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# Fcntl

<div class="pperl-badges"><span class="pperl-badge pperl-tier-min" title="Minimum feature tier: min. Available in all builds from \`--features native-min\` upward.">📦 min</span></div>

Symbolic constants for `fcntl(2)`, `flock(2)`, `seek(2)`, and file-mode
bits — everything you need to call `sysopen`, `fcntl`, `flock`, `seek`,
and to interpret the mode word returned by `stat`.

Import individual names, or a named group via one of the export tags:
`:DEFAULT` (open and file-descriptor flags), `:flock` (advisory locks),
`:seek` (whence positions), `:mode` (mode bits and `S_IS*` predicates),
`:Fcompat` (BSD-style `flock` aliases).

## Open flags (sysopen)

- `O_RDONLY`, `O_WRONLY`, `O_RDWR` — access mode.
- `O_CREAT` — create the file if it does not exist.
- `O_EXCL` — with `O_CREAT`, fail if the file already exists.
- `O_TRUNC` — truncate to zero length on open.
- `O_APPEND` — every write seeks to end of file first.
- `O_NONBLOCK` — open without blocking; I/O returns `EAGAIN` instead.
- `O_NOCTTY`, `O_NOFOLLOW`, `O_BINARY` — additional platform flags
  (`O_BINARY` is a no-op on Linux).

## File-descriptor control (fcntl)

- `F_DUPFD` — duplicate a descriptor.
- `F_GETFD`, `F_SETFD` — read / set the close-on-exec flag `FD_CLOEXEC`.
- `F_GETFL`, `F_SETFL` — read / set the open-file status flags.

## Lock types (flock)

- `LOCK_SH` — shared lock.
- `LOCK_EX` — exclusive lock.
- `LOCK_UN` — release lock.
- `LOCK_NB` — OR with the above to request a non-blocking attempt.

## Seek whence

- `SEEK_SET` — from start of file.
- `SEEK_CUR` — from current position.
- `SEEK_END` — from end of file.

## File mode bits (stat)

- Type mask and values: `S_IFMT`, `S_IFREG`, `S_IFDIR`, `S_IFCHR`,
  `S_IFBLK`, `S_IFIFO`, `S_IFLNK`, `S_IFSOCK`.
- Permission triplets: `S_IRWXU`, `S_IRWXG`, `S_IRWXO` and the
  per-bit `S_IRUSR` / `S_IWUSR` / `S_IXUSR` (and the `GRP`, `OTH`
  variants).
- Setuid / setgid / sticky bits: `S_ISUID`, `S_ISGID`, `S_ISVTX`.

## Mode test helpers

Function-form predicates over a stat-mode value: `S_ISREG`, `S_ISDIR`,
`S_ISCHR`, `S_ISBLK`, `S_ISFIFO`, `S_ISLNK`, `S_ISSOCK`, plus
`S_IFMT` (extract file-type bits) and `S_IMODE` (extract permission
bits).

## Performance

Every constant in this module is resolved at compile time — using
`O_CREAT` in an expression costs exactly what writing the integer
literal would cost. No per-call lookup.

## Functions

### Mode test helpers

#### [`s_ifmt`](Fcntl/s_ifmt.md)

Extract the file-type bits from a stat-mode value.

#### [`s_imode`](Fcntl/s_imode.md)

Extract the permission bits from a stat-mode value.

#### [`s_isreg`](Fcntl/s_isreg.md)

True if a stat-mode value names a regular file.

#### [`s_isdir`](Fcntl/s_isdir.md)

True if a stat-mode value names a directory.

#### [`s_ischr`](Fcntl/s_ischr.md)

True if a stat-mode value names a character-special device.

#### [`s_isblk`](Fcntl/s_isblk.md)

True if a stat-mode value names a block-special device.

#### [`s_isfifo`](Fcntl/s_isfifo.md)

True if a stat-mode value names a named pipe (FIFO).

#### [`s_islnk`](Fcntl/s_islnk.md)

True if a stat-mode value names a symbolic link.

#### [`s_issock`](Fcntl/s_issock.md)

True if a stat-mode value names a socket.