FCNTL(2)

NAME
fcntl - file control

SYNOPSIS
#include <fcntl.h>
int
fcntl(int fd, int cmd, int arg);

DESCRIPTION
Fcntl() provides for control over descriptors. The argument fd is a descriptor to be operated on by cmd as follows:

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 associated with the new file descriptor is set to remain open across execv(2) system calls.

F_GETFD
    Get the close-on-exec flag associated with the file descriptor fd. If the low-order bit of the returned value 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 the low order bit of arg (0 or 1 as 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_PREALLOCATE
    Preallocate file storage space.

F_SETSIZE
    Truncate a file without zeroing space. The calling process must have root privileges.

F_RDADVISE
    Issue an advisory read async with no copy to user.

F_RDAHEAD
    Turn read ahead off/on. A zero value in arg disables read ahead. A non-zero value in arg turns read ahead on.

F_READBOOTSTRAP
    Read bootstrap from disk.

F_WRITEBOOTSTRAP
    Write bootstrap on disk. The calling process must have root privileges.

F_NOCACHE
    Turns data caching off/on. A non-zero value in arg turns data caching off. A value of zero in arg turns data caching on.

F_LOG2PHYS
    Get disk device information. Currently this only includes the disk device address that corresponds to the current file offset.

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 call, or if a write 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_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 */
            };

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 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 EACCES.

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.

This interface follows the completely stupid semantics of System V and p1003.1-88 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 getpwname(3) to retrieve a record, the lock will be lost because 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 fork(2) function. The flock(2) interface has much more rational last close semantics and allows locks to be inherited by child processes. 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(2) 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.

The F_PREALLOCATE command operates on the following structure:

            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;

The flags (fst_flags) for the F_PREALLOCATE command are as follows:

F_ALLOCATECONTIG
    Allocate contiguous space.

F_ALLOCATEALL
    Allocate all requested space or no space at all.

The position modes (fst_posmode) for the F_PREALLOCATE command indicate how to use the offset field. The modes are as follows:

F_PEOFPOSMODE
    Allocate from the physical end of file.

F_VOLPOSMODE
    Allocate from the volume offset.

The F_RDADVISE command operates on the following structure which holds information passed from the user to the system:

            struct radvisory {
               off_t   ra_offset;  /* offset into the file */
               int     ra_count;   /* size of the read     */
            };

The F_READBOOTSTRAP and F_WRITEBOOTSTRAP commands operate on the following structure.

            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;

The F_LOG2PHYS command operates on the following structure.

            struct log2phys {
    	    u_int32_t	l2p_flags;		/* unused so far */
    	    off_t	l2p_contigbytes;	/* unused so far */
    	    off_t	l2p_devoffset;	    /* bytes into device */
            };

RETURN VALUES
Upon successful completion, the value returned depends on cmd as follows:

F_DUPFD
    A new file descriptor.

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
Fcntl() will fail if:

[EACCES]
    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.

    The argument cmd is either F_SETSIZE or F_WRITEBOOTSTRAP and the calling process does not have root privileges.

[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.

    The argument cmd is F_PREALLOCATE and the calling process does not have file write permission.

    The argument cmd is F_LOG2PHYS and fildes is not a valid file descriptor open for reading.

[EMFILE]
    Cmd is F_DUPFD and the maximum allowed number of file descriptors are currently open.

[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]
    Cmd 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, or fildes refers to a file that does not support locking.

    The argument cmd is F_PREALLOCATE and the fst_posmode is not a valid mode, or when F_PEOFPOSMODE is set and fst_offset is a non-zero value, or when F_VOLPOSMODE is set and fst_offset is a negative or zero value.

    The argument cmd is either F_READBOOTSTRAP or F_WRITEBOOTSTRAP and the operation was attempted on a non-HFS disk type.

[EMFILE]
    The argument cmd is 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 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.

[ESRCH]
    Cmd is F_SETOWN and the process ID given as argument is not in use.

SEE ALSO
close(2), execve(2), flock(2), getdtablesize(2), open(2), sigaction(3)

HISTORY
The fcntl() function call appeared in 4.2BSD.