fcntl

FCNTL(3P)                  POSIX Programmer's Manual                 FCNTL(3P)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the corresponding
       Linux manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.


NAME
       fcntl — file control

SYNOPSIS
       #include <fcntl.h>

       int fcntl(int fildes, int cmd, ...);

DESCRIPTION
       The fcntl() function shall perform the operations described below on
       open files. The fildes argument is a file descriptor.

       The available values for cmd are defined in <fcntl.h> and are as
       follows:

       F_DUPFD       Return a new file descriptor which shall be the lowest
                     numbered available (that is, not already open) file
                     descriptor greater than or equal to the third argument,
                     arg, taken as an integer of type int.  The new file
                     descriptor shall refer to the same open file description
                     as the original file descriptor, and shall share any
                     locks. The FD_CLOEXEC flag associated with the new file
                     descriptor shall be cleared to keep the file open across
                     calls to one of the exec functions.

       F_DUPFD_CLOEXEC
                     Like F_DUPFD, but the FD_CLOEXEC flag associated with the
                     new file descriptor shall be set.

       F_GETFD       Get the file descriptor flags defined in <fcntl.h> that
                     are associated with the file descriptor fildes.  File
                     descriptor flags are associated with a single file
                     descriptor and do not affect other file descriptors that
                     refer to the same file.

       F_SETFD       Set the file descriptor flags defined in <fcntl.h>, that
                     are associated with fildes, to the third argument, arg,
                     taken as type int.  If the FD_CLOEXEC flag in the third
                     argument is 0, the file descriptor shall remain open
                     across the exec functions; otherwise, the file descriptor
                     shall be closed upon successful execution of one of the
                     exec functions.

       F_GETFL       Get the file status flags and file access modes, defined
                     in <fcntl.h>, for the file description associated with
                     fildes.  The file access modes can be extracted from the
                     return value using the mask O_ACCMODE, which is defined
                     in <fcntl.h>.  File status flags and file access modes
                     are associated with the file description and do not
                     affect other file descriptors that refer to the same file
                     with different open file descriptions. The flags returned
                     may include non-standard file status flags which the
                     application did not set, provided that these additional
                     flags do not alter the behavior of a conforming
                     application.

       F_SETFL       Set the file status flags, defined in <fcntl.h>, for the
                     file description associated with fildes from the
                     corresponding bits in the third argument, arg, taken as
                     type int.  Bits corresponding to the file access mode and
                     the file creation flags, as defined in <fcntl.h>, that
                     are set in arg shall be ignored. If any bits in arg other
                     than those mentioned here are changed by the application,
                     the result is unspecified. If fildes does not support
                     non-blocking operations, it is unspecified whether the
                     O_NONBLOCK flag will be ignored.

       F_GETOWN      If fildes refers to a socket, get the process or process
                     group ID specified to receive SIGURG signals when out-of-
                     band data is available. Positive values indicate a
                     process ID; negative values, other than −1, indicate a
                     process group ID. If fildes does not refer to a socket,
                     the results are unspecified.

       F_SETOWN      If fildes refers to a socket, set the process or process
                     group ID specified to receive SIGURG signals when out-of-
                     band data is available, using the value of the third
                     argument, arg, taken as type int.  Positive values
                     indicate a process ID; negative values, other than −1,
                     indicate a process group ID. If fildes does not refer to
                     a socket, the results are unspecified.

       The following values for cmd are available for advisory record locking.
       Record locking shall be supported for regular files, and may be
       supported for other files.

       F_GETLK       Get the first lock which blocks the lock description
                     pointed to by the third argument, arg, taken as a pointer
                     to type struct flock, defined in <fcntl.h>.  The
                     information retrieved shall overwrite the information
                     passed to fcntl() in the structure flock.  If no lock is
                     found that would prevent this lock from being created,
                     then the structure shall be left unchanged except for the
                     lock type which shall be 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 type struct flock, defined in <fcntl.h>.
                     F_SETLK can establish shared (or read) locks (F_RDLCK) or
                     exclusive (or write) locks (F_WRLCK), as well as to
                     remove either type of lock (F_UNLCK). F_RDLCK, F_WRLCK,
                     and F_UNLCK are defined in <fcntl.h>.  If a shared or
                     exclusive lock cannot be set, fcntl() shall return
                     immediately with a return value of −1.

       F_SETLKW      This command shall be equivalent to F_SETLK except that
                     if a shared or exclusive lock is blocked by other locks,
                     the thread shall wait until the request can be satisfied.
                     If a signal that is to be caught is received while
                     fcntl() is waiting for a region, fcntl() shall be
                     interrupted. Upon return from the signal handler, fcntl()
                     shall return −1 with errno set to [EINTR], and the lock
                     operation shall not be done.

       Additional implementation-defined values for cmd may be defined in
       <fcntl.h>.  Their names shall start with F_.

       When a shared lock is set on a segment of a file, other processes shall
       be able to 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 shall
       fail if the file descriptor was not opened with read access.

       An exclusive lock shall prevent 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 shall fail if the file descriptor was not
       opened with write access.

       The structure flock describes the type (l_type), starting offset
       (l_whence), relative offset (l_start), size (l_len), and process ID
       (l_pid) of the segment of the file to be affected.

       The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END, to indicate
       that the relative offset l_start bytes shall 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. The
       value of l_len may be negative (where the definition of off_t permits
       negative values of l_len).  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, when a blocking lock is found, the values
       returned in the flock structure shall be as follows:

       l_type    Type of blocking lock found.

       l_whence  SEEK_SET.

       l_start   Start of the blocking lock.

       l_len     Length of the blocking lock.

       l_pid     Process ID of the process that holds the blocking lock.

       If the command is F_SETLKW and the process must wait for another
       process to release a lock, then the range of bytes to be locked shall
       be determined before the fcntl() function blocks. If the file size or
       file descriptor seek offset change while fcntl() is blocked, this shall
       not affect the range of bytes locked.

       If l_len is positive, the area affected shall start at l_start and end
       at l_start+l_len−1.  If l_len is negative, the area affected shall
       start at l_start+l_len and end at l_start−1.  Locks may start and
       extend beyond the current end of a file, but shall not extend before
       the beginning of the file. A lock shall be set to extend to the largest
       possible value of the file offset for that file by setting l_len to 0.
       If such a lock also has l_start set to 0 and l_whence is set to
       SEEK_SET, the whole file shall be locked.

       There shall be 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 shall be 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 (respectively) shall fail or
       block 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.

       All locks associated with a file for a given process shall be removed
       when a file descriptor for that file is closed by that process or the
       process holding that file descriptor terminates. Locks are not
       inherited by a child process.

       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. If the system detects that sleeping until a locked
       region is unlocked would cause a deadlock, fcntl() shall fail with an
       [EDEADLK] error.

       An unlock (F_UNLCK) request in which l_len is non-zero and the offset
       of the last byte of the requested segment is the maximum value for an
       object of type off_t, when the process has an existing lock in which
       l_len is 0 and which includes the last byte of the requested segment,
       shall be treated as a request to unlock from the start of the requested
       segment with an l_len equal to 0. Otherwise, an unlock (F_UNLCK)
       request shall attempt to unlock only the requested segment.

       When the file descriptor fildes refers to a shared memory object, the
       behavior of fcntl() shall be the same as for a regular file except the
       effect of the following values for the argument cmd shall be
       unspecified: F_SETFL, F_GETLK, F_SETLK, and F_SETLKW.

       If fildes refers to a typed memory object, the result of the fcntl()
       function is unspecified.

RETURN VALUE
       Upon successful completion, the value returned shall depend on cmd as
       follows:

       F_DUPFD     A new file descriptor.

       F_DUPFD_CLOEXEC
                   A new file descriptor.

       F_GETFD     Value of flags defined in <fcntl.h>.  The return value
                   shall not be negative.

       F_SETFD     Value other than −1.

       F_GETFL     Value of file status flags and access modes. The return
                   value is not negative.

       F_SETFL     Value other than −1.

       F_GETLK     Value other than −1.

       F_SETLK     Value other than −1.

       F_SETLKW    Value other than −1.

       F_GETOWN    Value of the socket owner process or process group; this
                   will not be −1.

       F_SETOWN    Value other than −1.

       Otherwise, −1 shall be returned and errno set to indicate the error.

ERRORS
       The fcntl() function shall fail if:

       EACCES or EAGAIN
              The cmd argument is F_SETLK; the type of lock (l_type) is a
              shared (F_RDLCK) or exclusive (F_WRLCK) lock 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 fildes argument is not a valid open file descriptor, or 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, or the type of lock, l_type, is an
              exclusive lock (F_WRLCK), and fildes is not a valid file
              descriptor open for writing.

       EINTR  The cmd argument is F_SETLKW and the function was interrupted by
              a signal.

       EINVAL The cmd argument is invalid, or the cmd argument is F_DUPFD or
              F_DUPFD_CLOEXEC and arg is negative or greater than or equal to
              {OPEN_MAX}, or the cmd argument is F_GETLK, F_SETLK, or F_SETLKW
              and the data pointed to by arg is not valid, or fildes refers to
              a file that does not support locking.

       EMFILE The argument cmd is F_DUPFD or F_DUPFD_CLOEXEC and all file
              descriptors available to the process are currently open, 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.

       EOVERFLOW
              One of the values to be returned cannot be represented
              correctly.

       EOVERFLOW
              The cmd argument is F_GETLK, F_SETLK, or F_SETLKW and the
              smallest or, if l_len is non-zero, the largest offset of any
              byte in the requested segment cannot be represented correctly in
              an object of type off_t.

       The fcntl() function may fail if:

       EDEADLK
              The cmd argument is F_SETLKW, the lock is blocked by a lock from
              another process, and putting the calling process to sleep to
              wait for that lock to become free would cause a deadlock.

       The following sections are informative.

EXAMPLES
   Locking and Unlocking a File
       The following example demonstrates how to place a lock on bytes 100 to
       109 of a file and then later remove it. F_SETLK is used to perform a
       non-blocking lock request so that the process does not have to wait if
       an incompatible lock is held by another process; instead the process
       can take some other action.

           #include <stdlib.h>
           #include <unistd.h>
           #include <fcntl.h>
           #include <errno.h>
           #include <stdio.h>

           int
           main(int argc, char *argv[])
           {
               int fd;
               struct flock fl;

               fd = open("testfile", O_RDWR);
               if (fd == -1)
                   /* Handle error */;

               /* Make a non-blocking request to place a write lock
                  on bytes 100-109 of testfile */

               fl.l_type = F_WRLCK;
               fl.l_whence = SEEK_SET;
               fl.l_start = 100;
               fl.l_len = 10;

               if (fcntl(fd, F_SETLK, &fl) == −1) {
                   if (errno == EACCES || errno == EAGAIN) {
                       printf("Already locked by another process\n");

                       /* We can't get the lock at the moment */

                   } else {
                       /* Handle unexpected error */;
                   }
               } else { /* Lock was granted... */

                   /* Perform I/O on bytes 100 to 109 of file */

                   /* Unlock the locked bytes */

                   fl.l_type = F_UNLCK;
                   fl.l_whence = SEEK_SET;
                   fl.l_start = 100;
                   fl.l_len = 10;
                   if (fcntl(fd, F_SETLK, &fl) == −1)
                       /* Handle error */;
               }
               exit(EXIT_SUCCESS);
           } /* main */

   Setting the Close-on-Exec Flag
       The following example demonstrates how to set the close-on-exec flag
       for the file descriptor fd.

           #include <unistd.h>
           #include <fcntl.h>
           ...
               int flags;

               flags = fcntl(fd, F_GETFD);
               if (flags == −1)
                   /* Handle error */;
               flags |= FD_CLOEXEC;
               if (fcntl(fd, F_SETFD, flags) == −1)
                   /* Handle error */;"

APPLICATION USAGE
       The arg values to F_GETFD, F_SETFD, F_GETFL, and F_SETFL all represent
       flag values to allow for future growth. Applications using these
       functions should do a read-modify-write operation on them, rather than
       assuming that only the values defined by this volume of POSIX.1‐2008
       are valid. It is a common error to forget this, particularly in the
       case of F_SETFD. Some implementations set additional file status flags
       to advise the application of default behavior, even though the
       application did not request these flags.

RATIONALE
       The ellipsis in the SYNOPSIS is the syntax specified by the ISO C
       standard for a variable number of arguments. It is used because System
       V uses pointers for the implementation of file locking functions.

       This volume of POSIX.1‐2008 permits concurrent read and write access to
       file data using the fcntl() function; this is a change from the 1984
       /usr/group standard and early proposals. Without concurrency controls,
       this feature may not be fully utilized without occasional loss of data.

       Data losses occur in several ways. One case occurs when several
       processes try to update the same record, without sequencing controls;
       several updates may occur in parallel and the last writer ``wins''.
       Another case is a bit-tree or other internal list-based database that
       is undergoing reorganization. Without exclusive use to the tree segment
       by the updating process, other reading processes chance getting lost in
       the database when the index blocks are split, condensed, inserted, or
       deleted. While fcntl() is useful for many applications, it is not
       intended to be overly general and does not handle the bit-tree example
       well.

       This facility is only required for regular files because it is not
       appropriate for many devices such as terminals and network connections.

       Since fcntl() works with ``any file descriptor associated with that
       file, however it is obtained'', the file descriptor may have been
       inherited through a fork() or exec operation and thus may affect a file
       that another process also has open.

       The use of the open file description to identify what to lock requires
       extra calls and presents problems if several processes are sharing an
       open file description, but there are too many implementations of the
       existing mechanism for this volume of POSIX.1‐2008 to use different
       specifications.

       Another consequence of this model is that closing any file descriptor
       for a given file (whether or not it is the same open file description
       that created the lock) causes the locks on that file to be relinquished
       for that process. Equivalently, any close for any file/process pair
       relinquishes the locks owned on that file for that process. But note
       that while an open file description may be shared through fork(), locks
       are not inherited through fork().  Yet locks may be inherited through
       one of the exec functions.

       The identification of a machine in a network environment is outside the
       scope of this volume of POSIX.1‐2008. Thus, an l_sysid member, such as
       found in System V, is not included in the locking structure.

       Changing of lock types can result in a previously locked region being
       split into smaller regions.

       Mandatory locking was a major feature of the 1984 /usr/group standard.

       For advisory file record locking to be effective, all processes that
       have access to a file must cooperate and use the advisory mechanism
       before doing I/O on the file. Enforcement-mode record locking is
       important when it cannot be assumed that all processes are cooperating.
       For example, if one user uses an editor to update a file at the same
       time that a second user executes another process that updates the same
       file and if only one of the two processes is using advisory locking,
       the processes are not cooperating. Enforcement-mode record locking
       would protect against accidental collisions.

       Secondly, advisory record locking requires a process using locking to
       bracket each I/O operation with lock (or test) and unlock operations.
       With enforcement-mode file and record locking, a process can lock the
       file once and unlock when all I/O operations have been completed.
       Enforcement-mode record locking provides a base that can be enhanced;
       for example, with sharable locks. That is, the mechanism could be
       enhanced to allow a process to lock a file so other processes could
       read it, but none of them could write it.

       Mandatory locks were omitted for several reasons:

        1. Mandatory lock setting was done by multiplexing the set-group-ID
           bit in most implementations; this was confusing, at best.

        2. The relationship to file truncation as supported in 4.2 BSD was not
           well specified.

        3. Any publicly readable file could be locked by anyone. Many
           historical implementations keep the password database in a publicly
           readable file. A malicious user could thus prohibit logins. Another
           possibility would be to hold open a long-distance telephone line.

        4. Some demand-paged historical implementations offer memory mapped
           files, and enforcement cannot be done on that type of file.

       Since sleeping on a region is interrupted with any signal, alarm() may
       be used to provide a timeout facility in applications requiring it.
       This is useful in deadlock detection. Since implementation of full
       deadlock detection is not always feasible, the [EDEADLK] error was made
       optional.

FUTURE DIRECTIONS
       None.

SEE ALSO
       alarm(), close(), exec, open(), sigaction()

       The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <signal.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be obtained online
       at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2013                            FCNTL(3P)