engine(3) OpenSL engine(3)
NAME
engine - ENGINE cryptographic module support
SYNOPSIS
#include
ENGINE *ENGINEgetfirst(void);
ENGINE *ENGINEgetlast(void);
ENGINE *ENGINEgetnext(ENGINE *e);
ENGINE *ENGINEgetprev(ENGINE *e);
int ENGINEadd(ENGINE *e);
int ENGINEremove(ENGINE *e);
ENGINE *ENGINEbyid(const char *id);
int ENGINEinit(ENGINE *e);
int ENGINEfinish(ENGINE *e);
void ENGINEloadopenssl(void);
void ENGINEloaddynamic(void);
void ENGINEloadcswift(void);
void ENGINEloadchil(void);
void ENGINEloadatalla(void);
void ENGINEloadnuron(void);
void ENGINEloadubsec(void);
void ENGINEloadaep(void);
void ENGINEloadsureware(void);
void ENGINEload4758cca(void);
void ENGINEloadopenbsddevcrypto(void);
void ENGINEloadbuiltinengines(void);
void ENGINEcleanup(void);
ENGINE *ENGINEgetdefaultRSA(void);
ENGINE *ENGINEgetdefaultDSA(void);
ENGINE *ENGINEgetdefaultDH(void);
ENGINE *ENGINEgetdefaultRAND(void);
ENGINE *ENGINEgetcipherengine(int nid);
ENGINE *ENGINEgetdigestengine(int nid);
int ENGINEsetdefaultRSA(ENGINE *e);
int ENGINEsetdefaultDSA(ENGINE *e);
int ENGINEsetdefaultDH(ENGINE *e);
int ENGINEsetdefaultRAND(ENGINE *e);
int ENGINEsetdefaultciphers(ENGINE *e);
int ENGINEsetdefaultdigests(ENGINE *e);
int ENGINEsetdefaultstring(ENGINE *e, const char *list);
int ENGINEsetdefault(ENGINE *e, unsigned int flags);
unsigned int ENGINEgettableflags(void);
void ENGINEsettableflags(unsigned int flags);
int ENGINEregisterRSA(ENGINE *e);
void ENGINEunregisterRSA(ENGINE *e);
void ENGINEregisterallRSA(void);
int ENGINEregisterDSA(ENGINE *e);
void ENGINEunregisterDSA(ENGINE *e);
void ENGINEregisterallDSA(void);
int ENGINEregisterDH(ENGINE *e);
void ENGINEunregisterDH(ENGINE *e);
void ENGINEregisterallDH(void);
int ENGINEregisterRAND(ENGINE *e);
void ENGINEunregisterRAND(ENGINE *e);
void ENGINEregisterallRAND(void);
int ENGINEregisterciphers(ENGINE *e);
void ENGINEunregisterciphers(ENGINE *e);
void ENGINEregisterallciphers(void);
int ENGINEregisterdigests(ENGINE *e);
void ENGINEunregisterdigests(ENGINE *e);
void ENGINEregisteralldigests(void);
int ENGINEregistercomplete(ENGINE *e);
int ENGINEregisterallcomplete(void);
int ENGINEctrl(ENGINE *e, int cmd, long i, void *p, void (*f)());
int ENGINEcmdisexecutable(ENGINE *e, int cmd);
int ENGINEctrlcmd(ENGINE *e, const char *cmdname,
long i, void *p, void (*f)(), int cmdoptional);
int ENGINEctrlcmdstring(ENGINE *e, const char *cmdname, const char *arg,
int cmdoptional);
int ENGINEsetexdata(ENGINE *e, int idx, void *arg);
void *ENGINEgetexdata(const ENGINE *e, int idx);
int ENGINEgetexnewindex(long argl, void *argp, CRYPTOEXnew *newfunc,
CRYPTOEXdup *dupfunc, CRYPTOEXfree *freefunc);
ENGINE *ENGINEnew(void);
int ENGINEfree(ENGINE *e);
int ENGINEsetid(ENGINE *e, const char *id);
int ENGINEsetname(ENGINE *e, const char *name);
int ENGINEsetRSA(ENGINE *e, const RSAMETHOD *rsameth);
int ENGINEsetDSA(ENGINE *e, const DSAMETHOD *dsameth);
int ENGINEsetDH(ENGINE *e, const DHMETHOD *dhmeth);
int ENGINEsetRAND(ENGINE *e, const RANDMETHOD *randmeth);
int ENGINEsetdestroyfunction(ENGINE *e, ENGINEGENINTFUNCPTR destroyf);
int ENGINEsetinitfunction(ENGINE *e, ENGINEGENINTFUNCPTR initf);
int ENGINEsetfinishfunction(ENGINE *e, ENGINEGENINTFUNCPTR finishf);
int ENGINEsetctrlfunction(ENGINE *e, ENGINECTRLFUNCPTR ctrlf);
int ENGINEsetloadprivkeyfunction(ENGINE *e, ENGINELOADKEYPTR loadprivf);
int ENGINEsetloadpubkeyfunction(ENGINE *e, ENGINELOADKEYPTR loadpubf);
int ENGINEsetciphers(ENGINE *e, ENGINECIPHERSPTR f);
int ENGINEsetdigests(ENGINE *e, ENGINEDIGESTSPTR f);
int ENGINEsetflags(ENGINE *e, int flags);
int ENGINEsetcmddefns(ENGINE *e, const ENGINECMDEFN *defns);
const char *ENGINEgetid(const ENGINE *e);
const char *ENGINEgetname(const ENGINE *e);
const RSAMETHOD *ENGINEgetRSA(const ENGINE *e);
const DSAMETHOD *ENGINEgetDSA(const ENGINE *e);
const DHMETHOD *ENGINEgetDH(const ENGINE *e);
const RANDMETHOD *ENGINEgetRAND(const ENGINE *e);
ENGINEGENINTFUNCPTR ENGINEgetdestroyfunction(const ENGINE *e);
ENGINEGENINTFUNCPTR ENGINEgetinitfunction(const ENGINE *e);
ENGINEGENINTFUNCPTR ENGINEgetfinishfunction(const ENGINE *e);
ENGINECTRLFUNCPTR ENGINEgetctrlfunction(const ENGINE *e);
ENGINELOADKEYPTR ENGINEgetloadprivkeyfunction(const ENGINE *e);
ENGINELOADKEYPTR ENGINEgetloadpubkeyfunction(const ENGINE *e);
ENGINECIPHERSPTR ENGINEgetciphers(const ENGINE *e);
ENGINEDIGESTSPTR ENGINEgetdigests(const ENGINE *e);
const EVPCIPHER *ENGINEgetcipher(ENGINE *e, int nid);
const EVPMD *ENGINEgetdigest(ENGINE *e, int nid);
int ENGINEgetflags(const ENGINE *e);
const ENGINECMDEFN *ENGINEgetcmddefns(const ENGINE *e);
EVPKEY *ENGINEloadprivatekey(ENGINE *e, const char *keyid,
UIMETHOD *uimethod, void *callbackdata);
EVPKEY *ENGINEloadpublickey(ENGINE *e, const char *keyid,
UIMETHOD *uimethod, void *callbackdata);
void ENGINEaddconfmodule(void);
DESCRIPTION
These functions create, manipulate, and use cryptographic modules in
the form of ENGINE objects. These objects act as containers for imple-
mentations of cryptographic algorithms, and support a reference-counted
mechanism to allow them to be dynamically loaded in and out of the run-
ning application.
The cryptographic functionality that can be provided by an ENGINE
implementation includes the following abstractions;
RSAMETHOD - for providing alternative RSA implementations
DSAMETHOD, DHMETHOD, RANDMETHOD - alternative DSA, DH, and RAND
EVPCIPHER - potentially multiple cipher algorithms (indexed by 'nid')
EVPDIGEST - potentially multiple hash algorithms (indexed by 'nid')
key-loading - loading public and/or private EVPKEY keys
Reference counting and handles
Due to the modular nature of the ENGINE API, pointers to ENGINEs need
to be treated as handles - ie. not only as pointers, but also as refer-
ences to the underlying ENGINE object. Ie. you should obtain a new ref-
erence when making copies of an ENGINE pointer if the copies will be
used (and released) independantly.
ENGINE objects have two levels of reference-counting to match the way
in which the objects are used. At the most basic level, each ENGINE
pointer is inherently a structural reference - you need a structural
reference simply to refer to the pointer value at all, as this kind of
reference is your guarantee that the structure can not be deallocated
until you release your reference.
However, a structural reference provides no guarantee that the ENGINE
has been initiliased to be usable to perform any of its cryptographic
implementations - and indeed it's quite possible that most ENGINEs will
not initialised at all on standard setups, as ENGINEs are typically
used to support specialised hardware. To use an ENGINE's functionality,
you need a functional reference. This kind of reference can be consid-
ered a specialised form of structural reference, because each func-
tional reference implicitly contains a structural reference as well -
however to avoid difficult-to-find programming bugs, it is recommended
to treat the two kinds of reference independantly. If you have a func-
tional reference to an ENGINE, you have a guarantee that the ENGINE has
been initialised ready to perform cryptographic operations and will not
be uninitialised or cleaned up until after you have released your ref-
erence.
We will discuss the two kinds of reference separately, including how to
tell which one you are dealing with at any given point in time (after
all they are both simply (ENGINE *) pointers, the difference is in the
way they are used).
Structural references
This basic type of reference is typically used for creating new ENGINEs
dynamically, iterating across OpenSL's internal linked-list of loaded
ENGINEs, reading information about an ENGINE, etc. Essentially a struc-
tural reference is sufficient if you only need to query or manipulate
the data of an ENGINE implementation rather than use its functionality.
The ENGINEnew() function returns a structural reference to a new
(empty) ENGINE object. Other than that, structural references come from
return values to various ENGINE API functions such as; ENGINEbyid(),
ENGINEgetfirst(), ENGINEgetlast(), ENGINEgetnext(),
ENGINEgetprev(). All structural references should be released by a
corresponding to call to the ENGINEfree() function - the ENGINE object
itself will only actually be cleaned up and deallocated when the last
structural reference is released.
It should also be noted that many ENGINE API function calls that accept
a structural reference will internally obtain another reference - typi-
cally this happens whenever the supplied ENGINE will be needed by
OpenSL after the function has returned. Eg. the function to add a new
ENGINE to OpenSL's internal list is ENGINEadd() - if this function
returns success, then OpenSL will have stored a new structural refer-
ence internally so the caller is still responsible for freeing their
own reference with ENGINEfree() when they are finished with it. In a
similar way, some functions will automatically release the structural
reference passed to it if part of the function's job is to do so. Eg.
the ENGINEgetnext() and ENGINEgetprev() functions are used for
iterating across the internal ENGINE list - they will return a new
structural reference to the next (or previous) ENGINE in the list or
NUL if at the end (or beginning) of the list, but in either case the
structural reference passed to the function is released on behalf of
the caller.
To clarify a particular function's handling of references, one should
always consult that function's documentation "man" page, or failing
that the openssl/engine.h header file includes some hints.
Functional references
As mentioned, functional references exist when the cryptographic func-
tionality of an ENGINE is required to be available. A functional refer-
ence can be obtained in one of two ways; from an existing structural
reference to the required ENGINE, or by asking OpenSL for the default
operational ENGINE for a given cryptographic purpose.
To obtain a functional reference from an existing structural reference,
call the ENGINEinit() function. This returns zero if the ENGINE was
not already operational and couldn't be successfully initialised (eg.
lack of system drivers, no special hardware attached, etc), otherwise
it will return non-zero to indicate that the ENGINE is now operational
and will have allocated a new functional reference to the ENGINE. In
this case, the supplied ENGINE pointer is, from the point of the view
of the caller, both a structural reference and a functional reference -
so if the caller intends to use it as a functional reference it should
free the structural reference with ENGINEfree() first. If the caller
wishes to use it only as a structural reference (eg. if the
ENGINEinit() call was simply to test if the ENGINE seems avail-
able/online), then it should free the functional reference; all func-
tional references are released by the ENGINEfinish() function.
The second way to get a functional reference is by asking OpenSL for a
default implementation for a given task, eg. by
ENGINEgetdefaultRSA(), ENGINEgetdefaultcipherengine(), etc.
These are discussed in the next section, though they are not usually
required by application programmers as they are used automatically when
creating and using the relevant algorithm-specific types in OpenSL,
such as RSA, DSA, EVPCIPHERCTX, etc.
Default implementations
For each supported abstraction, the ENGINE code maintains an internal
table of state to control which implementations are available for a
given abstraction and which should be used by default. These implemen-
tations are registered in the tables separated-out by an 'nid' index,
because abstractions like EVPCIPHER and EVPDIGEST support many dis-
tinct algorithms and modes - ENGINEs will support different numbers and
combinations of these. In the case of other abstractions like RSA, DSA,
etc, there is only one "algorithm" so all implementations implicitly
register using the same 'nid' index. ENGINEs can be registered into
these tables to make themselves available for use automatically by the
various abstractions, eg. RSA. For illustrative purposes, we continue
with the RSA example, though all comments apply similarly to the other
abstractions (they each get their own table and linkage to the corre-
sponding section of openssl code).
When a new RSA key is being created, ie. in RSAnewmethod(), a
"getdefault" call will be made to the ENGINE subsystem to process the
RSA state table and return a functional reference to an initialised
ENGINE whose RSAMETHOD should be used. If no ENGINE should (or can) be
used, it will return NUL and the RSA key will operate with a NUL
ENGINE handle by using the conventional RSA implementation in OpenSL
(and will from then on behave the way it used to before the ENGINE API
existed - for details see RSAnewmethod(3)).
Each state table has a flag to note whether it has processed this
"getdefault" query since the table was last modified, because to
process this question it must iterate across all the registered ENGINEs
in the table trying to initialise each of them in turn, in case one of
them is operational. If it returns a functional reference to an ENGINE,
it will also cache another reference to speed up processing future
queries (without needing to iterate across the table). Likewise, it
will cache a NUL response if no ENGINE was available so that future
queries won't repeat the same iteration unless the state table changes.
This behaviour can also be changed; if the ENGINETABLEFLAGNOINIT
flag is set (using ENGINEsettableflags()), no attempted initialisa-
tions will take place, instead the only way for the state table to
return a non-NUL ENGINE to the "getdefault" query will be if one is
expressly set in the table. Eg. ENGINEsetdefaultRSA() does the same
job as ENGINEregisterRSA() except that it also sets the state table's
cached response for the "getdefault" query.
In the case of abstractions like EVPCIPHER, where implementations are
indexed by 'nid', these flags and cached-responses are distinct for
each 'nid' value.
It is worth illustrating the difference between "registration" of
ENGINEs into these per-algorithm state tables and using the alternative
"setdefault" functions. The latter handles both "registration" and
also setting the cached "default" ENGINE in each relevant state table -
so registered ENGINEs will only have a chance to be initialised for use
as a default if a default ENGINE wasn't already set for the same state
table. Eg. if ENGINE X supports cipher nids {A,B} and RSA, ENGINE Y
supports ciphers {A} and DSA, and the following code is executed;
ENGINEregistercomplete(X);
ENGINEsetdefault(Y, ENGINEMETHODAL);
e1 = ENGINEgetdefaultRSA();
e2 = ENGINEgetcipherengine(A);
e3 = ENGINEgetcipherengine(B);
e4 = ENGINEgetdefaultDSA();
e5 = ENGINEgetcipherengine(C);
The results would be as follows;
assert(e1 == X);
assert(e2 == Y);
assert(e3 == X);
assert(e4 == Y);
assert(e5 == NUL);
Application requirements
This section will explain the basic things an application programmer
should support to make the most useful elements of the ENGINE function-
ality available to the user. The first thing to consider is whether the
programmer wishes to make alternative ENGINE modules available to the
application and user. OpenSL maintains an internal linked list of
"visible" ENGINEs from which it has to operate - at start-up, this list
is empty and in fact if an application does not call any ENGINE API
calls and it uses static linking against openssl, then the resulting
application binary will not contain any alternative ENGINE code at all.
So the first consideration is whether any/all available ENGINE imple-
mentations should be made visible to OpenSL - this is controlled by
calling the various "load" functions, eg.
/* Make the "dynamic" ENGINE available */
void ENGINEloaddynamic(void);
/* Make the CryptoSwift hardware acceleration support available */
void ENGINEloadcswift(void);
/* Make support for nCipher's "CHIL" hardware available */
void ENGINEloadchil(void);
...
/* Make AL ENGINE implementations bundled with OpenSL available */
void ENGINEloadbuiltinengines(void);
Having called any of these functions, ENGINE objects would have been
dynamically allocated and populated with these implementations and
linked into OpenSL's internal linked list. At this point it is impor-
tant to mention an important API function;
void ENGINEcleanup(void);
If no ENGINE API functions are called at all in an application, then
there are no inherent memory leaks to worry about from the ENGINE func-
tionality, however if any ENGINEs are "load"ed, even if they are never
registered or used, it is necessary to use the ENGINEcleanup() func-
tion to correspondingly cleanup before program exit, if the caller
wishes to avoid memory leaks. This mechanism uses an internal callback
registration table so that any ENGINE API functionality that knows it
requires cleanup can register its cleanup details to be called during
ENGINEcleanup(). This approach allows ENGINEcleanup() to clean up
after any ENGINE functionality at all that your program uses, yet
doesn't automatically create linker dependencies to all possible ENGINE
functionality - only the cleanup callbacks required by the functional-
ity you do use will be required by the linker.
The fact that ENGINEs are made visible to OpenSL (and thus are linked
into the program and loaded into memory at run-time) does not mean they
are "registered" or called into use by OpenSL automatically - that be-
haviour is something for the application to have control over. Some
applications will want to allow the user to specify exactly which
ENGINE they want used if any is to be used at all. Others may prefer to
load all support and have OpenSL automatically use at run-time any
ENGINE that is able to successfully initialise - ie. to assume that
this corresponds to acceleration hardware attached to the machine or
some such thing. There are probably numerous other ways in which appli-
cations may prefer to handle things, so we will simply illustrate the
consequences as they apply to a couple of simple cases and leave devel-
opers to consider these and the source code to openssl's builtin utili-
ties as guides.
Using a specific ENGINE implementation
Here we'll assume an application has been configured by its user or
admin to want to use the "ACME" ENGINE if it is available in the ver-
sion of OpenSL the application was compiled with. If it is available,
it should be used by default for all RSA, DSA, and symmetric cipher
operation, otherwise OpenSL should use its builtin software as per
usual. The following code illustrates how to approach this;
ENGINE *e;
const char *engineid = "ACME";
ENGINEloadbuiltinengines();
e = ENGINEbyid(engineid);
if(!e)
/* the engine isn't available */
return;
if(!ENGINEinit(e)) {
/* the engine couldn't initialise, release 'e' */
ENGINEfree(e);
return;
}
if(!ENGINEsetdefaultRSA(e))
/* This should only happen when 'e' can't initialise, but the previous
* statement suggests it did. */
abort();
ENGINEsetdefaultDSA(e);
ENGINEsetdefaultciphers(e);
/* Release the functional reference from ENGINEinit() */
ENGINEfinish(e);
/* Release the structural reference from ENGINEbyid() */
ENGINEfree(e);
Automatically using builtin ENGINE implementations
Here we'll assume we want to load and register all ENGINE implementa-
tions bundled with OpenSL, such that for any cryptographic algorithm
required by OpenSL - if there is an ENGINE that implements it and can
be initialise, it should be used. The following code illustrates how
this can work;
/* Load all bundled ENGINEs into memory and make them visible */
ENGINEloadbuiltinengines();
/* Register all of them for every algorithm they collectively implement */
ENGINEregisterallcomplete();
That's all that's required. Eg. the next time OpenSL tries to set up
an RSA key, any bundled ENGINEs that implement RSAMETHOD will be
passed to ENGINEinit() and if any of those succeed, that ENGINE will
be set as the default for use with RSA from then on.
Advanced configuration support
There is a mechanism supported by the ENGINE framework that allows each
ENGINE implementation to define an arbitrary set of configuration "com-
mands" and expose them to OpenSL and any applications based on
OpenSL. This mechanism is entirely based on the use of name-value
pairs and and assumes ASCI input (no unicode or UTF for now!), so it
is ideal if applications want to provide a transparent way for users to
provide arbitrary configuration "directives" directly to such ENGINEs.
It is also possible for the application to dynamically interrogate the
loaded ENGINE implementations for the names, descriptions, and input
flags of their available "control commands", providing a more flexible
configuration scheme. However, if the user is expected to know which
ENGINE device he/she is using (in the case of specialised hardware,
this goes without saying) then applications may not need to concern
themselves with discovering the supported control commands and simply
prefer to allow settings to passed into ENGINEs exactly as they are
provided by the user.
Before illustrating how control commands work, it is worth mentioning
what they are typically used for. Broadly speaking there are two uses
for control commands; the first is to provide the necessary details to
the implementation (which may know nothing at all specific to the host
system) so that it can be initialised for use. This could include the
path to any driver or config files it needs to load, required network
addresses, smart-card identifiers, passwords to initialise password-
protected devices, logging information, etc etc. This class of commands
typically needs to be passed to an ENGINE before attempting to ini-
tialise it, ie. before calling ENGINEinit(). The other class of com-
mands consist of settings or operations that tweak certain behaviour or
cause certain operations to take place, and these commands may work
either before or after ENGINEinit(), or in same cases both. ENGINE
implementations should provide indications of this in the descriptions
attached to builtin control commands and/or in external product docu-
mentation.
Issuing control commands to an ENGINE
Let's illustrate by example; a function for which the caller supplies
the name of the ENGINE it wishes to use, a table of string-pairs for
use before initialisation, and another table for use after initialisa-
tion. Note that the string-pairs used for control commands consist of a
command "name" followed by the command "parameter" - the parameter
could be NUL in some cases but the name can not. This function should
initialise the ENGINE (issuing the "pre" commands beforehand and the
"post" commands afterwards) and set it as the default for everything
except RAND and then return a boolean success or failure.
int genericloadenginefn(const char *engineid,
const char **precmds, int prenum,
const char **postcmds, int postnum)
{
ENGINE *e = ENGINEbyid(engineid);
if(!e) return 0;
while(prenum--) {
if(!ENGINEctrlcmdstring(e, precmds[0], precmds[1], 0)) {
fprintf(stderr, "Failed command (%s - %s:%s)\n", engineid,
precmds[0], precmds[1] ? precmds[1] : "(NUL)");
ENGINEfree(e);
return 0;
}
precmds ]= 2;
}
if(!ENGINEinit(e)) {
fprintf(stderr, "Failed initialisation\n");
ENGINEfree(e);
return 0;
}
/* ENGINEinit() returned a functional reference, so free the structural
* reference from ENGINEbyid(). */
ENGINEfree(e);
while(postnum--) {
if(!ENGINEctrlcmdstring(e, postcmds[0], postcmds[1], 0)) {
fprintf(stderr, "Failed command (%s - %s:%s)\n", engineid,
postcmds[0], postcmds[1] ? postcmds[1] : "(NUL)");
ENGINEfinish(e);
return 0;
}
postcmds ]= 2;
}
ENGINEsetdefault(e, ENGINEMETHODAL & ~ENGINEMETHODRAND);
/* Success */
return 1;
}
Note that ENGINEctrlcmdstring() accepts a boolean argument that can
relax the semantics of the function - if set non-zero it will only
return failure if the ENGINE supported the given command name but
failed while executing it, if the ENGINE doesn't support the command
name it will simply return success without doing anything. In this case
we assume the user is only supplying commands specific to the given
ENGINE so we set this to FALSE.
Discovering supported control commands
It is possible to discover at run-time the names, numerical-ids,
descriptions and input parameters of the control commands supported
from a structural reference to any ENGINE. It is first important to
note that some control commands are defined by OpenSL itself and it
will intercept and handle these control commands on behalf of the
ENGINE, ie. the ENGINE's ctrl() handler is not used for the control
command. openssl/engine.h defines a symbol, ENGINECMDBASE, that all
control commands implemented by ENGINEs from. Any command value lower
than this symbol is considered a "generic" command is handled directly
by the OpenSL core routines.
It is using these "core" control commands that one can discover the the
control commands implemented by a given ENGINE, specifically the com-
mands;
#define ENGINEHASCTRLFUNCTION 10
#define ENGINECTRLGETFIRSTCMDTYPE 11
#define ENGINECTRLGETNEXTCMDTYPE 12
#define ENGINECTRLGETCMDFROMNAME 13
#define ENGINECTRLGETNAMELENFROMCMD 14
#define ENGINECTRLGETNAMEFROMCMD 15
#define ENGINECTRLGETDESCLENFROMCMD 16
#define ENGINECTRLGETDESCFROMCMD 17
#define ENGINECTRLGETCMDFLAGS 18
Whilst these commands are automatically processed by the OpenSL frame-
work code, they use various properties exposed by each ENGINE by which
to process these queries. An ENGINE has 3 properties it exposes that
can affect this behaviour; it can supply a ctrl() handler, it can spec-
ify ENGINEFLAGSMANUALCMDCTRL in the ENGINE's flags, and it can
expose an array of control command descriptions. If an ENGINE speci-
fies the ENGINEFLAGSMANUALCMDCTRL flag, then it will simply pass
all these "core" control commands directly to the ENGINE's ctrl() han-
dler (and thus, it must have supplied one), so it is up to the ENGINE
to reply to these "discovery" commands itself. If that flag is not set,
then the OpenSL framework code will work with the following rules;
if no ctrl() handler supplied;
ENGINEHASCTRLFUNCTION returns FALSE (zero),
all other commands fail.
if a ctrl() handler was supplied but no array of control commands;
ENGINEHASCTRLFUNCTION returns TRUE,
all other commands fail.
if a ctrl() handler and array of control commands was supplied;
ENGINEHASCTRLFUNCTION returns TRUE,
all other commands proceed processing ...
If the ENGINE's array of control commands is empty then all other com-
mands will fail, otherwise; ENGINECTRLGETFIRSTCMDTYPE returns the
identifier of the first command supported by the ENGINE,
ENGINEGETNEXTCMDTYPE takes the identifier of a command supported by
the ENGINE and returns the next command identifier or fails if there
are no more, ENGINECMDFROMNAME takes a string name for a command and
returns the corresponding identifier or fails if no such command name
exists, and the remaining commands take a command identifier and return
properties of the corresponding commands. All except
ENGINECTRLGETFLAGS return the string length of a command name or
description, or populate a supplied character buffer with a copy of the
command name or description. ENGINECTRLGETFLAGS returns a bit-
wise-OR'd mask of the following possible values;
#define ENGINECMDFLAGNUMERIC (unsigned int)0x0001
#define ENGINECMDFLAGSTRING (unsigned int)0x0002
#define ENGINECMDFLAGNOINPUT (unsigned int)0x0004
#define ENGINECMDFLAGINTERNAL (unsigned int)0x0008
If the ENGINECMDFLAGINTERNAL flag is set, then any other flags are
purely informational to the caller - this flag will prevent the command
being usable for any higher-level ENGINE functions such as
ENGINEctrlcmdstring(). "INTERNAL" commands are not intended to be
exposed to text-based configuration by applications, administrations,
users, etc. These can support arbitrary operations via ENGINEctrl(),
including passing to and/or from the control commands data of any arbi-
trary type. These commands are supported in the discovery mechanisms
simply to allow applications determinie if an ENGINE supports certain
specific commands it might want to use (eg. application "foo" might
query various ENGINEs to see if they implement "FOGETVEN-
DORLOGOGIF" - and ENGINE could therefore decide whether or not to
support this "foo"-specific extension).
Future developments
The ENGINE API and internal architecture is currently being reviewed.
Slated for possible release in 0.9.8 is support for transparent loading
of "dynamic" ENGINEs (built as self-contained shared-libraries). This
would allow ENGINE implementations to be provided independantly of
OpenSL libraries and/or OpenSL-based applications, and would also
remove any requirement for applications to explicitly use the "dynamic"
ENGINE to bind to shared-library implementations.
SEE ALSO
rsa(3), dsa(3), dh(3), rand(3), RSAnewmethod(3)
0.9.7l 2002-12-15 engine(3)
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