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FCNTL(2) System Calls Manual FCNTL(2)
NAME
fcntl - file descriptor control
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int
fcntl(int fd, int cmd, ...);
DESCRIPTION
fcntl() provides for control over descriptors. The argument fd is a
descriptor to be operated on by cmd as described below. The third
parameter is called arg and is technically a pointer to void, but it is
interpreted as an int by some commands and ignored by others.
Commands are:
F_DUPFD Return a new descriptor as follows:
⊕ Lowest numbered available descriptor greater
than or equal to arg, which is interpreted as an
int.
⊕ 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 associated with the new
file descriptor is cleared to allow the file
descriptor to remain open across execve(2)
system calls.
⊕ The close-on-fork flag FD_CLOFORK associated
with the new file descriptor is cleared, so the
file descriptor will remain open across fork(2)
system calls.
F_DUPFD_CLOEXEC Same as F_DUPFD, but sets the close-on-exec property on
the file descriptor created.
F_DUPFD_CLOFORK Same as F_DUPFD, but sets the close-on-fork property on
the file descriptor created.
F_DUPFD_CLOBOTH Same as F_DUPFD, but sets both the close-on-exec and
close-on-fork properties on the file descriptor created.
F_GETFD Get the flags associated with the file descriptor fd
(arg is ignored). 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(). If the returned value ANDed with FD_CLOFORK
is not 0, the file will be closed upon execution of the
fork() family of system calls.
F_SETFD Set flags associated with fd. The available flags,
passed as arg (treated as an integer), are FD_CLOEXEC
and FD_CLOFORK.
F_GETFL Get descriptor status flags, as described below (arg is
ignored).
F_SETFL Set descriptor status flags to arg, which is interpreted
as an int.
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 argument arg is interpreted as an
int.
F_CLOSEM Close all file descriptors greater than or equal to fd,
(arg is ignored).
F_MAXFD Return the maximum file descriptor number currently open
by the process. (arg is ignored).
F_GETNOSIGPIPE Return 1 if the O_NOSIGPIPE flag is set in the file
descriptor, otherwise 0 (arg is ignored).
F_SETNOSIGPIPE If arg is 0, clear the O_NOSIGPIPE status in the file
descriptor, otherwise set it.
F_GETPATH Place a pathname corresponding to fd in the buffer
pointed to by arg. arg should be pointing to a buffer
of at least MAXPATHLEN bytes.
F_ADD_SEALS Add seals specified in arg to fd to restrict possible
operations on fd as described below. Like flags,
multiple seals can be specified at once. Additionally,
specifying seals that are already associated with fd is
a no-op.
F_GET_SEALS Get the seals currently associated with fd as described
below (arg is ignored).
The set of valid flags for the F_GETFL and F_SETFL flags are as follows:
O_APPEND, O_ASYNC, O_SYNC, O_NONBLOCK, O_DSYNC, O_RSYNC, O_ALT_IO,
O_DIRECT, O_NOSIGPIPE. These flags are described in open(2).
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 */
};
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 function call except
for the lock type l_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). As specified by the
value of l_type, 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, the result is undefined. The l_pid field is only used with
F_GETLK to return the process ID of the process holding a blocking lock.
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.
Possible seals are:
F_SEAL_SEAL Prevent any further seals from being added to fd.
F_SEAL_SHRINK Prevent the size of fd from decreasing.
F_SEAL_GROW Prevent the size of fd from increasing.
F_SEAL_WRITE Prevent any write operations to fd. F_SEAL_WRITE
cannot be applied if fd has any memory mappings.
F_SEAL_FUTURE_WRITE Like F_SEAL_WRITE but allow any current memory
mappings of fd to remain open, including those with
PROT_WRITE.
NOTES
For F_GETPATH:
⊕ For vnodes, functionality is implemented using the reverse namei(9)
cache. The implications of this are
⊕ For hard links where the file descriptor can resolve to multiple
pathnames, the first entry found in the cache is returned.
⊕ F_GETPATH may fail if the corresponding entry has been evicted
from the LRU namei(9) cache and return ENOENT.
⊕ For a file descriptor created by memfd_create(2), the name provided
at fd creation, with the prefix "memfd:" is used.
⊕ Other types of file descriptors are not handled, as well as symbolic
links since there is currently no way to obtain a file descriptor
pointing to a symbolic link.
RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (one or both of the bits FD_CLOEXEC and
FD_CLOFORK).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
F_MAXFD Value of the highest file descriptor open by the
process.
F_GET_SEALS Value of the seals currently associated with fd.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
COMPATIBILITY
This interface follows the completely stupid semantics of AT&T System V
UNIX 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) function. The
flock(2) interface has much more rational last close semantics and allows
locks to be inherited by child processes. Calling flock(2) 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. Note that flock(2) and fcntl locks may be safely used
concurrently.
All locks associated with a file for a given process are removed when the
process terminates.
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.
ERRORS
fcntl() will fail if:
[EACCES] The argument cmd is F_GETPATH and read or search
permission was denied for a component of the pathname.
[EAGAIN] The argument arg 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] fildes is not a valid open file descriptor.
The argument cmd is F_SETLK or F_SETLKW, the type of
lock (l_type) is a shared lock (F_RDLCK), and fildes
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
fildes is not a valid file descriptor open for
writing.
[EBUSY] The argument cmd is F_ADD_SEALS, arg contains
F_SEAL_WRITE and fd is currently mapped by mmap(2).
[EDEADLK] The argument cmd is F_SETLKW, and a deadlock condition
was detected.
[EINTR] The argument cmd is F_SETLKW, and the function was
interrupted by a signal.
[EINVAL] The argument cmd is invalid.
The argument cmd is F_DUPFD and arg is negative or
greater than the maximum allowable number (see
getdtablesize(3)).
The argument cmd is F_GETLK, F_SETLK, or F_SETLKW and
the data to which arg points is not valid, or fildes
refers to a file that does not support locking.
The argument cmd is F_ADD_SEALS or F_GET_SEALS and fd
does not support seals.
The argument cmd is F_ADD_SEALS and arg contains set
bits for unsupported seals.
[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.
[ENFILE] cmd is F_DUPFD and system-wide the maximum allowed
number of file descriptors are currently open.
[ENOENT] The argument cmd is F_GETPATH and a component of the
pathname no longer exists.
[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.
[ENOMEM] The argument cmd is F_GETPATH and insufficient memory
is available.
The argument cmd is F_GETLK, F_SETLK, or F_SETLKW, and
the file lock limit for the current unprivileged user
has been reached. It can be modified using the
kern.maxfiles sysctl(7).
[EPERM] The argument cmd is F_ADD_SEALS and fd already has
F_SEAL_SEAL.
[ERANGE] The argument cmd is F_GETPATH and the resulting path
would be greater than MAXPATHLEN.
[ESRCH] cmd is F_SETOWN and the process ID given as argument
is not in use.
SEE ALSO
close(2), execve(2), flock(2), open(2), sigaction(2), getdtablesize(3)
STANDARDS
The fcntl() function conforms to IEEE Std 1003.1-1990 ("POSIX.1").
HISTORY
The fcntl() function call appeared in 4.2BSD. The F_DUPFD_CLOBOTH
operation first appeared in NetBSD 11.0.
NetBSD 11.99 July 8, 2025 NetBSD 11.99