Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
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.Dd $Mdocdate: March 31 2022 $ .Dt SRP_ENTER 9 .Os .Sh NAME .Nm srp_init , .Nm srp_gc_init , .Nm srp_update , .Nm srp_update_locked , .Nm srp_swap , .Nm srp_swap_locked , .Nm srp_enter , .Nm srp_follow , .Nm srp_leave , .Nm srp_get_locked , .Nm srp_finalize , .Nm srp_gc_finalize , .Nm SRP_INITIALIZER , .Nm SRP_GC_INITIALIZER .Nd shared reference pointers .Sh SYNOPSIS n sys/srp.h .Ft void .Fn srp_init "struct srp *p" .Ft void .Fo srp_gc_init .Fa "struct srp_gc *gc" .Fa "void (*dtor)(void *, void *)" .Fa "void *ctx" .Fc .Ft void * .Fn srp_swap "struct srp *p" "void *v" .Ft void * .Fn srp_swap_locked "struct srp *p" "void *v" .Ft void .Fn srp_update "struct srp_gc *gc" "struct srp *p" "void *v" .Ft void .Fn srp_update_locked "struct srp_gc *gc" "struct srp *p" "void *v" .Ft void * .Fn srp_enter "struct srp_ref *sr" "struct srp *p" .Ft void * .Fn srp_follow "struct srp_ref *sr" "struct srp *n" .Ft void .Fn srp_leave "struct srp_ref *sr" .Ft void * .Fn srp_get_locked "struct srp *p" .Ft void .Fn srp_finalize "void *v" "const char *wmesg" .Ft void .Fn srp_gc_finalize "struct srp_gc *gc" .Fn SRP_INITIALIZER .Fo SRP_GC_INITIALIZER .Fa "void (*dtor)(void *, void *)" .Fa "void *ctx" .Fc .Sh DESCRIPTION An srp structure represents a pointer or reference to an object in memory. The srp API provides concurrent lock free access to these objects, and can guarantee that the data isn't destroyed while that reference is in use. It does not prevent concurrent modification of the referenced object.
p .Fn srp_init initialises the srp structure .Fa p to an empty state.
p .Fn srp_gc_init initialises the srp_gc structure .Fa gc so it can be used as a garbage collector for data that gets referenced by srp structures. An update to an srp structure will cause the old data to be destroyed when it is no longer referenced by any CPU in the system. The old data will be destroyed by the garbage collector by a call to .Fa dtor with .Fa ctx as the first argument and the pointer to the data as the second argument.
p .Fn srp_update and .Fn srp_update_locked replace the data referenced by the srp struct .Fa p with the data referenced by .Fa v . When the original data is no longer in use, it will be destroyed by the garbage collector .Fa gc . .Fn srp_update uses atomic CPU operations to change the references. .Fn srp_update_locked may be used if modifications to .Fa p are already serialised by the caller. Both .Fn srp_update and .Fn srp_update_locked may sleep.
p .Fn srp_swap and .Fn srp_swap_locked replace the data referenced by the srp struct .Fa p with the data referenced by .Fa v . When clearing or replacing the last reference to a data structure, .Fn srp_finalize must be used to ensure that the data is no longer in use via any srp structures. .Fn srp_swap uses atomic CPU operations to change the reference. .Fn srp_swap_locked may be used if modifications to .Fa p are already serialised by the caller.
p .Fn srp_enter returns a pointer to a data structure referenced by the srp struct .Fa p and guarantees it will remain available for use until it is released with a call to .Fn srp_leave or .Fn srp_follow . The reference is held via .Fa sr .
p .Fn srp_follow replaces the reference held via .Fa sr with a reference to the data structure represented by .Fa p .
p .Fn srp_leave releases the reference held via .Fa sr and makes it available for garbage collection.
p .Fn srp_get_locked provides access to the data referenced by the srp .Fa p if the caller has excluded updates to .Fa p .
p .Fn srp_finalize sleeps until there are no longer any references to .Fa v via any srp structure in the system.
p .Fn srp_gc_finalize sleeps until all references to data by srp structures using the garbage collector .Fa gc have completed. That in turn means the .Fa gc structure will no longer be referenced and can itself be destroyed.
p A srp structure declaration can be initialised with the .Fn SRP_INITIALIZER macro.
p A srp_gc structure declaration can be initialised with the .Fn SRP_GC_INITIALIZER macro. Data will be destroyed by the garbage collector by a call to .Fa dtor with .Fa ctx as the first argument and the pointer to the data as the second argument. .Sh CONTEXT .Fn srp_init , .Fn srp_gc_init , .Fn srp_update , .Fn srp_update_locked , .Fn srp_get_locked , .Fn srp_finalize , and .Fn srp_gc_finalize can be called during autoconf or from process context.
p .Fn srp_swap , .Fn srp_swap_locked , .Fn srp_enter , .Fn srp_follow , and .Fn srp_leave can be called during autoconf, from process context, or from interrupt context. .Sh RETURN VALUES .Fn srp_swap and .Fn srp_swap_locked return a pointer to the previous value referenced by the srp structure .Fa p .
p .Fn srp_enter , .Fn srp_follow , and .Fn srp_get_locked return a pointer to the data referenced by the srp structure .Fa p or .Dv NULL . .Sh HISTORY The srp API was originally written by .An Jonathan Matthew Aq Mt jmatthew@openbsd.org and .An David Gwynne Aq Mt dlg@openbsd.org . The srp API first appeared in .Ox 5.8 .