLCK. If the lease holder currently holds a write lease on the file, and the lease breaker is opening the file for reading, then it is sufficient for the lease holder to downgrade the lease to a read lease. This is done by performing an F_SETLEASE command specifying arg as F_RDLCK.
If the lease holder fails to downgrade or remove the lease within the number of seconds specified in /proc/sys/fs/lease-break-time then the kernel forcibly removes or downgrades the lease holder's lease.
Once the lease has been voluntarily or forcibly removed or downgraded, and assuming the lease breaker has not unblocked its system call, the kernel permits the lease breaker's system call to proceed.
If the lease breaker's blocked open(2) or truncate(2) is interrupted by a signal handler, then the system call fails with the error EINTR, but the other steps still occur as described above. If the lease breaker is killed by a signal while blocked in open(2) or truncate(2), then the other steps still occur as described above. If the lease breaker specifies the O_NONBLOCK flag when calling open(2), then the call immediately fails with the error EWOULDBLOCK, but the other steps still occur as described above.
The default signal used to notify the lease holder is SIGIO, but this can be changed using the F_SETSIG command to fcntl(). If a F_SETSIG command is performed (even one specifying SIGIO), and the signal handler is established using SA_SIGINFO, then the handler will receive a siginfo_t structure as its second argument, and the si_fd field of this argument will hold the descriptor of the leased file that has been accessed by another process. (This is useful if the caller holds leases against multiple files).
Directory notifications are normally "one-shot", and the application must reregister to receive further notifications. Alternatively, if DN_MULTISHOT is included in arg, then notification will remain in effect until explicitly removed.
A series of F_NOTIFY requests is cumulative, with the events in arg being added to the set already monitored. To disable notification of all events, make an F_NOTIFY call specifying arg as 0.
Notification occurs via delivery of a signal. The default signal is SIGIO, but this can be changed using the F_SETSIG command to fcntl(). In the latter case, the signal handler receives a siginfo_t structure as its second argument (if the handler was established using SA_SIGINFO) and the si_fd field of this structure contains the file descriptor which generated the notification (useful when establishing notification on multiple directories).
Especially when using DN_MULTISHOT, a real time signal should be used for notification, so that multiple notifications can be queued.
NOTE: New applications should use the inotify interface (available since kernel 2.6.13), which provides a much superior interface for obtaining notifications of file system events. See inotify(7).
On error, -1 is returned, and errno is set appropriately.
F_GETOWN and F_SETOWN are specified in POSIX.1-2001. (To get their definitions, define BSD_SOURCE, or _XOPEN_SOURCE with the value 500 or greater, or define _POSIX_C_SOURCE with the value 200809L or greater.)
F_DUPFD_CLOEXEC is specified in POSIX.1-2008. (To get this definition, define _POSIX_C_SOURCE with the value 200809L or greater, or _XOPEN_SOURCE with the value 700 or greater.)
F_GETOWN_EX, F_SETOWN_EX, F_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific. (Define the _GNU_SOURCE macro to obtain these definitions.)
Since kernel 2.0, there is no interaction between the types of lock placed by flock(2) and fcntl().
Several systems have more fields in struct flock such as, for example, l_sysid. Clearly, l_pid alone is not going to be very useful if the process holding the lock may live on a different machine.
In Linux 2.4 and earlier, there is bug that can occur when an unprivileged process uses F_SETOWN to specify the owner of a socket file descriptor as a process (group) other than the caller. In this case, fcntl() can return -1 with errno set to EPERM, even when the owner process (group) is one that the caller has permission to send signals to. Despite this error return, the file descriptor owner is set, and signals will be sent to the owner.
The implementation of mandatory locking in all known versions of Linux is subject to race conditions which render it unreliable: a write(2) call that overlaps with a lock may modify data after the mandatory lock is acquired; a read(2) call that overlaps with a lock may detect changes to data that were made only after a write lock was acquired. Similar races exist between mandatory locks and mmap(2). It is therefore inadvisable to rely on mandatory locking.
See also locks.txt, mandatory-locking.txt, and dnotify.txt in the kernel source directory Documentation/filesystems/. (On older kernels, these files are directly under the Documentation/ directory, and mandatory-locking.txt is called mandatory.txt.)