FCNTL(2) 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 argu- ment fd is a descriptor to be operated on by cmd as described below. De- pending on the value of cmd, fcntl() can take an additional third argument arg. Unless otherwise noted below for a specific operation, arg has type int. F_DUPFD Return a new descriptor as follows: * Lowest numbered available descriptor greater than or equal to arg. * Same object references as the original descrip- tor. * 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 descrip- tors 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. * The fork-on-exec flag FD_CLOFORK associated with the new file descriptor is cleared, so the file descriptor is to remain open across fork(2) sys- tem calls. * The FD_RESOLVE_BENEATH flag, described below, will be set if it was set on the original de- scriptor. 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_DUPFD_CLOFORK Like F_DUPFD, but the FD_CLOFORK flag associated with the new file descriptor is set, so the file descriptor is closed when fork(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_DUP3FD Used to implement the dup3() call. Do not use it. F_GETFD Get the flags associated with the file descriptor fd. The following flags are defined: FD_CLOEXEC The file will be closed upon execu- tion of exec() (arg is ignored). Otherwise, the file descriptor will remain open. FD_CLOFORK The file will be closed upon execu- tion of the fork() family of system calls. FD_RESOLVE_BENEATH All path name lookups relative to that file descriptor will behave as if the lookup had O_RESOLVE_BENEATH or AT_RESOLVE_BENEATH semantics. It is not permitted to call fchdir(2) or fchroot(2) on such a file descriptor. The FD_RESOLVE_BENEATH flag is sticky, meaning that it is preserved by dup(2) and similar operations, and opening a directory with openat(2) where the directory descriptor has the flag set causes the new directory descriptor to also have the flag set. F_SETFD Set flags associated with fd to arg. The available flags are FD_CLOEXEC, FD_CLOFORK and FD_RESOLVE_BENEATH. The FD_RESOLVE_BENEATH flag cannot be cleared once set. 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. F_ADD_SEALS Add seals to the file as described below, if the underly- ing filesystem supports seals. F_GET_SEALS Get seals associated with the file, if the underlying filesystem supports seals. F_ISUNIONSTACK Check if the vnode is part of a union stack (either the "union" flag from mount(2) or unionfs). This is a hack not intended to be used outside of libc. F_KINFO Fills a struct kinfo_file for the file referenced by the specified file descriptor. The arg argument should point to the storage for struct kinfo_file. The kf_structsize member of the passed structure must be initialized with the sizeof of struct kinfo_file, to allow for the inter- face versioning and evolution. The flags for the F_GETFL and F_SETFL commands 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 writ- ing. 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. O_SYNC Enable synchronous writes. Corresponds to the O_SYNC flag of open(2). O_FSYNC is an historical synonym for O_SYNC. O_DSYNC Enable synchronous data writes. Corresponds to the O_DSYNC flag of open(2). The seals that may be applied with F_ADD_SEALS are as follows: F_SEAL_SEAL Prevent any further seals from being applied to the file. F_SEAL_SHRINK Prevent the file from being shrunk with ftruncate(2). F_SEAL_GROW Prevent the file from being enlarged with ftruncate(2). F_SEAL_WRITE Prevent any further write(2) calls to the file. Any writes in progress will finish before fcntl() returns. If any writeable mappings exist, F_ADD_SEALS will fail and return EBUSY. Seals are on a per-inode basis and require support by the underlying filesystem. If the underlying filesystem does not support seals, F_ADD_SEALS and F_GET_SEALS will fail and return EINVAL. Several operations 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 */ }; These advisory file locking operations take a pointer to struct flock as the third argument arg. 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 informa- tion 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 descrip- tion 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 imme- diately 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 un- til 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 speci- fied 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 pre- vents 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 descrip- tor 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 nega- tive, 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 effi- cient. 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 descrip- tions of shared locks and exclusive locks, an F_SETLK or an F_SETLKW re- quest fails or blocks respectively when another process has existing locks on bytes in the specified region and the type of any of those locks con- flicts 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 en- sure 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. How- ever, 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 fc- ntl() 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 de- scriptor 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 flags. 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 er- ror. 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 exclu- sive-locked by another process; or the type is an exclu- sive 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. [EBUSY] The argument cmd is F_ADD_SEALS, attempting to set F_SEAL_WRITE, and writeable mappings of the file exist. [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 getdta- blesize(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 F_ADD_SEALS or F_GET_SEALS, and the underlying filesystem does not support sealing. 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. The cmd argument is F_ADD_SEALS and the F_SEAL_SEAL seal has already been set. [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 op- posed 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), sigac- tion(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3) STANDARDS The F_DUP2FD and F_DUP3FD constants are not portable. They are 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 con- tinue waiting during thread suspension such as a stop signal. However, in this implementation a call with F_SETLKW is restarted after catching a sig- nal 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. The F_DUPFD_CLOFORK and F_DUP3FD flags appeared in FreeBSD 15.0. FreeBSD 15.1 STABLE June 24, 2025 FCNTL(2)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY
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