| FCNTL(2) | System Calls Manual | FCNTL(2) | 
fcntl —
#include <fcntl.h>
int
  
  fcntl(int
    fd, int cmd,
    ...);
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_DUPFDF_DUPFD_CLOEXECF_DUPFD, but sets the close-on-exec
      property on the file descriptor created.F_GETFDFD_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_SETFDFD_CLOEXEC, as described above.F_GETFLF_SETFLF_GETOWNSIGIO and SIGURG signals;
      process groups are returned as negative values (arg
      is ignored).F_SETOWNSIGIO
      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_CLOSEMF_MAXFDF_GETNOSIGPIPEO_NOSIGPIPE flag is set in the file
      descriptor.F_SETNOSIGPIPEO_NOSIGPIPE in the file
      descriptor.F_GETPATHMAXPATHLEN.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_GETLKfcntl 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_SETLKF_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_SETLKWF_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.
F_GETPATH functionality is implemented using the
  reverse namei(9) cache. The
  implications of this are:
F_GETPATH
      may fail if the corresponding entry has been evicted from the LRU
      namei(9) cache and return
      ENOENT.Otherwise, a value of -1 is returned and errno is set to indicate the error.
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.
fcntl() will fail if:
EACCES]F_GETPATH and read or search permission was denied
      for a component of the pathname.EAGAIN]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 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.
EDEADLK]F_SETLKW, and a deadlock condition was
    detected.EINTR]F_SETLKW, and the function was interrupted by a
      signal.EINVAL]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.
EMFILE]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]F_DUPFD and
      system-wide the maximum allowed number of file descriptors are currently
      open.ENOENT]F_GETPATH and a component of the pathname no
      longer exists.ENOLCK]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]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).
ERANGE]F_GETPATH and the resulting path would be greater
      than MAXPATHLEN.ESRCH]F_SETOWN and the
      process ID given as argument is not in use.fcntl() function conforms to IEEE
  Std 1003.1-1990 (“POSIX.1”).
fcntl() function call appeared in
  4.2BSD.
| September 26, 2019 | NetBSD 10.0 |