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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.
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.
The argument cmd is invalid.
[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 Version 4 of the Single UNIX Specification (“SUSv4”), 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.
Debian Nov 15, 2018 FCNTL(2)