'\" t .\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" This manpage is Copyright (C) 1992 Drew Eckhardt; .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson; .\" and Copyright (C) 1998 Jamie Lokier; .\" and Copyright (C) 2002 Michael Kerrisk. .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified 1993-07-24 by Rik Faith .\" Modified 1995-09-26 by Andries Brouwer .\" and again on 960413 and 980804 and 981223. .\" Modified 1998-12-11 by Jamie Lokier .\" Applied correction by Christian Ehrhardt - aeb, 990712 .\" Modified 2002-04-23 by Michael Kerrisk .\" Added note on F_SETFL and O_DIRECT .\" Complete rewrite + expansion of material on file locking .\" Incorporated description of F_NOTIFY, drawing on .\" Stephen Rothwell's notes in Documentation/dnotify.txt. .\" Added description of F_SETLEASE and F_GETLEASE .\" Corrected and polished, aeb, 020527. .\" Modified 2004-03-03 by Michael Kerrisk .\" Modified description of file leases: fixed some errors of detail .\" Replaced the term "lease contestant" by "lease breaker" .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" Modified 2004-12-08, added O_NOATIME after note from Martin Pool .\" 2004-12-10, mtk, noted F_GETOWN bug after suggestion from aeb. .\" 2005-04-08 Jamie Lokier , mtk .\" Described behaviour of F_SETOWN/F_SETSIG in .\" multi-threaded processes, and generally cleaned .\" up the discussion of F_SETOWN. .\" 2005-05-20, Johannes Nicolai , .\" mtk: Noted F_SETOWN bug for socket file descriptor in Linux 2.4 .\" and earlier. Added text on permissions required to send signal. .\" .TH FCNTL 2 2005-05-20 "Linux 2.6.14" "Linux Programmer's Manual" .SH NAME fcntl \- manipulate file descriptor .SH SYNOPSIS .nf .B #include .B #include .sp .BI "int fcntl(int " fd ", int " cmd ); .BI "int fcntl(int " fd ", int " cmd ", long " arg ); .BI "int fcntl(int " fd ", int " cmd ", struct flock *" lock ); .fi .SH DESCRIPTION .BR fcntl () performs one of the operations described below on the open file descriptor .IR fd . The operation is determined by .IR cmd . .SS "Duplicating a file descriptor" .TP .B F_DUPFD Find the lowest numbered available file descriptor greater than or equal to .I arg and make it be a copy of .IR fd . This is different from .BR dup2 (2) which uses exactly the descriptor specified. .sp On success, the new descriptor is returned. .sp See .BR dup (2) for further details. .SS "File descriptor flags" The following commands manipulate the flags associated with a file descriptor. Currently, only one such flag is defined: .BR FD_CLOEXEC , the close-on-exec flag. If the .B FD_CLOEXEC bit is 0, the file descriptor will remain open across an .BR execve (2), otherwise it will be closed. .TP .B F_GETFD Read the file descriptor flags. .TP .B F_SETFD Set the file descriptor flags to the value specified by .IR arg . .SS "File status flags" Each open file description has certain associated status flags, initialized by .BR open (2) .\" or .\" .BR creat (2), and possibly modified by .BR fcntl (2). Duplicated file descriptors (made with .BR dup (), .BR fcntl (F_DUPFD), .BR fork (), etc.) refer to the same open file description, and thus share the same file status flags. .sp The file status flags and their semantics are described in .BR open (2). .TP .B F_GETFL Read the file status flags. .TP .B F_SETFL Set the file status flags to the value specified by .IR arg . File access mode .RB ( O_RDONLY ", " O_WRONLY ", " O_RDWR ) and file creation flags (i.e., .BR O_CREAT ", " O_EXCL ", " O_NOCTTY ", " O_TRUNC ) in .I arg are ignored. On Linux this command can only change the .BR O_APPEND , .BR O_ASYNC , .BR O_DIRECT , .BR O_NOATIME , and .BR O_NONBLOCK flags. .\" FIXME According to POSIX.1-2001, O_SYNC should also be modifiable .\" via fcntl(2), but currently Linux does not permit this .\" See http://bugzilla.kernel.org/show_bug.cgi?id=5994 .SS "Advisory locking" .BR 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 .I lock is a pointer to a structure that has at least the following fields (in unspecified order). .in +2n .nf .sp 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) */ ... }; .fi .in -2n .P The .IR l_whence ", " l_start ", and " l_len fields of this structure specify the range of bytes we wish to lock. .I l_start is the starting offset for the lock, and is interpreted relative to either: the start of the file (if .I l_whence is .BR SEEK_SET ); the current file offset (if .I l_whence is .BR SEEK_CUR ); or the end of the file (if .I l_whence is .BR SEEK_END ). In the final two cases, .I l_start can be a negative number provided the offset does not lie before the start of the file. .I l_len is a non-negative integer (but see the NOTES below) specifying the number of bytes to be locked. Bytes past the end of the file may be locked, but not bytes before the start of the file. Specifying 0 for .I l_len has the special meaning: lock all bytes starting at the location specified by .IR l_whence " and " l_start through to the end of file, no matter how large the file grows. .P The .I l_type field can be used to place a read .RB ( F_RDLCK ) or a write .RB ( 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.) .TP .B F_SETLK Acquire a lock (when .I l_type is .B F_RDLCK or .BR F_WRLCK ) or release a lock (when .I l_type is .BR F_UNLCK ) on the bytes specified by the .IR l_whence ", " l_start ", and " l_len fields of .IR lock . If a conflicting lock is held by another process, this call returns \-1 and sets .I errno to .B EACCES or .BR EAGAIN . .TP .B F_SETLKW As for .BR 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 .I errno set to .BR EINTR ). .TP .B F_GETLK On input to this call, .I lock describes a lock we would like to place on the file. If the lock could be placed, .BR fcntl () does not actually place it, but returns .B F_UNLCK in the .I l_type field of .I lock and leaves the other fields of the structure unchanged. If one or more incompatible locks would prevent this lock being placed, then .BR fcntl () returns details about one of these locks in the .IR l_type ", " l_whence ", " l_start ", and " l_len fields of .I lock and sets .I l_pid to be the PID of the process holding that lock. .P In order to place a read lock, .I fd must be open for reading. In order to place a write lock, .I fd must be open for writing. To place both types of lock, open a file read-write. .P As well as being removed by an explicit .BR F_UNLCK , record locks are automatically released when the process terminates or if it closes .I any file descriptor referring to a file on which locks are held. .\" (Additional file descriptors referring to the same file .\" may have been obtained by calls to .\" .BR open "(2), " dup "(2), " dup2 "(2), or " fcntl (2).) This is bad: it means that a process can lose the locks on a file like .I /etc/passwd or .I /etc/mtab when for some reason a library function decides to open, read and close it. .P Record locks are not inherited by a child created via .BR fork (2), but are preserved across an .BR execve (2). .P Because of the buffering performed by the .BR stdio (3) library, the use of record locking with routines in that package should be avoided; use .BR read (2) and .BR write (2) instead. .SS "Mandatory locking" (Non-POSIX.) The above record locks may be either advisory or mandatory, and are advisory by default. 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., .BR read (2) or .BR write (2)) on a file region that has an incompatible mandatory lock, then the result depends upon whether the .B O_NONBLOCK flag is enabled for its open file description. If the .B 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 .B O_NONBLOCK flag is enabled, then the system call fails with the error .BR EAGAIN or .BR EWOULDBLOCK. 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 .BR mount (8), or the .B MS_MANDLOCK flag for .BR 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 .BR chmod (1) and .BR chmod (2)). .SS "Managing signals" .BR F_GETOWN ", " F_SETOWN ", " F_GETSIG " and " F_SETSIG are used to manage I/O availability signals: .TP .B F_GETOWN Get the process ID or process group currently receiving SIGIO and SIGURG signals for events on file descriptor .IR fd . Process IDs are returned as positive values; process group IDs are returned as negative values (but see BUGS below). .TP .B F_SETOWN Set the process ID or process group ID that will receive SIGIO and SIGURG signals for events on file descriptor .IR 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, .I arg is specified as .BR getpid ()). .\" From glibc.info: If you set the .B O_ASYNC status flag on a file descriptor (either by providing this flag with the .BR open (2) .\" FIXME The statement that O_ASYNC can be used in open() does not .\" match reality; setting O_ASYNC via open() does not seem to be .\" effective. .\" See http://bugzilla.kernel.org/show_bug.cgi?id=5993 call, or by using the .B F_SETFL command of .BR fcntl ()), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor. .B 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 .B F_SETOWN is subject to the same permissions checks as are described for .BR kill (2), where the sending process is the one that employs .BR F_SETOWN (but see BUGS below). .sp If the file descriptor .I fd refers to a socket, .B 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 .BR select (2) would report the socket as having an "exceptional condition".) .\" The following appears to be rubbish. It doesn't seem to .\" be true according to the kernel source, and I can write .\" a program that gets a terminal-generated SIGIO even though .\" it is not the foreground process group of the terminal. .\" -- MTK, 8 Apr 05 .\" .\" If the file descriptor .\" .I fd .\" refers to a terminal device, then SIGIO .\" signals are sent to the foreground process group of the terminal. If a non-zero value is given to .B 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 .B F_SETOWN has a different meaning: .\" The relevant place in the (2.6) kernel source is the .\" 'switch' in fs/fcntl.c::send_sigio_to_task() -- MTK, Apr 2005 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 .B F_SETOWN the result of .BR gettid () instead of .BR getpid () to get sensible results when .B 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 .BR gettid () or .BR getpid () 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 .BR F_SETOWN . .\" send_sigurg()/send_sigurg_to_task() bypasses .\" kill_fasync()/send_sigio()/send_sigio_to_task() .\" to directly call send_group_sig_info() .\" -- MTK, Apr 2005 (kernel 2.6.11) Note also that Linux imposes a limit on the number of real-time signals that may be queued to a process (see .BR getrlimit (2) and .BR 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. .\" See fs/fcntl.c::send_sigio_to_task() (2.4/2.6) sources -- MTK, Apr 05 .TP .B F_GETSIG Get the signal sent when input or output becomes possible. A value of zero means SIGIO is sent. Any other value (including SIGIO) is the signal sent instead, and in this case additional info is available to the signal handler if installed with SA_SIGINFO. .TP .B F_SETSIG 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 SIGIO) is the signal to send instead, and in this case additional info is available to the signal handler if installed with SA_SIGINFO. .sp Additionally, passing a non-zero value to .B F_SETSIG changes the signal recipient from a whole process to a specific thread within a process. See the description of .B F_SETOWN for more details. .sp By using .B F_SETSIG with a non-zero value, and setting SA_SIGINFO for the signal handler (see .BR sigaction (2)), extra information about I/O events is passed to the handler in a .I siginfo_t structure. If the .I si_code field indicates the source is SI_SIGIO, the .I 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 .RB ( select (2), .BR poll (2), .BR read (2) with .B O_NONBLOCK set etc.) to determine which file descriptors are available for I/O. .sp 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. .PP Using these mechanisms, a program can implement fully asynchronous I/O without using .BR select (2) or .BR poll (2) most of the time. .PP The use of .BR O_ASYNC , .BR F_GETOWN , .B F_SETOWN is specific to BSD and Linux. .B F_GETSIG and .B F_SETSIG are Linux specific. POSIX has asynchronous I/O and the .I aio_sigevent structure to achieve similar things; these are also available in Linux as part of the GNU C Library (Glibc). .SS Leases .B F_SETLEASE and .B F_GETLEASE (Linux 2.4 onwards) are used (respectively) to establish and retrieve the current setting of the calling process's lease on the file referred to by .IR fd . A file lease provides a mechanism whereby the process holding the lease (the "lease holder") is notified (via delivery of a signal) when a process (the "lease breaker") tries to .BR open (2) or .BR truncate (2) that file. .TP .B F_SETLEASE Set or remove a file lease according to which of the following values is specified in the integer .IR arg : .RS .B F_RDLCK Take out a read lease. This will cause the calling process to be notified when the file is opened for writing or is truncated. .\" The following became true in kernel 2.6.10: .\" See the man-pages-2.09 Changelog for further info. A read lease can only be placed on a file descriptor that is opened read-only. .TP .B F_WRLCK Take out a write lease. This will cause the caller to be notified when the file is opened for reading or writing or is truncated. A write lease may be placed on a file only if no other process currently has the file open. .TP .B F_UNLCK Remove our lease from the file. .RE .P A process may hold only one type of lease on a file. .P Leases may only be taken out on regular files. An unprivileged process may only take out a lease on a file whose UID matches the file system UID of the process. A process with the .B CAP_LEASE capability may take out leases on arbitrary files. .TP .B F_GETLEASE Indicates what type of lease we hold on the file referred to by .I fd by returning either .BR 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 .BR fcntl () is omitted.) .PP When a process (the "lease breaker") performs an .BR open () or .BR truncate () that conflicts with a lease established via .BR 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 .B F_SETLEASE command specifying .I arg as .BR F_UNLCK . If we currently hold a write lease on the file, and the lease breaker is opening the file for reading, then it is sufficient to downgrade the lease to a read lease. This is done by performing an .B F_SETLEASE command specifying .I arg as .BR F_RDLCK . If the lease holder fails to downgrade or remove the lease within the number of seconds specified in .I /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 .BR open () or .BR truncate () is interrupted by a signal handler, then the system call fails with the error .BR EINTR , but the other steps still occur as described above. If the lease breaker is killed by a signal while blocked in .BR open () or .BR truncate (), then the other steps still occur as described above. If the lease breaker specifies the .B O_NONBLOCK flag when calling .BR open (), then the call immediately fails with the error .BR 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 .B F_SETSIG command to .BR fcntl (). If a .B F_SETSIG command is performed (even one specifying SIGIO), and the signal handler is established using SA_SIGINFO, then the handler will receive a .I siginfo_t structure as its second argument, and the .I 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). .SS "File and directory change notification (dnotify)" .TP .B F_NOTIFY (Linux 2.4 onwards) Provide notification when the directory referred to by .I fd or any of the files that it contains is changed. The events to be notified are specified in .IR arg , which is a bit mask specified by ORing together zero or more of the following bits: .TS 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]) .TE .sp (In order to obtain these definitions, the _GNU_SOURCE feature test macro must be defined.) .sp Directory notifications are normally "one-shot", and the application must re-register to receive further notifications. Alternatively, if .B DN_MULTISHOT is included in .IR arg , then notification will remain in effect until explicitly removed. .\" The following does seem a poor API-design choice... A series of .B F_NOTIFY requests is cumulative, with the events in .I arg being added to the set already monitored. To disable notification of all events, make an .B F_NOTIFY call specifying .I arg as 0. .sp Notification occurs via delivery of a signal. The default signal is SIGIO, but this can be changed using the .B F_SETSIG command to .BR fcntl (). In the latter case, the signal handler receives a .I siginfo_t structure as its second argument (if the handler was established using SA_SIGINFO) and the .I si_fd field of this structure contains the file descriptor which generated the notification (useful when establishing notification on multiple directories). .sp Especially when using .BR DN_MULTISHOT , a real time signal should be used for notification, so that multiple notifications can be queued. .B NOTE: New applications should consider using the .I inotify interface (available since kernel 2.6.13), which provides a superior interface for obtaining notifications of file system events. See .BR inotify (7). .SH "RETURN VALUE" For a successful call, the return value depends on the operation: .TP 0.9i .B F_DUPFD The new descriptor. .TP .B F_GETFD Value of flags. .TP .B F_GETFL Value of flags. .TP .B F_GETOWN Value of descriptor owner. .TP .B F_GETSIG Value of signal sent when read or write becomes possible, or zero for traditional SIGIO behaviour. .TP All other commands Zero. .PP On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .BR EACCES " or " EAGAIN Operation is prohibited by locks held by other processes. .TP .B EAGAIN The operation is prohibited because the file has been memory-mapped by another process. .TP .B EBADF .I fd is not an open file descriptor, or the command was .B F_SETLK or .B F_SETLKW and the file descriptor open mode doesn't match with the type of lock requested. .TP .B EDEADLK It was detected that the specified .B F_SETLKW command would cause a deadlock. .TP .B EFAULT .I lock is outside your accessible address space. .TP .B EINTR For .BR F_SETLKW , the command was interrupted by a signal. For .BR 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. .TP .B EINVAL For .BR F_DUPFD , .I arg is negative or is greater than the maximum allowable value. For .BR F_SETSIG , .I arg is not an allowable signal number. .TP .B EMFILE For .BR F_DUPFD , the process already has the maximum number of file descriptors open. .TP .B ENOLCK Too many segment locks open, lock table is full, or a remote locking protocol failed (e.g. locking over NFS). .TP .B EPERM Attempted to clear the .B O_APPEND flag on a file that has the append-only attribute set. .SH NOTES The errors returned by .BR dup2 () are different from those returned by .BR F_DUPFD . Since kernel 2.0, there is no interaction between the types of lock placed by .BR flock (2) and .BR fcntl (2). POSIX.1-2001 allows .I l_len to be negative. (And if it is, the interval described by the lock covers bytes .IR l_start + l_len up to and including .IR l_start \-1.) This is supported by Linux since Linux 2.4.21 and 2.5.49. Several systems have more fields in .I "struct flock" such as e.g. .IR l_sysid . Clearly, .I l_pid alone is not going to be very useful if the process holding the lock may live on a different machine. .SH BUGS A limitation of the Linux system call conventions on some architectures (notably x86) means that if a (negative) process group ID to be returned by .B F_GETOWN falls in the range \-1 to \-4095, then the return value is wrongly interpreted by glibc as an error in the system call; .\" glibc source: sysdeps/unix/sysv/linux/i386/sysdep.h that is, the return value of .BR fcntl () will be \-1, and .I errno will contain the (positive) process group ID. .\" FIXME Dec 04: some limited testing on alpha and ia64 seems to .\" indicate that ANY negative PGID value will cause F_GETOWN .\" to misinterpret the return as an error. Some other architectures .\" seem to have the same range check as x86. Must document .\" the reality on other architectures -- MTK In Linux 2.4 and earlier, there is bug that can occur when an unprivileged process uses .B F_SETOWN to specify the owner of a socket file descriptor as a process (group) other than the caller. In this case, .BR fcntl () can return \-1 with .I errno set to .BR 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. .SH "CONFORMING TO" SVr4, 4.3BSD, POSIX.1-2001. Only the operations F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK, F_SETLKW, F_GETOWN, and F_SETOWN are specified in POSIX.1-2001. F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux specific. (Define the _GNU_SOURCE macro to obtain these definitions.) .\" .PP .\" SVr4 documents additional EIO, ENOLINK and EOVERFLOW error conditions. .SH "SEE ALSO" .BR dup2 (2), .BR flock (2), .BR open (2), .BR socket (2), .BR lockf (3), .BR capabilities (7), .BR feature_test_macros (7) .P See also locks.txt, mandatory.txt, and dnotify.txt in /usr/src/linux/Documentation.