From 691c69e626e4ceb85567a5288348bd04378fd410 Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Mon, 30 Aug 2021 19:11:44 +0000 Subject: s6-svlink, s6-svunlink, .h changes Renamed from s6-svdir-(un)link. Doc added. Full functionality added. Still need to be tested. Unrelated: .h names simplified. Signed-off-by: Laurent Bercot --- doc/libs6/lock.html | 239 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 239 insertions(+) create mode 100644 doc/libs6/lock.html (limited to 'doc/libs6/lock.html') diff --git a/doc/libs6/lock.html b/doc/libs6/lock.html new file mode 100644 index 0000000..1cc896d --- /dev/null +++ b/doc/libs6/lock.html @@ -0,0 +1,239 @@ + + + + + + s6: the s6lock library interface + + + + + + +

+libs6
+s6
+Software
+skarnet.org +

+ +

The `s6lock` library interface

+ +

General information

+ +

+ s6lock is a C interface to timed locks. Unix natively provides +locks, but the locking primitives are synchronous, so either they are +unbounded in execution time or they require polling. s6lock provides +poll-free locks that can timeout during attempted acquisition. +

+ +

Programming

+ +

Check the s6/lock.h header +for the prototypes. The functions documented here are +often simplified macros, for instance relying on the STAMP global variable +to hold the current time. Fully reentrant functions with more control +options are usually available.
Given the nature of the s6lock library, it makes sense to use a +s6lockd service concurrently +accessed by several applications using such locks to gate shared +resources.
If you're not using an s6lockd service, +make sure your application is not disturbed by children it doesn't +know it has. Using nonblocking waits, ignoring pids you don't know, and +using a +self-pipe +if your application is built around an event loop, are good programming +practices.

+ +

Starting and ending a session

+ +

+s6lock_t a = S6LOCK_ZERO ;
+tain_t deadline ;
+
+tain_now_g() ;
+tain_addsec_g(&deadline, 2)
+
+char const *path = S6LOCK_IPCPATH ;
+s6lock_start_g(&a, path, &deadline) ;
+// char const *lockdir = "/tmp/lock" ;
+// s6lock_startf_g(&a, lockdir, &deadline) ;
+

+ +

+s6lock_start_g starts a session by connecting to an s6lockd service +listening on path. The working directory is set by the administrator +of the service.
+s6lock_startf_g starts a session with an s6lockd process as a child, +using lockdir as its working directory. +
+a is an s6lock_t structure that must be declared in the stack and +initialized to S6LOCK_ZERO. +If the session initialization fails, the function returns 0 and errno is set; +else the function returns 1. +

+If the absolute time deadline is reached and the function +has not returned yet, it immediately returns 0 with errno set to ETIMEDOUT. + +Only local interprocess communications are involved; unless your system is +heavily overloaded, the function should return near-instantly. One or two +seconds of delay between the current time and deadline should be +enough: if the function takes more than that to return, then there is a +problem with the underlying processes. +

+ +

+ You can have more than one session open in parallel, by declaring +several distinct s6lock_t structures and calling +s6lock_startf_g (or s6lock_start_g) more than once. +However, one single session can handle +virtually as many concurrent locks as your application needs, so +opening several sessions is only useful if you need to acquire locks +in various distinct lock directories. +

+ +

+s6lock_end(&a) ;
+

+ +

+s6lock_end frees all the resources used by the session. The +a structure is then reusable for another session. +

+ +

Acquiring and releasing locks

+ +

+uint16_t id ;
+char const *file = "lockfile" ;
+tain_t limit ;
+tain_t deadline ;
+
+int r = s6lock_acquire_sh_g (&a, &id, file, &limit, &deadline) ;
+/* int r = s6lock_acquire_ex_g (&a, &id, file, &limit, &deadline) ; */
+r = s6lock_release_g(&a, id, &deadline) ;
+

+ +

+s6lock_acquire_sh_g instructs the +s6lockd daemon, related to the open +session represented by the a handle, to try and acquire a +shared lock on the +file file located under that daemon's working directory +(typically /var/lock). file will be interpreted as +relative to the daemon's working directory even if it starts with a +slash; however, slashes in the middle of file are likely to +result in an error. +

+ +

+limit and deadline are two absolute dates. +deadline is a deadline for the execution of the +function: if by deadline the function has not returned, +then it instantly returns 0 and sets errno to ETIMEDOUT. The +function is normally near-instantaneous, so deadline can +be very close in the future and serves only as a protection against +malicious servers. limit is the acquisition deadline: if +by limit the daemon still has not been able to acquire a lock +on file, then it will report a timeout to the client. +

+ +

+The function returns 1 in case of success, or 0 if an error occurs, +with errno set to a suitable value. If it succeeds, then a 16-bit +number is stored into *id; this number serves as an identifier +for this lock. +

+ +

+s6lock_acquire_ex_g works just like s6lock_acquire_sh_g, +except that the daemon tries to acquire an exclusive lock. +

+ +

+s6lock_release_g releases the lock identified by id. +It normally returns 1. It can return 0 with errno set to a suitable +value if it fails. id is not valid after the corresponding +lock has been released. The function normally returns instantly, with +deadline as a safeguard. +

+ +

Asynchronously waiting for locks

+ +

+ (from now on, the functions are listed with their prototypes instead +of usage examples.) +

+ +

+int s6lock_fd (s6lock_t const *a)
+

+ +

+ Returns a file descriptor to select on for reading. Do not +read() it though. +

+ +

+int s6lock_update (s6lock_t *a)
+

+ +

+ Call this function whenever the fd checks readability: it will +update a's internal structures with information from the +s6lockd daemon. It returns -1 if an error +occurs; in case of success, it returns the number of identifiers for +which something happened. +

+ +

+ When s6lock_update returns, +genalloc_s(uint16_t, &a->list) points to an array of +genalloc_len(uint16_t, &a->list) 16-bit unsigned +integers. Those integers are ids waiting to be passed to +s6lock_check. +

+ +

+int s6lock_check (s6lock_t *a, uint16_t id, char *what)
+

+ +

+ Checks whether the lock identified by id has +been acquired. Use after a call to s6lock_update(). +

+ +

If an error occurred, returns -1 and sets errno. The error +number may have been transmitted from +s6lockd.
If the lock has not been acquired yet, returns 0.
If the lock has been acquired, returns 1.

+ +

Synchronously waiting for locks

+ +

+ int s6lock_wait_or_g (s6lock_t *a, uint16_t const *idlist, unsigned int n, tain_t const *deadline)
+Synchronously waits for one of the locks represented by the array pointed to +by idlist of length n to be acquired. Returns -1 if it fails, +or a nonnegative number on success, which is the index in idlist of the +acquired lock's id. If no result has been obtained by deadline, the +function returns -1 ETIMEDOUT. +

+ +

+ int s6lock_wait_and_g (s6lock_t *a, uint16_t const *idlist, unsigned int n, tain_t const *deadline)
+Synchronously waits for all of the locks represented by the array pointed to +by idlist of length n to be acquired. Returns -1 if it fails and +0 if it succeeds. If no result has been obtained by deadline, the +function returns -1 ETIMEDOUT. +

+ + + -- cgit v1.2.3