fcntl

Find the lowest numbered available file descriptor greater than or equal to arg and make it be a copy of R fd . This is different from dup2(2) which uses exactly the descriptor specified. On success, the new descriptor is returned. See dup(2) for further details.

File descriptor flags

The following commands manipulate the flags associated with a file descriptor. Currently, only one such flag is defined: R FD_CLOEXEC , the close-on-exec flag. If the FD_CLOEXEC bit is 0, the file descriptor will remain open across an execve(2), otherwise it will be closed.

Read the file descriptor flags.

Set the file descriptor flags to the value specified by R arg .

File status flags

Each open file description has certain associated status flags, initialized by open(2) and possibly modified by fcntl(2). Duplicated file descriptors (made with dup(2), R fcntl (F_DUPFD), fork(2), etc.) refer to the same open file description, and thus share the same file status flags. The file status flags and their semantics are described in open(2).

Read the file status flags.

Set the file status flags to the value specified by R arg . File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags (i.e., R O_CREAT , O_EXCL , O_NOCTTY , O_TRUNC ) in arg are ignored. On Linux this command can only change the R O_APPEND , R O_ASYNC , R O_DIRECT , R O_NOATIME , and R O_NONBLOCK flags.

Advisory locking

R F_GETLK , F_SETLK and F_SETLKW are used to acquire, release, and test for the existence of record locks (also known as file-segment or file-region locks). The third argument lock is a pointer to a structure that has at least the following fields (in unspecified order).

 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) */
    ...
};

Acquire a lock (when l_type is F_RDLCK or R F_WRLCK ) or release a lock (when l_type is R F_UNLCK ) on the bytes specified by the R l_whence , l_start , and l_len fields of R lock . If a conflicting lock is held by another process, this call returns -1 and sets errno to EACCES or R EAGAIN .

As for R F_SETLK , but if a conflicting lock is held on the file, then wait for that lock to be released. If a signal is caught while waiting, then the call is interrupted and (after the signal handler has returned) returns immediately (with return value -1 and errno set to R EINTR ).

On input to this call, lock describes a lock we would like to place on the file. If the lock could be placed, R fcntl () does not actually place it, but returns F_UNLCK in the l_type field of lock and leaves the other fields of the structure unchanged. If one or more incompatible locks would prevent this lock being placed, then R fcntl () returns details about one of these locks in the R l_type , l_whence , l_start , and l_len fields of lock and sets l_pid to be the PID of the process holding that lock.

Get the process ID or process group currently receiving SIGIO and SIGURG signals for events on file descriptor R fd . Process IDs are returned as positive values; process group IDs are returned as negative values (but see BUGS below).

Set the process ID or process group ID that will receive SIGIO and SIGURG signals for events on file descriptor R fd . A process ID is specified as a positive value; a process group ID is specified as a negative value. Most commonly, the calling process specifies itself as the owner (that is, arg is specified as getpid(2)). If you set the O_ASYNC status flag on a file descriptor by using the F_SETFL command of R 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 R 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 R 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".) If a non-zero value is given to F_SETSIG in a multi-threaded process running with a threading library that supports thread groups (e.g., NPTL), then a positive value given to F_SETOWN has a different meaning: instead of being a process ID identifying a whole process, it is a thread ID identifying a specific thread within a process. Consequently, it may be necessary to pass F_SETOWN the result of gettid(2) instead of getpid(2) to get sensible results when F_SETSIG is used. (In current Linux threading implementations, a main thread's thread ID is the same as its process ID. This means that a single-threaded program can equally use gettid(2) or getpid(2) in this scenario.) Note, however, that the statements in this paragraph do not apply to the SIGURG signal generated for out-of-band data on a socket: this signal is always sent to either a process or a process group, depending on the value given to R F_SETOWN . Note also 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 R SIGIO , and this signal is delivered to the entire process rather than to a specific thread.

Get the signal sent when input or output becomes possible. A value of zero means SIGIO is sent. Any other value (including R SIGIO ) is the signal sent instead, and in this case additional info is available to the signal handler if installed with R SA_SIGINFO .

Sets the signal sent when input or output becomes possible. A value of zero means to send the default SIGIO signal. Any other value (including R SIGIO ) is the signal to send instead, and in this case additional info is available to the signal handler if installed with R SA_SIGINFO . Additionally, passing a non-zero value to F_SETSIG changes the signal recipient from a whole process to a specific thread within a process. See the description of F_SETOWN for more details. By using F_SETSIG with a non-zero 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 R 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 >= R 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.

Set or remove a file lease according to which of the following values is specified in the integer R arg :

F_WRLCK

F_UNLCK

Indicates what type of lease we hold on the file referred to by fd by returning either R F_RDLCK , F_WRLCK , or F_UNLCK, indicating, respectively, that the calling process holds a read, a write, or no lease on the file. (The third argument to R fcntl () is omitted.)

(Linux 2.4 onwards) Provide notification when the directory referred to by fd or any of the files that it contains is changed. The events to be notified are specified in R arg , which is a bit mask specified by ORing together zero or more of the following bits: l l ---- lB l. Bit Description (event in directory) DN_ACCESS A file was accessed (read, pread, readv) DN_MODIFY A file was modified (write, pwrite, writev, truncate, ftruncate) DN_CREATE A file was created (open, creat, mknod, mkdir, link, symlink, rename) DN_DELETE A file was unlinked (unlink, rename to another directory, rmdir) DN_RENAME A file was renamed within this directory (rename) DN_ATTRIB The attributes of a file were changed (chown, chmod, utime[s]) (In order to obtain these definitions, the _GNU_SOURCE feature test macro must be defined.) Directory notifications are normally "one-shot", and the application must re-register to receive further notifications. Alternatively, if DN_MULTISHOT is included in R 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 R SIGIO , but this can be changed using the F_SETSIG command to R fcntl (). In the latter case, the signal handler receives a siginfo_t structure as its second argument (if the handler was established using R 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 R DN_MULTISHOT , a real time signal should be used for notification, so that multiple notifications can be queued. NOTE: New applications should consider using the inotify interface (available since kernel 2.6.13), which provides a superior interface for obtaining notifications of file system events. See inotify(7).

The new descriptor.

Value of flags.

Value of flags.

Value of descriptor owner.

Value of signal sent when read or write becomes possible, or zero for traditional SIGIO behavior.

Zero.

Operation is prohibited by locks held by other processes.

The operation is prohibited because the file has been memory-mapped by another process.

fd is not an open file descriptor, or the command was F_SETLK or F_SETLKW and the file descriptor open mode doesn't match with the type of lock requested.

It was detected that the specified F_SETLKW command would cause a deadlock.

lock is outside your accessible address space.

For R F_SETLKW , the command was interrupted by a signal. For R F_GETLK and F_SETLK , the command was interrupted by a signal before the lock was checked or acquired. Most likely when locking a remote file (e.g., locking over NFS), but can sometimes happen locally.

For R F_DUPFD , arg is negative or is greater than the maximum allowable value. For R F_SETSIG , arg is not an allowable signal number.

For R F_DUPFD , the process already has the maximum number of file descriptors open.

Too many segment locks open, lock table is full, or a remote locking protocol failed (e.g., locking over NFS).

Attempted to clear the O_APPEND flag on a file that has the append-only attribute set.