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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.
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)).
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.
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.
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.
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.
[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.
HISTORY
The fcntl() system call appeared in 4.2BSD.
The F_DUP2FD constant first appeared in FreeBSD 7.1.