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# Mixing flock() and close() on Various Unices
https://stackoverflow.com/questions/70045539/file-retains-lock-after-mmap-and-close
This turned out to be a bigger problem than I thought. Well, too late to fix the
kernel now.
Added `fcntl()` as control.
## INSTALL
```sh
mkdir flock_mmap
cd flock_mmap
wget \
https://github.com/dxdxdt/gists/raw/master/flock_mmap/tests.sh \
https://github.com/dxdxdt/gists/raw/master/flock_mmap/Makefile \
https://github.com/dxdxdt/gists/raw/master/flock_mmap/flock_mmap.c \
https://github.com/dxdxdt/gists/raw/master/flock_mmap/README.md
chmod 755 tests.sh
```
## Run it
```sh
make 2> /dev/null
```
### Result A
- Linux: glibc, ulibc, musl
```
./tests.sh
[TEST]fcntl(fd, F_SETLK, ...) and close() unlocks the file: YES
[TEST]fcntl(fd, F_SETLK, ...) and holding unlocks the file: NO
[TEST]flock() and close() unlocks the file*: NO
[TEST]flock() and holding unlocks the file : NO
```
### Result B
- FreeBSD
- OpenBSD
- mac
```
./tests.sh
[TEST]fcntl(fd, F_SETLK, ...) and close() unlocks the file: YES
[TEST]fcntl(fd, F_SETLK, ...) and holding unlocks the file: NO
[TEST]flock() and close() unlocks the file*: YES
[TEST]flock() and holding unlocks the file : NO
```
## WHY?
Not sure. Just don't mix `mmap()` with `flock()`, I guess.
```
HISTORY
4.4BSD (the flock() call first appeared in 4.2BSD). A version of
flock(), possibly implemented in terms of fcntl(2), appears on most
UNIX systems.
```
```
VERSIONS
Since Linux 2.0, flock() is implemented as a system call in its own
right rather than being emulated in the GNU C library as a call to fc‐
ntl(2). With this implementation, there is no interaction between the
types of lock placed by flock() and fcntl(2), and flock() does not de‐
tect deadlock. (Note, however, that on some systems, such as the mod‐
ern BSDs, flock() and fcntl(2) locks do interact with one another.)
```
They're the same picture:
- https://github.com/openbsd/src/blob/eeb276dfee031771095d62f976689174e966054b/sys/kern/kern_descrip.c#L1292
- https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/bsd/kern/kern_descrip.c#L5704
This is not:
- https://github.com/torvalds/linux/blob/8e938e39866920ddc266898e6ae1fffc5c8f51aa/fs/locks.c#L2117
|
