fcntl() - Unix, Linux System Call

Unix - Home
Unix - Getting Started
Unix - File Management
Unix - Directories
Unix - File Permission
Unix - Environment
Unix - Basic Utilities
Unix - Pipes & Filters
Unix - Processes
Unix - Communication
Unix - The vi Editor
Unix - What is Shell?
Unix - Using Variables
Unix - Special Variables
Unix - Using Arrays
Unix - Basic Operators
Unix - Decision Making
Unix - Shell Loops
Unix - Loop Control
Unix - Shell Substitutions
Unix - Quoting Mechanisms
Unix - IO Redirections
Unix - Shell Functions
Unix - Manpage Help
Unix - Regular Expressions
Unix - File System Basics
Unix - User Administration
Unix - System Performance
Unix - System Logging
Unix - Signals and Traps
Unix - Useful Commands
Unix - Quick Guide
Unix - Builtin Functions
Unix - System Calls
Unix - Commands List
Unix Useful Resources
Computer Glossary
Who is Who
Copyright © 2014 by tutorialspoint
fcntl - manipulate file descriptor
#include <unistd.h> 
#include <fcntl.h> 

int fcntl(int fd, int cmd); 
int fcntl(int fd, int cmd, long arg); 
int fcntl(int fd, int cmd, struct flock *lock); 


int fcntl(int fd, int cmd); 
int fcntl(int fd, int cmd, long arg); 
int fcntl(int fd, int cmd, struct flock *lock); 

fcntl() performs one of the operations described below on the open file descriptor
fd. The operation is determined by
cmd. 

On success, the new descriptor is returned.


See
dup(2)
for further details.

The following commands manipulate the flags associated with
a file descriptor. 
Currently, only one such flag is defined:
FD_CLOEXEC, the close-on-exec flag.
If the
FD_CLOEXEC bit is 0, the file descriptor will remain open across an
execve(2),
otherwise it will be closed.
Each open file description has certain associated status flags,
initialized by
open(2)
and possibly modified by
fcntl(2).
Duplicated file descriptors
(made with
dup(), fcntl(F_DUPFD), fork(), etc.) refer to the same open file description, and thus
share the same file status flags.
The file status flags and their semantics are described in
open(2).
F_GETLK, F_SETLK and F_SETLKW are used to acquire, release, and test for the existence of record
locks (also known as file-segment or file-region locks).
The third argument
lock is a pointer to a structure that has at least the following fields
(in unspecified order).

struct flock {
    ...
    short l_type;    /* Type of lock: F_RDLCK,
                        F_WRLCK, F_UNLCK */
    short l_whence;  /* How to interpret l_start:
                        SEEK_SET, SEEK_CUR, SEEK_END */
    off_t l_start;   /* Starting offset for lock */
    off_t l_len;     /* Number of bytes to lock */
    pid_t l_pid;     /* PID of process blocking our lock
                        (F_GETLK only) */
    ...
};


struct flock {
    ...
    short l_type;    /* Type of lock: F_RDLCK,
                        F_WRLCK, F_UNLCK */
    short l_whence;  /* How to interpret l_start:
                        SEEK_SET, SEEK_CUR, SEEK_END */
    off_t l_start;   /* Starting offset for lock */
    off_t l_len;     /* Number of bytes to lock */
    pid_t l_pid;     /* PID of process blocking our lock
                        (F_GETLK only) */
    ...
};

The l_whence, l_start, and l_len fields of this structure specify the range of bytes we wish to lock.
l_start is the starting offset for the lock, and is interpreted
relative to either:
the start of the file (if
l_whence is
SEEK_SET); the current file offset (if
l_whence is
SEEK_CUR); or the end of the file (if
l_whence is
SEEK_END). In the final two cases,
l_start can be a negative number provided the
offset does not lie before the start of the file.
l_len is a non-negative integer (but see the NOTES below) specifying
the number of bytes to be locked.
Bytes past the end of the file may be locked,
but not bytes before the start of the file.
Specifying 0 for
l_len has the special meaning: lock all bytes starting at the
location specified by
l_whence and l_start through to the end of file, no matter how large the file grows.
The l_type field can be used to place a read
(F_RDLCK) or a write
(F_WRLCK) lock on a file.
Any number of processes may hold a read lock (shared lock)
on a file region, but only one process may hold a write lock
(exclusive lock). An exclusive lock excludes all other locks,
both shared and exclusive.
A single process can hold only one type of lock on a file region;
if a new lock is applied to an already-locked region,
then the existing lock is converted to the new lock type.
(Such conversions may involve splitting, shrinking, or coalescing with
an existing lock if the byte range specified by the new lock does not
precisely coincide with the range of the existing lock.)
In order to place a read lock,
fd must be open for reading.
In order to place a write lock,
fd must be open for writing.
To place both types of lock, open a file read-write.

As well as being removed by an explicit
F_UNLCK, record locks are automatically released when the process
terminates or if it closes
any file descriptor referring to a file on which locks are held.
This is bad: it means that a process can lose the locks on
a file like
/etc/passwd or
/etc/mtab when for some reason a library function decides to open, read
and close it.
Record locks are not inherited by a child created via
fork(2),
but are preserved across an
execve(2).
Because of the buffering performed by the
stdio(3)
library, the use of record locking with routines in that package
should be avoided; use
read(2)
and
write(2)
instead.
Advisory locks are not enforced and are useful only between
cooperating processes.
Mandatory locks are enforced for all processes. 
If a process tries to perform an incompatible access (e.g.,
read(2)
or
write(2))
on a file region that has an incompatible mandatory lock,
then the result depends upon whether the
O_NONBLOCK flag is enabled for its open file description.
If the
O_NONBLOCK flag is not enabled, then
system call is blocked until the lock is removed
or converted to a mode that is compatible with the access.
If the
O_NONBLOCK flag is enabled, then the system call fails with the error
EAGAIN or
EWOULDBLOCK. 
To make use of mandatory locks, mandatory locking must be enabled
both on the file system that contains the file to be locked,
and on the file itself.
Mandatory locking is enabled on a file system
using the "-o mand" option to
mount(8),
or the
MS_MANDLOCK flag for
mount(2).
Mandatory locking is enabled on a file by disabling
group execute permission on the file and enabling the set-group-ID
permission bit (see
chmod(1)
and
chmod(2)).
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O availability signals:

If you set the
O_ASYNC status flag on a file descriptor (either by providing this flag with the
open(2)
call, or by using the
F_SETFL command of
fcntl()), a SIGIO signal is sent whenever input or output becomes possible
on that file descriptor.
F_SETSIG can be used to obtain delivery of a signal other than SIGIO.
If this permission check fails, then the signal is
silently discarded.


Sending a signal to the owner process (group) specified by
F_SETOWN is subject to the same permissions checks as are described for
kill(2),
where the sending process is the one that employs
F_SETOWN (but see BUGS below).


If the file descriptor
fd refers to a socket,
F_SETOWN also selects
the recipient of SIGURG signals that are delivered when out-of-band
data arrives on that socket.  (SIGURG is sent in any situation where
select(2)
would report the socket as having an "exceptional condition".)


If a non-zero value is given to
F_SETSIG in a multi-threaded process running with a threading library
that supports thread groups (e.g., NPTL),
then a positive value given to
F_SETOWN has a different meaning:
instead of being a process ID identifying a whole process,
it is a thread ID identifying a specific thread within a process.
Consequently, it may be necessary to pass
F_SETOWN the result of
gettid() instead of
getpid() to get sensible results when
F_SETSIG is used.
(In current Linux threading implementations,
a main thread’s thread ID is the same as its process ID.
This means that a single-threaded program can equally use
gettid() or
getpid() in this scenario.)
Note, however, that the statements in this paragraph do not apply
to the SIGURG signal generated for out-of-band data on a socket:
this signal is always sent to either a process or a process group,
depending on the value given to
F_SETOWN. Note also that Linux imposes a limit on the
number of real-time signals that may be queued to a
process (see
getrlimit(2)
and
signal(7))
and if this limit is reached, then the kernel reverts to
delivering SIGIO, and this signal is delivered to the entire
process rather than to a specific thread.


Additionally, passing a non-zero value to
F_SETSIG changes the signal recipient from a whole process to a specific thread
within a process.
See the description of
F_SETOWN for more details.


By using
F_SETSIG with a non-zero value, and setting SA_SIGINFO for the
signal handler (see
sigaction(2)),
extra information about I/O events is passed to
the handler in a
siginfo_t structure.
If the
si_code field indicates the source is SI_SIGIO, the
si_fd field gives the file descriptor associated with the event.  Otherwise,
there is no indication which file descriptors are pending, and you
should use the usual mechanisms
(select(2), poll(2),
read(2)
with
O_NONBLOCK set etc.) to determine which file descriptors are available for I/O.


By selecting a real time signal (value >= SIGRTMIN), multiple
I/O events may be queued using the same signal numbers.  (Queuing is
dependent on available memory).  Extra information is available
if SA_SIGINFO is set for the signal handler, as above.

Using these mechanisms, a program can implement fully asynchronous I/O
without using
select(2)
or
poll(2)
most of the time.
The use of
O_ASYNC, F_GETOWN, F_SETOWN is specific to BSD and Linux.
F_GETSIG and
F_SETSIG are Linux-specific.  POSIX has asynchronous I/O and the
aio_sigevent structure to achieve similar things; these are also available
in Linux as part of the GNU C Library (Glibc).


When a process (the "lease breaker") performs an 
open() or
truncate() that conflicts with a lease established via
F_SETLEASE, the system call is blocked by the kernel and
the kernel notifies the lease holder by sending it a signal
(SIGIO by default).
The lease holder should respond to receipt of this signal by doing
whatever cleanup is required in preparation for the file to be
accessed by another process (e.g., flushing cached buffers) and
then either remove or downgrade its lease.
A lease is removed by performing an
F_SETLEASE command specifying
arg as
F_UNLCK. If we currently hold a write lease on the file,
and the lease breaker is opening the file for reading,
then it is sufficient to downgrade the lease to a read lease.
This is done by performing an
F_SETLEASE command specifying
arg as
F_RDLCK. 
If the lease holder fails to downgrade or remove the lease within
the number of seconds specified in
/proc/sys/fs/lease-break-time then the kernel forcibly removes or downgrades the lease holder’s lease.
Once the lease has been voluntarily or forcibly removed or downgraded,
and assuming the lease breaker has not unblocked its system call,
the kernel permits the lease breaker’s system call to proceed.
If the lease breaker’s blocked 
open() or
truncate() is interrupted by a signal handler,
then the system call fails with the error
EINTR, but the other steps still occur as described above.
If the lease breaker is killed by a signal while blocked in
open() or
truncate(), then the other steps still occur as described above.
If the lease breaker specifies the
O_NONBLOCK flag when calling
open(), then the call immediately fails with the error
EWOULDBLOCK, but the other steps still occur as described above.
The default signal used to notify the lease holder is SIGIO,
but this can be changed using the
F_SETSIG command to
fcntl(). If a
F_SETSIG command is performed (even one specifying SIGIO), and the signal
handler is established using SA_SIGINFO, then the handler will
receive a
siginfo_t structure as its second argument, and the
si_fd field of this argument will hold the descriptor of the leased file
that has been accessed by another process.
(This is useful if the caller holds leases against multiple files).


(In order to obtain these definitions, the _GNU_SOURCE feature test macro
must be defined.)
Directory notifications are normally "one-shot", and the application
must re-register to receive further notifications.
Alternatively, if
DN_MULTISHOT is included in
arg, then notification will remain in effect until explicitly removed.v

A series of F_NOTIFY requests is cumulative, with the events in
arg being added to the set already monitored.
To disable notification of all events, make an
F_NOTIFY call specifying
arg as 0.
Notification occurs via delivery of a signal.
The default signal is SIGIO, but this can be changed using the
F_SETSIG command to
fcntl(). In the latter case, the signal handler receives a
siginfo_t structure as its second argument (if the handler was
established using SA_SIGINFO) and the
si_fd field of this structure contains the file descriptor which
generated the notification (useful when establishing notification
on multiple directories).
Especially when using DN_MULTISHOT, a real time signal should be used for notification,
so that multiple notifications can be queued.
NOTE: New applications should consider using the inotify interface (available since kernel 2.6.13),
which provides a superior interface for obtaining notifications of
file system events.  See inotify(7).
For a successful call, the return value depends on the operation:
On error, -1 is returned, and
errno is set appropriately.
The errors returned by
dup2() are different from those returned by
F_DUPFD. 
Since kernel 2.0, there is no interaction between the types of lock
placed by
flock(2)
and
fcntl(2).
POSIX.1-2001 allows
l_len to be negative. (And if it is, the interval described by the lock
covers bytes
l_start+l_len up to and including
l_start-1.) This is supported by Linux since Linux 2.4.21 and 2.5.49.
Several systems have more fields in
struct flock such as e.g.
l_sysid. Clearly,
l_pid alone is not going to be very useful if the process holding the lock
may live on a different machine.
A limitation of the Linux system call conventions on some
architectures (notably x86) means that if a (negative)
process group ID to be returned by
F_GETOWN falls in the range -1 to -4095, then the return value is wrongly
interpreted by glibc as an error in the system call;
that is, the return value of fcntl() will be -1, and
errno will contain the (positive) process group ID.
In Linux 2.4 and earlier, there is bug that can occur
when an unprivileged process uses
F_SETOWN to specify the owner
of a socket file descriptor
as a process (group) other than the caller.
In this case,
fcntl() can return -1 with
errno set to
EPERM, even when the owner process (group) is one that the caller
has permission to send signals to.
Despite this error return, the file descriptor owner is set,
and signals will be sent to the owner.
SVr4, 4.3BSD, POSIX.1-2001.  Only the operations F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK, F_SETLKW,
F_GETOWN, and F_SETOWN are specified in POSIX.1-2001.
F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE
are Linux specific. (Define the _GNU_SOURCE macro to obtain these definitions.)
dup2 (2)
dup2 (2)
flock (2)
flock (2)
open (2)
open (2)
socket (2)
socket (2)

Advertisements


    129 Lectures 
     23 hours 

 Eduonix Learning Solutions

    5 Lectures 
     4.5 hours 

 Frahaan Hussain

    35 Lectures 
     2 hours 

 Pradeep D

    41 Lectures 
     2.5 hours 

 Musab Zayadneh

    46 Lectures 
     4 hours 

 GUHARAJANM

    6 Lectures 
     4 hours 

 Uplatz
 Print
 Add Notes
 Bookmark this page