Fcntl(3perl) Perl Programmers Reference Guide Fcntl(3perl)
NAME
Fcntl - various flag constants and helper functions from C's fcntl.h
SYNOPSIS
use Fcntl;
use Fcntl qw(:DEFAULT :flock);
use Fcntl qw(F_GETFD F_SETFD FD_CLOEXEC);
DESCRIPTION
This module provides flags and helper functions for use with "chmod" in
perlfunc (S_*), "fcntl" in perlfunc (F_*), "flock" in perlfunc (LOCK_*),
"seek" in perlfunc (SEEK_*), "stat" in perlfunc (S_*), "sysopen" in
perlfunc (O_*), and "sysseek" in perlfunc (SEEK_*). They correspond to
the C macros defined in fcntl.h.
Not all symbols are available on all systems. Except where noted
otherwise, the constants and functions provided by this module will
throw a runtime exception if the corresponding C macro is not available.
Consult your system documentation to see the full description of each
symbol and whether it is available on your platform: chmod(2), fcntl(2),
flock(2), lseek(2), open(2), stat(2).
(In particular, some of the F_* symbols are highly non-portable because
they only exist on a single platform or require system-specific C data
structures to be passed as the third argument to "fcntl", which can't be
portably constructed in pure Perl.)
EXPORTED SYMBOLS
Default exports and export tags
The full list of default exports can be found below in "APPENDIX A".
In addition, the following export tags are available (see Exporter for
more information on export tags):
":DEFAULT"
Equivalent to the list of default export symbols (see "APPENDIX A").
":flock"
Equivalent to all LOCK_* symbols listed below.
":mode"
Equivalent to all S_* symbols listed below.
":seek"
Equivalent to all SEEK_* symbols listed below.
":Fcompat"
Equivalent to "qw(FAPPEND FASYNC FCREAT FDEFER FDSYNC FEXCL
FLARGEFILE FNDELAY FNONBLOCK FRSYNC FSYNC FTRUNC)". These only exist
for compatibility with old code (if your platform defines them at
all) and should not be used in new code.
Symbols for use with "fcntl"
"F_ALLOCSP"
File storage manipulation.
"F_ALLOCSP64"
File storage manipulation.
"F_DUP2FD"
Duplicate a file descriptor to the number specified in the third
argument to "fcntl" (if it refers to an open file, it is
automatically closed first).
"F_DUPFD"
Duplicate a file descriptor to the lowest unused number greater than
or equal to the third argument of "fcntl".
"F_FREESP"
File storage manipulation.
"F_FREESP64"
File storage manipulation.
"F_FSYNC"
Synchronize file data to disk.
"F_FSYNC64"
Synchronize file data to disk.
"F_GETFD"
Return (as a number) the set of file descriptor flags, in which the
following bits may be set:
"FD_CLOEXEC"
During a successful "exec" call, the file descriptor will be
closed automatically.
"F_GETFL"
Return (as a number) the set of file description status flags (O_*)
as set by "open" and "fcntl". To determine the file access mode,
perform a bitwise AND with ""O_ACCMODE"" and see whether the result
is equal to "O_RDONLY", "O_WRONLY", or "O_RDWR".
"F_GETLEASE"
Indicate the type of lease associated with the filehandle (if any)
by returning one of the following flags:
"F_RDLCK"
A read lease.
"F_WRLCK"
A write lease.
"F_UNLCK"
No lease.
"F_GETLK"
Test for the existence of record locks on the file.
"F_GETLK64"
Test for the existence of record locks on the file.
"F_GETOWN"
Return the ID of the process (as a positive number) or group (as a
negative number) that is currently receiving signals for events on
the file descriptor.
"F_GETPIPE_SZ"
Return the capacity of the pipe associated with the filehandle.
"F_GETSIG"
Return the number of the signal sent when input or output becomes
possible on the filehandle. A return value of 0 means "SIGIO".
"F_NOTIFY"
File and directory change notification with signals.
"DN_ACCESS"
"DN_ATTRIB"
"DN_CREATE"
"DN_DELETE"
"DN_MODIFY"
"DN_MULTISHOT"
"DN_RENAME"
"F_SETFD"
Set the file descriptor flags. See ""F_GETFD"" for the list of
available flags.
"F_SETFL"
Set the file description status flags (O_*). Only some flags can be
changed this way.
"F_SETLEASE"
Set a file lease as specified by the third "fnctl" argument, which
must be one of the following:
"F_RDLCK"
Set a read lease.
"F_WRLCK"
Set a write lease.
"F_UNLCK"
Remove a lease.
"F_SETLK"
Acquire a record lock.
"F_SETLK64"
Acquire a record lock.
"F_SETLKW"
Acquire a record lock and wait for conflicting locks to be released.
"F_SETLKW64"
Acquire a record lock and wait for conflicting locks to be released.
"F_SETOWN"
Set the ID of the process (as a positive number) or group (as a
negative number) that will receive signals for events on the file
descriptor.
"F_SETPIPE_SZ"
Set the capacity of the pipe associated with the filehandle. Return
the actual capacity reserved for the pipe, which may be higher than
requested.
"F_SETSIG"
Set the number of the signal sent when input or output becomes
possible on the filehandle. An argument of 0 means "SIGIO".
"F_SHARE"
Set share reservation.
"F_UNSHARE"
Remove share reservation.
"F_COMPAT"
"F_EXLCK"
"F_NODNY"
"F_POSIX"
"F_RDACC"
"F_RDDNY"
"F_RWACC"
"F_RWDNY"
"F_SHLCK"
"F_WRACC"
"F_WRDNY"
Symbols for use with "flock"
"LOCK_EX"
Request an exclusive lock.
"LOCK_MAND"
Request a mandatory lock.
"LOCK_NB"
Make lock request non-blocking (can be combined with other LOCK_*
flags using bitwise OR).
"LOCK_READ"
With "LOCK_MAND": Allow concurrent reads.
"LOCK_RW"
With "LOCK_MAND": Allow concurrent reads and writes.
"LOCK_SH"
Request a shared lock.
"LOCK_UN"
Release a held lock.
"LOCK_WRITE"
With "LOCK_MAND": Allow concurrent writes.
Symbols for use with "sysopen"
"O_ACCMODE"
Bit mask for extracting the file access mode (read-only, write-only,
or read/write) from the other flags. This is mainly useful in
combination with ""F_GETFL"".
"O_ALIAS"
(Mac OS) Open alias file (instead of the file that the alias refers
to).
"O_ALT_IO"
(NetBSD) Use alternative I/O semantics.
"O_APPEND"
Open the file in append mode. Writes always go to the end of the
file.
"O_ASYNC"
Enable signal-based I/O. When the file becomes readable or writable,
a signal is sent.
"O_BINARY"
(Windows) Open the file in binary mode.
"O_CREAT"
If the file to be opened does not exist yet, create it.
"O_DEFER"
(AIX) Changes to the file are kept in memory and not written to disk
until the program performs an explicit "$fh->sync()".
"O_DIRECT"
Perform direct I/O to/from user-space buffers; avoid caching at the
OS level.
"O_DIRECTORY"
Fail if the filename to be opened does not refer to a directory.
"O_DSYNC"
Synchronize file data immediately, like calling fdatasync(2) after
each write.
"O_EVTONLY"
(Mac OS) Open the file for event notifications, not reading or
writing.
"O_EXCL"
If the file already exists, fail and set $! to "EEXIST" (this only
makes sense in combination with "O_CREAT").
"O_EXLOCK"
When the file is opened, atomically obtain an exclusive lock.
"O_IGNORE_CTTY"
(Hurd) If the file to be opened is the controlling terminal for this
process, don't recognize it as such. Operations on this filehandle
won't trigger job control signals.
"O_LARGEFILE"
On 32-bit platforms, allow opening files whose size exceeds 2 GiB
(2,147,483,647 bytes).
"O_NDELAY"
Compatibility symbol. Use "O_NONBLOCK" instead.
"O_NOATIME"
Don't update the access time of the file when reading from it.
"O_NOCTTY"
If the process does not have a controlling terminal and the file to
be opened is a terminal device, don't make it the controlling
terminal of the process.
"O_NOFOLLOW"
If the final component of the filename is a symbolic link, fail and
set $! to "ELOOP".
"O_NOINHERIT"
(Windows) Don't let child processes inherit the opened file
descriptor.
"O_NOLINK"
(Hurd) If the file to be opened is a symbolic link, don't follow it;
open the link itself.
"O_NONBLOCK"
Open the file in non-blocking mode. Neither the open itself nor any
read/write operations on the filehandle will block. (This is mainly
useful for pipes and sockets. It has no effect on regular files.)
"O_NOSIGPIPE"
If the file to be opened is a pipe, then don't raise "SIGPIPE" for
write operations when the read end of the pipe is closed; make the
write fail with "EPIPE" instead.
"O_NOTRANS"
(Hurd) If the file to be opened is specially translated, don't
invoke the translator; open the bare file itself.
"O_RANDOM"
(Windows) Indicate that the program intends to access the file
contents randomly (without a predictable pattern). This is an
optimization hint for the file cache (but may cause excessive memory
use on large files).
"O_RAW"
(Windows) Same as "O_BINARY".
"O_RDONLY"
Open the file for reading (only).
"O_RDWR"
Open the file for reading and writing.
"O_RSRC"
(Mac OS) Open the resource fork of the file.
"O_RSYNC"
Extend the effects of "O_SYNC" and "O_DSYNC" to read operations. In
particular, reading from a filehandle opened with "O_SYNC | O_RSYNC"
will wait until the access time of the file has been modified on
disk.
"O_SEQUENTIAL"
(Windows) Indicate that the program intends to access the file
contents sequentially. This is an optimization hint for the file
cache.
"O_SHLOCK"
When the file is opened, atomically obtain a shared lock.
"O_SYMLINK"
If the file to be opened is a symbolic link, don't follow it; open
the link itself.
"O_SYNC"
Synchronize file data and metadata immediately, like calling
fsync(2) after each write.
"O_TEMPORARY"
(Windows) Delete the file when its last open file descriptor is
closed.
"O_TEXT"
(Windows) Open the file in text mode.
"O_TMPFILE"
Create an unnamed temporary file. The filename argument specifies
the directory the unnamed file should be placed in.
"O_TRUNC"
If the file already exists, truncate its contents to length 0.
"O_TTY_INIT"
If the file to be opened is a terminal that is not already open in
any process, initialize its termios parameters.
"O_WRONLY"
Open the file for writing (only).
"FAPPEND"
Compatibility symbol. Use "O_APPEND" instead.
"FASYNC"
Compatibility symbol. Use "O_ASYNC" instead.
"FCREAT"
Compatibility symbol. Use "O_CREAT" instead.
"FDEFER"
Compatibility symbol. Use "O_DEFER" instead.
"FDSYNC"
Compatibility symbol. Use "O_DSYNC" instead.
"FEXCL"
Compatibility symbol. Use "O_EXCL" instead.
"FLARGEFILE"
Compatibility symbol. Use "O_LARGEFILE" instead.
"FNDELAY"
Compatibility symbol. Use "O_NDELAY" instead.
"FNONBLOCK"
Compatibility symbol. Use "O_NONBLOCK" instead.
"FRSYNC"
Compatibility symbol. Use "O_RSYNC" instead.
"FSYNC"
Compatibility symbol. Use "O_SYNC" instead.
"FTRUNC"
Compatibility symbol. Use "O_TRUNC" instead.
Symbols for use with "seek" and "sysseek"
"SEEK_CUR"
File offsets are relative to the current position in the file.
"SEEK_END"
File offsets are relative to the end of the file (i.e. mostly
negative).
"SEEK_SET"
File offsets are absolute (i.e. relative to the beginning of the
file).
Symbols for use with "stat" and "chmod"
"S_ENFMT"
Enforce mandatory file locks. (This symbol typically shares its
value with "S_ISGID".)
"S_IEXEC"
Compatibility symbol. Use "S_IXUSR" instead.
"S_IFBLK"
File type: Block device.
"S_IFCHR"
File type: Character device.
"S_IFDIR"
File type: Directory.
"S_IFIFO"
File type: Fifo/pipe.
"S_IFLNK"
File type: Symbolic link.
"S_IFMT"
Bit mask for extracting the file type bits. This symbol can also be
used as a function: S_IFMT($mode) acts like "$mode & S_IFMT". The
result will be equal to one of the other S_IF* constants.
"_S_IFMT"
Bit mask for extracting the file type bits. This symbol is an actual
constant and cannot be used as a function; otherwise it is identical
to "S_IFMT".
"S_IFREG"
File type: Regular file.
"S_IFSOCK"
File type: Socket.
"S_IFWHT"
File type: Whiteout file (used to mark the absence/deletion of a
file in overlays).
"S_IMODE"
Function for extracting the permission bits from a file mode.
"S_IREAD"
Compatibility symbol. Use "S_IRUSR" instead.
"S_IRGRP"
Permissions: Readable by group.
"S_IROTH"
Permissions: Readable by others.
"S_IRUSR"
Permissions: Readable by owner.
"S_IRWXG"
Bit mask for extracting group permissions.
"S_IRWXO"
Bit mask for extracting other permissions.
"S_IRWXU"
Bit mask for extracting owner ("user") permissions.
"S_ISBLK"
Convenience function to check for block devices: S_ISBLK($mode) is
equivalent to "S_IFMT($mode) == S_IFBLK".
"S_ISCHR"
Convenience function to check for character devices: S_ISCHR($mode)
is equivalent to "S_IFMT($mode) == S_IFCHR".
"S_ISDIR"
Convenience function to check for directories: S_ISDIR($mode) is
equivalent to "S_IFMT($mode) == S_IFDIR".
"S_ISENFMT"
Broken function; do not use. (S_ISENFMT($mode) should always return
false, anyway.)
"S_ISFIFO"
Convenience function to check for fifos: S_ISFIFO($mode) is
equivalent to "S_IFMT($mode) == S_IFIFO".
"S_ISGID"
Permissions: Set effective group ID from file (when running
executables); mandatory locking (on non-group-executable files); new
files inherit their group from the directory (on directories).
"S_ISLNK"
Convenience function to check for symbolic links: S_ISLNK($mode) is
equivalent to "S_IFMT($mode) == S_IFLNK".
"S_ISREG"
Convenience function to check for regular files: S_ISREG($mode) is
equivalent to "S_IFMT($mode) == S_IFREG".
"S_ISSOCK"
Convenience function to check for sockets: S_ISSOCK($mode) is
equivalent to "S_IFMT($mode) == S_IFSOCK".
"S_ISTXT"
Compatibility symbol. Use "S_ISVTX" instead.
"S_ISUID"
Permissions: Set effective user ID from file (when running
executables).
"S_ISVTX"
Permissions: Files in this directory can only be deleted/renamed by
their owner (or the directory's owner), even if other users have
write permissions to the directory ("sticky bit").
"S_ISWHT"
Convenience function to check for whiteout files: S_ISWHT($mode) is
equivalent to "S_IFMT($mode) == S_IFWHT".
"S_IWGRP"
Permissions: Writable by group.
"S_IWOTH"
Permissions: Writable by others.
"S_IWRITE"
Compatibility symbol. Use "S_IWUSR" instead.
"S_IWUSR"
Permissions: Writable by owner.
"S_IXGRP"
Permissions: Executable/searchable by group.
"S_IXOTH"
Permissions: Executable/searchable by others.
"S_IXUSR"
Permissions: Executable/searchable by owner.
SEE ALSO
"chmod" in perlfunc, chmod(2), "fcntl" in perlfunc, fcntl(2), "flock" in
perlfunc, flock(2), "seek" in perlfunc, fseek(3), "stat" in perlfunc,
stat(2), "sysopen" in perlfunc, open(2), "sysseek" in perlfunc, lseek(2)
APPENDIX A
By default, if you say "use Fcntl;", the following symbols are exported:
FD_CLOEXEC
F_ALLOCSP
F_ALLOCSP64
F_COMPAT
F_DUP2FD
F_DUPFD
F_EXLCK
F_FREESP
F_FREESP64
F_FSYNC
F_FSYNC64
F_GETFD
F_GETFL
F_GETLK
F_GETLK64
F_GETOWN
F_NODNY
F_POSIX
F_RDACC
F_RDDNY
F_RDLCK
F_RWACC
F_RWDNY
F_SETFD
F_SETFL
F_SETLK
F_SETLK64
F_SETLKW
F_SETLKW64
F_SETOWN
F_SHARE
F_SHLCK
F_UNLCK
F_UNSHARE
F_WRACC
F_WRDNY
F_WRLCK
O_ACCMODE
O_ALIAS
O_APPEND
O_ASYNC
O_BINARY
O_CREAT
O_DEFER
O_DIRECT
O_DIRECTORY
O_DSYNC
O_EXCL
O_EXLOCK
O_LARGEFILE
O_NDELAY
O_NOCTTY
O_NOFOLLOW
O_NOINHERIT
O_NONBLOCK
O_RANDOM
O_RAW
O_RDONLY
O_RDWR
O_RSRC
O_RSYNC
O_SEQUENTIAL
O_SHLOCK
O_SYNC
O_TEMPORARY
O_TEXT
O_TRUNC
O_WRONLY
perl v5.40.1 2026-08-30 Fcntl(3perl)
Generated by dwww version 1.16 on Sat Oct 3 07:35:24 CEST 2026.