fcntl

FCNTL(2)                     BSD System Calls Manual                    FCNTL(2)

NAME
     fcntl — file control

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <fcntl.h>

     int
     fcntl(int fd, int cmd, ...);

DESCRIPTION
     The fcntl() system call provides for control over descriptors.  The
     argument fd is a descriptor to be operated on by cmd as described below.
     Depending on the value of cmd, fcntl() can take an additional third
     argument int arg.

     F_DUPFD           Return a new descriptor as follows:

                           Lowest numbered available descriptor greater than
                               or equal to arg.
                           Same object references as the original
                               descriptor.
                           New descriptor shares the same file offset if the
                               object was a file.
                           Same access mode (read, write or read/write).
                           Same file status flags (i.e., both file
                               descriptors share the same file status flags).
                           The close-on-exec flag FD_CLOEXEC associated with
                               the new file descriptor is cleared, so the file
                               descriptor is to remain open across execve(2)
                               system calls.

     F_DUPFD_CLOEXEC   Like F_DUPFD, but the FD_CLOEXEC flag associated with the
                       new file descriptor is set, so the file descriptor is
                       closed when execve(2) system call executes.

     F_DUP2FD          It is functionally equivalent to

                             dup2(fd, arg)

     F_DUP2FD_CLOEXEC  Like F_DUP2FD, but the FD_CLOEXEC flag associated with
                       the new file descriptor is set.

                       The F_DUP2FD and F_DUP2FD_CLOEXEC constants are not
                       portable, so they should not be used if portability is
                       needed.  Use dup2() instead of F_DUP2FD.

     F_GETFD           Get the close-on-exec flag associated with the file
                       descriptor fd as FD_CLOEXEC.  If the returned value ANDed
                       with FD_CLOEXEC is 0, the file will remain open across
                       exec(), otherwise the file will be closed upon execution
                       of exec() (arg is ignored).

     F_SETFD           Set the close-on-exec flag associated with fd to arg,
                       where arg is either 0 or FD_CLOEXEC, as described above.

     F_GETFL           Get descriptor status flags, as described below (arg is
                       ignored).

     F_SETFL           Set descriptor status flags to arg.

     F_GETOWN          Get the process ID or process group currently receiving
                       SIGIO and SIGURG signals; process groups are returned as
                       negative values (arg is ignored).

     F_SETOWN          Set the process or process group to receive SIGIO and
                       SIGURG signals; process groups are specified by supplying
                       arg as negative, otherwise arg is interpreted as a
                       process ID.

     F_READAHEAD       Set or clear the read ahead amount for sequential access
                       to the third argument, arg, which is rounded up to the
                       nearest block size.  A zero value in arg turns off read
                       ahead, a negative value restores the system default.

     F_RDAHEAD         Equivalent to Darwin counterpart which sets read ahead
                       amount of 128KB when the third argument, arg is non-zero.
                       A zero value in arg turns off read ahead.

     The flags for the F_GETFL and F_SETFL flags are as follows:

     O_NONBLOCK   Non-blocking I/O; if no data is available to a read(2) system
                  call, or if a write(2) operation would block, the read or
                  write call returns -1 with the error EAGAIN.

     O_APPEND     Force each write to append at the end of file; corresponds to
                  the O_APPEND flag of open(2).

     O_DIRECT     Minimize or eliminate the cache effects of reading and
                  writing.  The system will attempt to avoid caching the data
                  you read or write.  If it cannot avoid caching the data, it
                  will minimize the impact the data has on the cache.  Use of
                  this flag can drastically reduce performance if not used with
                  care.

     O_ASYNC      Enable the SIGIO signal to be sent to the process group when
                  I/O is possible, e.g., upon availability of data to be read.

     Several commands are available for doing advisory file locking; they all
     operate on the following structure:

     struct flock {
             off_t   l_start;        /* starting offset */
             off_t   l_len;          /* len = 0 means until end of file */
             pid_t   l_pid;          /* lock owner */
             short   l_type;         /* lock type: read/write, etc. */
             short   l_whence;       /* type of l_start */
             int     l_sysid;        /* remote system id or zero for local */
     };
     The commands available for advisory record locking are as follows:

     F_GETLK    Get the first lock that blocks the lock description pointed to
                by the third argument, arg, taken as a pointer to a struct flock
                (see above).  The information retrieved overwrites the
                information passed to fcntl() in the flock structure.  If no
                lock is found that would prevent this lock from being created,
                the structure is left unchanged by this system call except for
                the lock type which is set to F_UNLCK.

     F_SETLK    Set or clear a file segment lock according to the lock
                description pointed to by the third argument, arg, taken as a
                pointer to a struct flock (see above).  F_SETLK is used to
                establish shared (or read) locks (F_RDLCK) or exclusive (or
                write) locks, (F_WRLCK), as well as remove either type of lock
                (F_UNLCK).  If a shared or exclusive lock cannot be set, fcntl()
                returns immediately with EAGAIN.

     F_SETLKW   This command is the same as F_SETLK except that if a shared or
                exclusive lock is blocked by other locks, the process waits
                until the request can be satisfied.  If a signal that is to be
                caught is received while fcntl() is waiting for a region, the
                fcntl() will be interrupted if the signal handler has not
                specified the SA_RESTART (see sigaction(2)).

     When a shared lock has been set on a segment of a file, other processes can
     set shared locks on that segment or a portion of it.  A shared lock
     prevents any other process from setting an exclusive lock on any portion of
     the protected area.  A request for a shared lock fails if the file
     descriptor was not opened with read access.

     An exclusive lock prevents any other process from setting a shared lock or
     an exclusive lock on any portion of the protected area.  A request for an
     exclusive lock fails if the file was not opened with write access.

     The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that
     the relative offset, l_start bytes, will be measured from the start of the
     file, current position, or end of the file, respectively.  The value of
     l_len is the number of consecutive bytes to be locked.  If l_len is
     negative, l_start means end edge of the region.  The l_pid and l_sysid
     fields are only used with F_GETLK to return the process ID of the process
     holding a blocking lock and the system ID of the system that owns that
     process.  Locks created by the local system will have a system ID of zero.
     After a successful F_GETLK request, the value of l_whence is SEEK_SET.

     Locks may start and extend beyond the current end of a file, but may not
     start or extend before the beginning of the file.  A lock is set to extend
     to the largest possible value of the file offset for that file if l_len is
     set to zero.  If l_whence and l_start point to the beginning of the file,
     and l_len is zero, the entire file is locked.  If an application wishes
     only to do entire file locking, the flock(2) system call is much more
     efficient.

     There is at most one type of lock set for each byte in the file.  Before a
     successful return from an F_SETLK or an F_SETLKW request when the calling
     process has previously existing locks on bytes in the region specified by
     the request, the previous lock type for each byte in the specified region
     is replaced by the new lock type.  As specified above under the
     descriptions of shared locks and exclusive locks, an F_SETLK or an F_SETLKW
     request fails or blocks respectively when another process has existing
     locks on bytes in the specified region and the type of any of those locks
     conflicts with the type specified in the request.

     The queuing for F_SETLKW requests on local files is fair; that is, while
     the thread is blocked, subsequent requests conflicting with its requests
     will not be granted, even if these requests do not conflict with existing
     locks.

     This interface follows the completely stupid semantics of System V and IEEE
     Std 1003.1-1988 (“POSIX.1”) that require that all locks associated with a
     file for a given process are removed when any file descriptor for that file
     is closed by that process.  This semantic means that applications must be
     aware of any files that a subroutine library may access.  For example if an
     application for updating the password file locks the password file database
     while making the update, and then calls getpwnam(3) to retrieve a record,
     the lock will be lost because getpwnam(3) opens, reads, and closes the
     password database.  The database close will release all locks that the
     process has associated with the database, even if the library routine never
     requested a lock on the database.  Another minor semantic problem with this
     interface is that locks are not inherited by a child process created using
     the fork(2) system call.  The flock(2) interface has much more rational
     last close semantics and allows locks to be inherited by child processes.
     The flock(2) system call is recommended for applications that want to
     ensure the integrity of their locks when using library routines or wish to
     pass locks to their children.

     The fcntl(), flock(2), and lockf(3) locks are compatible.  Processes using
     different locking interfaces can cooperate over the same file safely.
     However, only one of such interfaces should be used within the same
     process.  If a file is locked by a process through flock(2), any record
     within the file will be seen as locked from the viewpoint of another
     process using fcntl() or lockf(3), and vice versa.  Note that
     fcntl(F_GETLK) returns -1 in l_pid if the process holding a blocking lock
     previously locked the file descriptor by flock(2).

     All locks associated with a file for a given process are removed when the
     process terminates.

     All locks obtained before a call to execve(2) remain in effect until the
     new program releases them.  If the new program does not know about the
     locks, they will not be released until the program exits.

     A potential for deadlock occurs if a process controlling a locked region is
     put to sleep by attempting to lock the locked region of another process.
     This implementation detects that sleeping until a locked region is unlocked
     would cause a deadlock and fails with an EDEADLK error.

RETURN VALUES
     Upon successful completion, the value returned depends on cmd as follows:

           F_DUPFD    A new file descriptor.

           F_DUP2FD   A file descriptor equal to arg.

           F_GETFD    Value of flag (only the low-order bit is defined).

           F_GETFL    Value of flags.

           F_GETOWN   Value of file descriptor owner.

           other      Value other than -1.

     Otherwise, a value of -1 is returned and errno is set to indicate the
     error.

ERRORS
     The fcntl() system call will fail if:

     [EAGAIN]           The argument cmd is F_SETLK, the type of lock (l_type)
                        is a shared lock (F_RDLCK) or exclusive lock (F_WRLCK),
                        and the segment of a file to be locked is already
                        exclusive-locked by another process; or the type is an
                        exclusive lock and some portion of the segment of a file
                        to be locked is already shared-locked or exclusive-
                        locked by another process.

     [EBADF]            The fd argument is not a valid open file descriptor.

                        The argument cmd is F_DUP2FD, and arg is not a valid
                        file descriptor.

                        The argument cmd is F_SETLK or F_SETLKW, the type of
                        lock (l_type) is a shared lock (F_RDLCK), and fd is not
                        a valid file descriptor open for reading.

                        The argument cmd is F_SETLK or F_SETLKW, the type of
                        lock (l_type) is an exclusive lock (F_WRLCK), and fd is
                        not a valid file descriptor open for writing.

     [EDEADLK]          The argument cmd is F_SETLKW, and a deadlock condition
                        was detected.

     [EINTR]            The argument cmd is F_SETLKW, and the system call was
                        interrupted by a signal.

     [EINVAL]           The cmd argument is F_DUPFD and arg is negative or
                        greater than the maximum allowable number (see
                        getdtablesize(2)).

                        The argument cmd is F_GETLK, F_SETLK or F_SETLKW and the
                        data to which arg points is not valid.

     [EMFILE]           The argument cmd is F_DUPFD and the maximum number of
                        file descriptors permitted for the process are already
                        in use, or no file descriptors greater than or equal to
                        arg are available.

     [ENOTTY]           The fd argument is not a valid file descriptor for the
                        requested operation.  This may be the case if fd is a
                        device node, or a descriptor returned by kqueue(2).

     [ENOLCK]           The argument cmd is F_SETLK or F_SETLKW, and satisfying
                        the lock or unlock request would result in the number of
                        locked regions in the system exceeding a system-imposed
                        limit.

     [EOPNOTSUPP]       The argument cmd is F_GETLK, F_SETLK or F_SETLKW and fd
                        refers to a file for which locking is not supported.

     [EOVERFLOW]        The argument cmd is F_GETLK, F_SETLK or F_SETLKW and an
                        off_t calculation overflowed.

     [EPERM]            The cmd argument is F_SETOWN and the process ID or
                        process group given as an argument is in a different
                        session than the caller.

     [ESRCH]            The cmd argument is F_SETOWN and the process ID given as
                        argument is not in use.

     In addition, if fd refers to a descriptor open on a terminal device (as
     opposed to a descriptor open on a socket), a cmd of F_SETOWN can fail for
     the same reasons as in tcsetpgrp(3), and a cmd of F_GETOWN for the reasons
     as stated in tcgetpgrp(3).

SEE ALSO
     close(2), dup2(2), execve(2), flock(2), getdtablesize(2), open(2),
     sigaction(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3)

STANDARDS
     The F_DUP2FD constant is non portable.  It is provided for compatibility
     with AIX and Solaris.

     Per , a call with F_SETLKW should fail with [EINTR] after any caught signal
     and should continue waiting during thread suspension such as a stop signal.
     However, in this implementation a call with F_SETLKW is restarted after
     catching a signal with a SA_RESTART handler or a thread suspension such as
     a stop signal.

HISTORY
     The fcntl() system call appeared in 4.2BSD.

     The F_DUP2FD constant first appeared in FreeBSD 7.1.

BSD                                May 2, 2018                               BSD