fcntl(2) fcntl(2)
NAME [Toc] [Back]
fcntl - file control
SYNOPSIS [Toc] [Back]
#include <fcntl.h>
int fcntl(int fildes, int cmd, ... /* arg */);
Remarks [Toc] [Back]
The ANSI C ", ..." construct denotes a variable length argument list
whose optional [or required] members are given in the associated
comment (/* */).
DESCRIPTION [Toc] [Back]
fcntl() provides for control over open files. fildes is an open file
descriptor.
The following are possible values for the cmd argument:
F_DUPFD Return a new file descriptor having the following
characteristics:
+ Lowest numbered available file descriptor
greater than or equal to the third
argument, arg, taken as an integer of type
int.
+ Same open file (or pipe) as the original
file.
+ Same file pointer as the original file
(that is, both file descriptors share one
file pointer).
+ Same access mode (read, write or
read/write).
+ Same file status flags (that is, both file
descriptors share the same file status
flags).
+ The close-on-exec flag associated with the
new file descriptor is set to remain open
across exec(2) system calls.
F_GETFD Get the close-on-exec flag associated with the
file descriptor fildes. If the low-order bit is 0
the file will remain open across exec(2),
otherwise the file will be closed upon execution
of exec(2).
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fcntl(2) fcntl(2)
F_SETFD Set the close-on-exec flag associated with fildes
to the low-order bit of the third argument, arg,
taken as an integer of type int (see F_GETFD).
F_GETFL Get file status flags and access modes; see
fcntl(5).
F_SETFL Set file status flags to the third argument, arg,
taken as an integer of type int. Only certain
flags can be set; see fcntl(5). It is not
possible to set both O_NDELAY and O_NONBLOCK.
F_GETLK Get the first lock that blocks the lock described
by the variable of type struct flock pointed to by
the third argument, arg, taken as a pointer to
type struct flock. 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 passed back unchanged, except that
the lock type is set to F_UNLCK.
F_SETLK Set or clear a file segment lock according to the
variable of type struct flock pointed to by the
third argument, arg, taken as a pointer to type
struct flock (see fcntl(5)). The cmd F_SETLK is
used to establish read (F_RDLCK) and write
(F_WRLCK) locks, as well as to remove either type
of lock (F_UNLCK). If a read or write lock cannot
be set, fcntl() returns immediately with an error
value of -1.
F_SETLKW This cmd is the same as F_SETLK except that if a
read or write lock is blocked by other locks, the
process will sleep until the segment is free to be
locked.
F_GETOWN If fildes refers to a socket, fcntl() returns the
process or process group ID specified to receive
SIGURG signals when out-of-band data is available.
Positive values indicate a process ID; negative
values, other than -1, indicate a process group
ID.
F_SETOWN If fildes refers to a socket, fcntl() sets the
process or process group ID specified to receive
SIGURG signals when out-of-band data is available,
using the value of the third argument, arg, taken
as type int. Positive values indicate a process
ID; negative values, other than -1, indicate a
process group ID.
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fcntl(2) fcntl(2)
F_GETLK64 Same as F_GETLK, except arg is a pointer to struct
flock64 instead of struct flock.
F_SETLK64 Same as F_SETLK, except arg is a pointer to struct
flock64 instead of struct flock.
F_SETLKW64 Same as F_SETLKW, except arg is a pointer to
struct flock64 instead of struct flock.
A read lock prevents any other process from write-locking the
protected area. More than one read lock can exist for a given
segment of a file at a given time. The file descriptor on which
a read lock is being placed must have been opened with read
access.
A write lock prevents any other process from read-locking or
write-locking the protected area. Only one write lock may exist
for a given segment of a file at a given time. The file
descriptor on which a write lock is being placed must have been
opened with write access.
The structure flock describes the type (l_type), starting offset
(l_whence), relative offset (l_start), size (l_len), and process
ID (l_pid) of the segment of the file to be affected. The
process ID field is only used with the F_GETLK cmd to return the
value of a block in lock. Locks can start and extend beyond the
current end of a file, but cannot be negative relative to the
beginning of the file. A lock can be set to always extend to the
end of file by setting l_len to zero (0). If such a lock also
has l_start set to zero (0), the whole file will be locked.
Changing or unlocking a segment from the middle of a larger
locked segment leaves two smaller segments for either end.
Locking a segment already locked by the calling process causes
the old lock type to be removed and the new lock type to take
effect. All locks associated with a file for a given process are
removed when a file descriptor for that file is closed by that
process or the process holding that file descriptor terminates.
Locks are not inherited by a child process in a fork(2) system
call.
When enforcement-mode file and record locking is activated on a
file (see chmod(2)), future read() and write() system calls on
the file are affected by the record locks in effect.
Application Usage [Toc] [Back]
Because in the future the external variable errno will be set to
[EAGAIN] rather than [EACCES] when a section of a file is already
locked by another process, portable application programs should expect
and test for either value. For example:
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fcntl(2) fcntl(2)
flk->l_type = F_RDLCK;
if (fcntl(fd, F_SETLK, flk) == -1)
if ((errno == EACCES) || (errno == EAGAIN))
/*
* section locked by another process,
* check for either EAGAIN or EACCES
* due to different implementations
*/
else if ...
/*
* check for other errors
*/
NETWORKING FEATURES [Toc] [Back]
NFS
The advisory record-locking capabilities of fcntl() are implemented
throughout the network by the ``network lock daemon'' (see lockd(1M)).
If the file server crashes and is rebooted, the lock daemon attempts
to recover all locks associated with the crashed server. If a lock
cannot be reclaimed, the process that held the lock is issued a
SIGLOST signal.
Record locking, as implemented for NFS files, is only advisory.
RETURN VALUE [Toc] [Back]
Upon successful completion, the value returned depends on cmd as
follows:
F_DUPFD A new file descriptor.
F_GETFD Value of close-on-exec flag (only the low-order
bit is defined).
F_SETFD Value other than -1.
F_GETFL Value of file status flags and access modes.
F_SETFL Value other than -1.
F_GETLK Value other than -1.
F_SETLK Value other than -1.
F_SETLKW Value other than -1.
F_GETOWN Value of process or process group ID specified to
receive SIGURG signals when out-of-band data is
available.
F_SETOWN Value other than -1.
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fcntl(2) fcntl(2)
F_GETLK64 Value other than -1.
F_SETLK64 Value other than -1.
F_SETLKW64 Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
ERRORS [Toc] [Back]
fcntl() fails if any of the following conditions occur:
[EBADF] fildes is not a valid open file descriptor, or was
not opened for reading when setting a read lock or
for writing when setting a write lock.
[EMFILE] cmd is F_DUPFD and the maximum number of file
descriptors is currently open.
[EMFILE] cmd is F_SETLK or F_SETLKW, the type of lock is a
read or write lock, and no more file-locking
headers are available (too many files have
segments locked).
[EINVAL] cmd is F_DUPFD and arg is greater than or equal to
the maximum number of file descriptors.
[EINVAL] cmd is F_DUPFD and arg is negative.
[EINVAL] cmd is F_GETLK, F_SETLK, or F_SETLKW, and arg or
the data it points to is not valid, or fildes
refers to a file that does not support locking.
[EINVAL] cmd is not a valid command.
[EINVAL] cmd is F_SETFL and both O_NONBLOCK and O_NDELAY
are specified.
[EINTR] cmd is F_SETLKW and the call was interrupted by a
signal.
[EACCES] cmd is F_SETLK, the type of lock (l_type) is a
read lock (F_RDLCK) or write lock (F_WRLCK) and
the segment of a file to be locked is already
write-locked by another process, or the type is a
write lock (F_WRLCK) and the segment of a file to
be locked is already read- or write-locked by
another process.
[ENOLCK] cmd is F_SETLK or F_SETLKW, the type of lock is a
read or write lock, and no more file-locking
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fcntl(2) fcntl(2)
headers are available (too many files have
segments locked), or no more record locks are
available (too many file segments locked).
[ENOLCK] cmd is F_SETLK or F_SETLKW, the type of lock
(l_type) is a read lock (F_RDLCK) or write lock
(F_WRLCK) and the file is an NFS file with access
bits set for enforcement mode.
[ENOLCK] cmd is F_GETLK, F_SETLK, or F_SETLKW, the file is
an NFS file, and a system error occurred on the
remote node.
[EOVERFLOW] cmd is F_GETLK and the blocking lock's starting
offset or length would not fit in the caller's
structure.
[EDEADLK] cmd is F_SETLKW, when the lock is blocked by a
lock from another process and sleeping (waiting)
for that lock to become free. This causes a
deadlock situation.
[EAGAIN] cmd is F_SETLK or F_SETLKW, and the file is mapped
in to virtual memory via the mmap() system call
(see mmap(2)).
[EFAULT] cmd is either F_GETLK, F_SETLK, or F_SETLKW, and
arg points to an illegal address.
[ENOTSOCK] cmd is F_GETOWN or F_SETOWN, and fildes does not
refer to a socket.
AUTHOR [Toc] [Back]
fcntl() was developed by HP, AT&T and the University of California,
Berkeley.
SEE ALSO [Toc] [Back]
lockd(1M), statd(1M), chmod(2), close(2), creat64(2), exec(2),
lockf(2), open(2), read(2), write(2), fcntl(5).
STANDARDS CONFORMANCE [Toc] [Back]
fcntl(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
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