OpenSL engine(3openssl)
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);
#ifndef OPENSLNOSTATICENGINE
void ENGINEload4758cca(void);
void ENGINEloadaep(void);
void ENGINEloadatalla(void);
void ENGINEloadchil(void);
void ENGINEloadcswift(void);
void ENGINEloadgmp(void);
void ENGINEloadnuron(void);
void ENGINEloadsureware(void);
void ENGINEloadubsec(void);
#endif
void ENGINEloadcryptodev(void);
void ENGINEloadbuiltinengines(void);
void ENGINEcleanup(void);
ENGINE *ENGINEgetdefaultRSA(void);
ENGINE *ENGINEgetdefaultDSA(void);
ENGINE *ENGINEgetdefaultECDH(void);
ENGINE *ENGINEgetdefaultECDSA(void);
ENGINE *ENGINEgetdefaultDH(void);
ENGINE *ENGINEgetdefaultRAND(void);
ENGINE *ENGINEgetcipherengine(int nid);
ENGINE *ENGINEgetdigestengine(int nid);
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OpenSL engine(3openssl)
int ENGINEsetdefaultRSA(ENGINE *e);
int ENGINEsetdefaultDSA(ENGINE *e);
int ENGINEsetdefaultECDH(ENGINE *e);
int ENGINEsetdefaultECDSA(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 ENGINEregisterECDH(ENGINE *e);
void ENGINEunregisterECDH(ENGINE *e);
void ENGINEregisterallECDH(void);
int ENGINEregisterECDSA(ENGINE *e);
void ENGINEunregisterECDSA(ENGINE *e);
void ENGINEregisterallECDSA(void);
int ENGINEregisterDH(ENGINE *e);
void ENGINEunregisterDH(ENGINE *e);
void ENGINEregisterallDH(void);
int ENGINEregisterRAND(ENGINE *e);
void ENGINEunregisterRAND(ENGINE *e);
void ENGINEregisterallRAND(void);
int ENGINEregisterSTORE(ENGINE *e);
void ENGINEunregisterSTORE(ENGINE *e);
void ENGINEregisterallSTORE(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)(void));
int ENGINEcmdisexecutable(ENGINE *e, int cmd);
int ENGINEctrlcmd(ENGINE *e, const char *cmdname,
long i, void *p, void (*f)(void), int cmdoptional);
int ENGINEctrlcmdstring(ENGINE *e, const char *cmdname, const char *arg,
int cmdoptional);
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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 ENGINEupref(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 ENGINEsetECDH(ENGINE *e, const ECDHMETHOD *dhmeth);
int ENGINEsetECDSA(ENGINE *e, const ECDSAMETHOD *dhmeth);
int ENGINEsetDH(ENGINE *e, const DHMETHOD *dhmeth);
int ENGINEsetRAND(ENGINE *e, const RANDMETHOD *randmeth);
int ENGINEsetSTORE(ENGINE *e, const STOREMETHOD *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 ECDHMETHOD *ENGINEgetECDH(const ENGINE *e);
const ECDSAMETHOD *ENGINEgetECDSA(const ENGINE *e);
const DHMETHOD *ENGINEgetDH(const ENGINE *e);
const RANDMETHOD *ENGINEgetRAND(const ENGINE *e);
const STOREMETHOD *ENGINEgetSTORE(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);
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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 implementations of cryptographic algorithms,
and support a reference-counted mechanism to allow them to
be dynamically loaded in and out of the running 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, ECDHMETHOD, ECDSAMETHOD,
STOREMETHOD - similarly for other OpenSL APIs
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 references to the underlying ENGINE
object. Ie. one should obtain a new reference 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 - a structural reference is required to use the
pointer value at all, as this kind of reference is a
guarantee that the structure can not be deallocated until
the reference is released.
However, a structural reference provides no guarantee that
the ENGINE is initiliased and able to use any of its
cryptographic implementations. Indeed it's quite possible
that most ENGINEs will not initialise at all in typical
environments, 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
considered a specialised form of structural reference,
because each functional reference implicitly contains a
structural reference as well - however to avoid difficult-
to-find programming bugs, it is recommended to treat the two
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OpenSL engine(3openssl)
kinds of reference independantly. If you have a functional
reference to an ENGINE, you have a guarantee that the ENGINE
has been initialised ready to perform cryptographic
operations and will remain uninitialised until after you
have released your reference.
Structural references
This basic type of reference is used for instantiating new
ENGINEs, iterating across OpenSL's internal linked-list of
loaded ENGINEs, reading information about an ENGINE, etc.
Essentially a structural 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. There are other ENGINE API
functions that return structural references 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 - typically 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 reference 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
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As mentioned, functional references exist when the
cryptographic functionality of an ENGINE is required to be
available. A functional reference 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. All functional references are released by calling
ENGINEfinish() (which removes the implicit structural
reference as well).
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 implementations are registered in the
tables and indexed by an 'nid' value, because abstractions
like EVPCIPHER and EVPDIGEST support many distinct
algorithms and modes, and ENGINEs can support arbitrarily
many of them. 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.
When a default ENGINE is requested for a given
abstraction/algorithm/mode, (eg. when calling
RSAnewmethod(NUL)), a "getdefault" call will be made to
the ENGINE subsystem to process the corresponding state
table and return a functional reference to an initialised
ENGINE whose implementation should be used. If no ENGINE
should (or can) be used, it will return NUL and the caller
will operate with a NUL ENGINE handle - this usually
equates to using the conventional software implementation.
In the latter case, OpenSL will from then on behave the way
it used to before the ENGINE API existed.
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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
initialisations 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.
Application requirements
This section will explain the basic things an application
programmer should support to make the most useful elements
of the ENGINE functionality 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
implementations 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
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implementations and linked into OpenSL's internal linked
list. At this point it is important 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 functionality, however if any
ENGINEs are loaded, even if they are never registered or
used, it is necessary to use the ENGINEcleanup() function
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 functionality 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 behaviour is something
for the application to control. 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 applications may
prefer to handle things, so we will simply illustrate the
consequences as they apply to a couple of simple cases and
leave developers to consider these and the source code to
openssl's builtin utilities 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 version 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;
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OpenSL engine(3openssl)
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
implementations 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
RSA use 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 "commands" and expose them to OpenSL and
any applications based on OpenSL. This mechanism is
entirely based on the use of name-value pairs 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
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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 pass settings 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 protected devices, logging
information, etc etc. This class of commands typically needs
to be passed to an ENGINE before attempting to initialise
it, ie. before calling ENGINEinit(). The other class of
commands 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 some cases both. ENGINE implementations
should provide indications of this in the descriptions
attached to builtin control commands and/or in external
product documentation.
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 initialisation. 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.
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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 by an ENGINE using a structural
reference. Note that some control commands are defined by
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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 an index, ENGINECMDBASE, that all
control commands implemented by ENGINEs should be numbered
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 commands;
#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 framework code, they use various properties exposed
by each ENGINE to process these queries. An ENGINE has 3
properties it exposes that can affect how this behaves; it
can supply a ctrl() handler, it can specify
ENGINEFLAGSMANUALCMDCTRL in the ENGINE's flags, and it
can expose an array of control command descriptions. If an
ENGINE specifies the ENGINEFLAGSMANUALCMDCTRL flag, then
it will simply pass all these "core" control commands
directly to the ENGINE's ctrl() handler (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 commands will fail, otherwise;
ENGINECTRLGETFIRSTCMDTYPE returns the identifier of the
first command supported by the ENGINE,
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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
bitwise-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 arbitrary 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
"FOGETVENDORLOGOGIF" - 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)
OpenSL-0.9.8 Last change: Oct 11 2005 13
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