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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94 .\" .Dd August 8, 2018 .Dt FCNTL 2 .Os BSD 4.2 .Sh NAME .Nm fcntl .Nd file control .Sh SYNOPSIS .Fd #include .Ft int .Fo fcntl .Fa "int fildes" .Fa "int cmd" .Fa "..." .Fc .Sh DESCRIPTION .Fn fcntl provides for control over descriptors. The argument .Fa fildes is a descriptor to be operated on by .Fa cmd as follows: .Bl -tag -width F_WRITEBOOTSTRAPX .It Dv F_DUPFD Return a new descriptor as follows: .Pp .Bl -bullet -compact -offset 4n .It Lowest numbered available descriptor greater than or equal to .Fa arg . .It Same object references as the original descriptor. .It New descriptor shares the same file offset if the object was a file. .It Same access mode (read, write or read/write). .It Same file status flags (i.e., both file descriptors share the same file status flags). .It The close-on-exec flag associated with the new file descriptor is cleared so that the descriptor remains open across an .Xr execv 2 system call. .El .It Dv F_DUPFD_CLOEXEC Like .Dv F_DUPFD , except that the close-on-exec flag associated with the new file descriptor is set. .It Dv F_GETFD Get the flags associated with the file descriptor .Fa fildes , as described below .Fa ( arg is ignored). .It Dv F_SETFD Set the file descriptor flags to .Fa arg . .It Dv F_GETFL Get descriptor status flags, as described below .Fa ( arg is ignored). .It Dv F_SETFL Set descriptor status flags to .Fa arg . .It Dv F_GETOWN Get the process ID or process group currently receiving .Dv SIGIO and .Dv SIGURG signals; process groups are returned as negative values .Fa ( arg is ignored). .It Dv F_SETOWN Set the process or process group to receive .Dv SIGIO and .Dv SIGURG signals; process groups are specified by supplying .Fa arg as negative, otherwise .Fa arg is interpreted as a process ID. .It Dv F_GETPATH Get the path of the file descriptor .Fa Fildes . The argument must be a buffer of size .Sy MAXPATHLEN or greater. .It Dv F_PREALLOCATE Preallocate file storage space. Note: upon success, the space that is allocated can be the size requested, larger than the size requested, or (if the .Dv F_ALLOCATEALL flag is not provided) smaller than the space requested. .It Dv F_PUNCHHOLE Deallocate a region and replace it with a hole. Subsequent reads of the affected region will return bytes of zeros that are usually not backed by physical blocks. This will not change the actual file size. Holes must be aligned to file system block boundaries. This will fail on file systems that do not support this interface. .It Dv F_SETSIZE Truncate a file without zeroing space. The calling process must have root privileges. .It Dv F_RDADVISE Issue an advisory read async with no copy to user. .It Dv F_RDAHEAD Turn read ahead off/on. A zero value in .Fa arg disables read ahead. A non-zero value in .Fa arg turns read ahead on. .It Dv F_READBOOTSTRAP Read bootstrap from disk. .It Dv F_WRITEBOOTSTRAP Write bootstrap on disk. The calling process must have root privileges. .It Dv F_NOCACHE Turns data caching off/on. A non-zero value in .Fa arg turns data caching off. A value of zero in .Fa arg turns data caching on. .It Dv F_LOG2PHYS Get disk device information. Currently this only returns the disk device address that corresponds to the current file offset. Note that the system may return -1 as the disk device address if the file is not backed by physical blocks. This is subject to change. .It Dv F_LOG2PHYS_EXT Variant of F_LOG2PHYS that uses the passed in file offset and length. .It Dv F_FULLFSYNC Does the same thing as .Xr fsync 2 then asks the drive to flush all buffered data to the permanent storage device .Fa ( arg is ignored). This is currently implemented on HFS, MS-DOS (FAT), and Universal Disk Format (UDF) file systems. The operation may take quite a while to complete. Certain FireWire drives have also been known to ignore the request to flush their buffered data. .It Dv F_SETNOSIGPIPE Determines whether a .Dv SIGPIPE signal will be generated when a write fails on a pipe or socket for which there is no reader. If .Fa arg is non-zero, .Dv SIGPIPE generation is disabled for descriptor .Fa fildes , while an .Fa arg of zero enables it (the default). .It Dv F_GETNOSIGPIPE Returns whether a .Dv SIGPIPE signal will be generated when a write fails on a pipe or socket for which there is no reader. The semantics of the return value match those of the .Fa arg of .Dv F_SETNOSIGPIPE . .El .Pp The flags for the .Dv F_GETFD and .Dv F_SETFD commands are as follows: .Bl -tag -width FD_CLOEXECX -offset indent .It Dv FD_CLOEXEC Close-on-exec; the given file descriptor will be automatically closed in the successor process image when one of the .Xr execv 2 or .Xr posix_spawn 2 family of system calls is invoked. .El .Pp The flags for the .Dv F_GETFL and .Dv F_SETFL commands are as follows: .Bl -tag -width O_NONBLOCKX -offset indent .It Dv O_NONBLOCK Non-blocking I/O; if no data is available to a .Xr read call, or if a .Xr write operation would block, the read or write call returns -1 with the error .Er EAGAIN . .It Dv O_APPEND Force each write to append at the end of file; corresponds to the .Dv O_APPEND flag of .Xr open 2 . .It Dv O_ASYNC Enable the .Dv SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be read. .El .Pp Several commands are available for doing advisory file locking; they all operate on the following structure: .ne 7v .Bd -literal 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 */ }; .Ed .Pp The commands available for advisory record locking are as follows: .Bl -tag -width F_SETLKWX .It Dv F_GETLK Get the first lock that blocks the lock description pointed to by the third argument, .Fa arg , taken as a pointer to a .Fa "struct flock" (see above). The information retrieved overwrites the information passed to .Nm fcntl in the .Fa 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 which is set to .Dv F_UNLCK . .It Dv F_SETLK Set or clear a file segment lock according to the lock description pointed to by the third argument, .Fa arg , taken as a pointer to a .Fa "struct flock" (see above). .Dv F_SETLK is used to establish shared (or read) locks .Dv (F_RDLCK) or exclusive (or write) locks, .Dv (F_WRLCK) , as well as remove either type of lock .Dv (F_UNLCK) . If a shared or exclusive lock cannot be set, .Nm fcntl returns immediately with .Er EAGAIN . .It Dv F_SETLKW This command is the same as .Dv 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 .Nm fcntl is waiting for a region, the .Nm fcntl will be interrupted if the signal handler has not specified the .Dv SA_RESTART (see .Xr sigaction 2 ) . .El .Pp 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. .Pp 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. .Pp The value of .Fa l_whence is .Dv SEEK_SET , .Dv SEEK_CUR , or .Dv SEEK_END to indicate that the relative offset, .Fa l_start bytes, will be measured from the start of the file, current position, or end of the file, respectively. The value of .Fa l_len is the number of consecutive bytes to be locked. If .Fa l_len is negative, the result is undefined. The .Fa l_pid field is only used with .Dv F_GETLK to return the process ID of the process holding a blocking lock. After a successful .Dv F_GETLK request, the value of .Fa l_whence is .Dv SEEK_SET . .Pp 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 .Fa l_len is set to zero. If .Fa l_whence and .Fa l_start point to the beginning of the file, and .Fa l_len is zero, the entire file is locked. If an application wishes only to do entire file locking, the .Xr flock 2 system call is much more efficient. .Pp There is at most one type of lock set for each byte in the file. Before a successful return from an .Dv F_SETLK or an .Dv 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 .Dv F_SETLK or an .Dv 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. .Pp This interface follows the completely stupid semantics of System V and .St -p1003.1-88 that require that all locks associated with a file for a given process are removed when \fIany\fP 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 .Xr getpwname 3 to retrieve a record, the lock will be lost because .Xr getpwname 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 .Xr fork 2 function. The .Xr flock 2 interface has much more rational last close semantics and allows locks to be inherited by child processes. .Xr 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 .Xr flock 2 and .Xr fcntl 2 locks may be safely used concurrently. .Pp All locks associated with a file for a given process are removed when the process terminates. .Pp 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 .Er EDEADLK error. .Pp The .Dv F_PREALLOCATE command operates on the following structure: .ne 7v .Bd -literal typedef struct fstore { u_int32_t fst_flags; /* IN: flags word */ int fst_posmode; /* IN: indicates offset field */ off_t fst_offset; /* IN: start of the region */ off_t fst_length; /* IN: size of the region */ off_t fst_bytesalloc; /* OUT: number of bytes allocated */ } fstore_t; .Ed .Pp The flags (fst_flags) for the .Dv F_PREALLOCATE command are as follows: .Bl -tag -width F_ALLOCATECONTIGX -offset indent .It Dv F_ALLOCATECONTIG Allocate contiguous space. .It Dv F_ALLOCATEALL Allocate all requested space or no space at all. .El .Pp The position modes (fst_posmode) for the .Dv F_PREALLOCATE command indicate how to use the offset field. The modes are as follows: .Bl -tag -width F_PEOFPOSMODEX -offset indent .It Dv F_PEOFPOSMODE Allocate from the physical end of file. In this case, .Fa fst_length indicates the number of newly allocated bytes desired. .It Dv F_VOLPOSMODE Allocate from the volume offset. .El .Pp The .Dv F_PUNCHHOLE command operates on the following structure: .ne 7v .Bd -literal typedef struct fpunchhole { u_int32_t fp_flags; /* unused */ u_int32_t reserved; /* (to maintain 8-byte alignment) */ off_t fp_offset; /* IN: start of the region */ off_t fp_length; /* IN: size of the region */ } fpunchhole_t; .Ed .Pp The .Dv F_RDADVISE command operates on the following structure which holds information passed from the user to the system: .ne 7v .Bd -literal struct radvisory { off_t ra_offset; /* offset into the file */ int ra_count; /* size of the read */ }; .Ed .Pp The .Dv F_READBOOTSTRAP and F_WRITEBOOTSTRAP commands operate on the following structure. .ne 7v .Bd -literal typedef struct fbootstraptransfer { off_t fbt_offset; /* IN: offset to start read/write */ size_t fbt_length; /* IN: number of bytes to transfer */ void *fbt_buffer; /* IN: buffer to be read/written */ } fbootstraptransfer_t; .Ed .Pp The .Dv F_LOG2PHYS command operates on the following structure: .ne 7v .Bd -literal struct log2phys { u_int32_t l2p_flags; /* unused so far */ off_t l2p_contigbytes; /* unused so far */ off_t l2p_devoffset; /* bytes into device */ }; .Ed .Pp The .Dv F_LOG2PHYS_EXT command operates on the same structure as F_LOG2PHYS but treats it as an in/out: .ne 7v .Bd -literal struct log2phys { u_int32_t l2p_flags; /* unused so far */ off_t l2p_contigbytes; /* IN: number of bytes to be queried; OUT: number of contiguous bytes allocated at this position */ off_t l2p_devoffset; /* IN: bytes into file; OUT: bytes into device */ }; .Ed .Pp If .Fa fildes is a socket, then the .Dv F_SETNOSIGPIPE and .Dv F_GETNOSIGPIPE commands are directly analogous, and fully interoperate with the .Dv SO_NOSIGPIPE option of .Xr setsockopt 2 and .Xr getsockopt 2 respectively. .Sh RETURN VALUES Upon successful completion, the value returned depends on .Fa cmd as follows: .Bl -tag -width F_GETOWNX -offset indent .It Dv F_DUPFD A new file descriptor. .It Dv F_GETFD Value of flag (only the low-order bit is defined). .It Dv F_GETFL Value of flags. .It Dv F_GETOWN Value of file descriptor owner. .It other Value other than -1. .El .Pp Otherwise, a value of -1 is returned and .Va errno is set to indicate the error. .Sh ERRORS The .Fn fcntl system call will fail if: .Bl -tag -width Er .\" ========== .It Bq Er EAGAIN The argument .Fa cmd is .Dv F_SETLK , the type of lock .Fa (l_type) is a shared lock .Dv (F_RDLCK) or exclusive lock .Dv (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. .It Bq Er EACCESS The argument .Fa cmd is either .Dv F_SETSIZE or .Dv F_WRITEBOOTSTRAP and the calling process does not have root privileges. .\" ========== .It Bq Er EBADF .Fa Fildes is not a valid open file descriptor. .Pp The argument .Fa cmd is .Dv F_SETLK or .Dv F_SETLKW , the type of lock .Fa (l_type) is a shared lock .Dv (F_RDLCK) , and .Fa fildes is not a valid file descriptor open for reading. .Pp The argument .Fa cmd is .Dv F_SETLK or .Dv F_SETLKW , the type of lock .Fa (l_type) is an exclusive lock .Dv (F_WRLCK) , and .Fa fildes is not a valid file descriptor open for writing. .Pp The argument .Fa cmd is .Dv F_PREALLOCATE and the calling process does not have file write permission. .Pp The argument .Fa cmd is .Dv F_LOG2PHYS or .Dv F_LOG2PHYS_EXT and .Fa fildes is not a valid file descriptor open for reading. .\" ========== .It Bq Er EDEADLK The argument .Fa cmd is .Dv F_SETLKW , and a deadlock condition was detected. .\" ========== .It Bq Er EINTR The argument .Fa cmd is .Dv F_SETLKW , and the function was interrupted by a signal. .\" ========== .It Bq Er EINVAL .Fa Cmd is .Dv F_DUPFD and .Fa arg is negative or greater than the maximum allowable number (see .Xr getdtablesize 2 ) . .Pp The argument .Fa cmd is .Dv F_GETLK , .Dv F_SETLK , or .Dv F_SETLKW and the data to which .Fa arg points is not valid, or .Fa fildes refers to a file that does not support locking. .Pp The argument .Fa cmd is .Dv F_PREALLOCATE and the .Fa fst_posmode is not a valid mode, or when .Dv F_PEOFPOSMODE is set and .Fa fst_offset is a non-zero value, or when .Dv F_VOLPOSMODE is set and .Fa fst_offset is a negative or zero value. .Pp The argument .Fa cmd is .Dv F_PUNCHHOLE and either .Fa fp_offset or .Fa fp_length are negative, or both .Fa fp_offset and .Fa fp_length are not multiples of the file system block size. .Pp The argument .Fa cmd is either .Dv F_READBOOTSTRAP or .Dv F_WRITEBOOTSTRAP and the operation was attempted on a non-HFS disk type. .\" ========== .It Bq Er EMFILE .Fa Cmd is .Dv F_DUPFD and the maximum allowed number of file descriptors are currently open. .\" ========== .It Bq Er EMFILE The argument .Fa cmd is .Dv F_DUPED and the maximum number of file descriptors permitted for the process are already in use, or no file descriptors greater than or equal to .Fa arg are available. .\" ========== .It Bq Er ENOLCK The argument .Fa cmd is .Dv F_SETLK or .Dv 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. .\" ========== .It Bq Er ENOSPC The argument .Fa cmd is .Dv F_PREALLOCATE and either there is no space available on the volume containing .Fa fildes or .Fa fst_flags contains .Dv F_ALLOCATEALL and there is not enough space available on the volume containing .Fa fildes to satisfy the entire request. .Pp The argument .Fa cmd is .Dv F_PUNCHHOLE and there is not enough space available on the volume containing .Fa fildes to satisfy the request. As an example, a filesystem that supports cloned files may return this error if punching a hole requires the creation of a clone and there is not enough space available to do so. .\" ========== .It Bq Er EOVERFLOW A return value would overflow its representation. For example, .Fa cmd is F_GETLK, F_SETLK, or F_SETLKW and the smallest (or, if l_len is non-zero, the largest) offset of a byte in the requested segment will not fit in an object of type off_t. .\" ========== .It Bq Er EPERM The argument cmd is .Dv F_PUNCHHOLE and the calling process does not have file write permission. .\" ========== .It Bq Er ESRCH .Fa Cmd is .Dv F_SETOWN and the process ID given as argument is not in use. .El .Sh SEE ALSO .Xr close 2 , .Xr execve 2 , .Xr flock 2 , .Xr getdtablesize 2 , .Xr open 2 , .Xr pipe 2 , .Xr socket 2 , .Xr setsockopt 2 , .Xr sigaction 3 .Sh HISTORY The .Fn fcntl function call appeared in .Bx 4.2 .