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=pod
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=head1 NAME
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OSSL_PARAM - a structure to pass or request object parameters
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=head1 SYNOPSIS
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#include <openssl/core.h>
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typedef struct ossl_param_st OSSL_PARAM;
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struct ossl_param_st {
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const char *key; /* the name of the parameter */
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unsigned char data_type; /* declare what kind of content is in buffer */
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void *buffer; /* value being passed in or out */
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size_t buffer_size; /* buffer size */
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size_t *return_size; /* OPTIONAL: address to content size */
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};
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=head1 DESCRIPTION
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C<OSSL_PARAM> is a type that allows passing arbitrary data for some
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object between two parties that have no or very little shared
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knowledge about their respective internal structures for that object.
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A typical usage example could be an application that wants to set some
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parameters for an object, or wants to find out some parameters of an
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object.
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Arrays of this type can be used for two purposes:
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=over 4
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=item *
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Setting parameters for some object.
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The caller sets up the C<OSSL_PARAM> array and calls some function
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(the I<setter>) that has intimate knowledge about the object that can
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take the data from the C<OSSL_PARAM> array and assign them in a
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suitable form for the internal structure of the object.
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=item *
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Request parameters of some object.
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The caller (the I<requestor>) sets up the C<OSSL_PARAM> array and
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calls some function (the I<responder>) that has intimate knowledge
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about the object, which can take the internal data of the object and
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copy (possibly convert) that to the buffers prepared by the
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I<requestor>.
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=back
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=head2 C<OSSL_PARAM> fields
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=over 4
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=item C<key>
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The identity of the parameter in the form of a string.
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=item C<data_type>
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=for comment It's still debated if this field should be present, or if
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the type should always be implied by how it's used.
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Either way, these data types will have to be passed together with the
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names as an array of OSSL_ITEM, for discovery purposes.
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The C<data_type> is a value that describes the type and organization of
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the data.
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See L</Supported types> below for a description of the types.
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=item C<buffer>
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=item C<buffer_size>
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C<buffer> is a pointer to the memory where the parameter data is (when
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setting parameters) or shall (when requesting parameters) be stored,
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and C<buffer_size> is its size in bytes.
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The organization of the data depends on the parameter type and flag.
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=item C<return_size>
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When an array of C<OSSL_PARAM> is used to request data, the
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I<responder> must set this field to indicate the actual size of the
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parameter data.
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In case the C<buffer_size> is too small for the data, the I<responder>
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must still set this field to indicate the minimum buffer size
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required.
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=back
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B<NOTE:>
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The key names and associated types are defined by the entity that
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offers these parameters, i.e. names for parameters provided by the
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OpenSSL libraries are defined by the libraries, and names for
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parameters provided by providers are defined by those providers,
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except for the pointer form of strings (see data type descriptions
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below).
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Entities that want to set or request parameters need to know what
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those keys are and of what type, any functionality between those two
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entities should remain oblivious and just pass the C<OSSL_PARAM> array
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along.
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=head2 Supported types
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The C<data_type> field can be one of the following types:
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=over 4
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=item C<OSSL_PARAM_INTEGER>
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=item C<OSSL_PARAM_UNSIGNED_INTEGER>
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The parameter data is an integer (signed or unsigned) of arbitrary
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length, organized in native form, i.e. most significant byte first on
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Big-Endian systems, and least significant byte first on Little-Endian
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systems.
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=item C<OSSL_PARAM_REAL>
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=for comment It's still debated if we need this or not.
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The parameter data is a floating point value in native form.
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=item C<OSSL_PARAM_UTF8_STRING>
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The parameter data is a printable string.
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=item C<OSSL_PARAM_OCTET_STRING>
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The parameter data is an arbitrary string of bytes.
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=back
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Additionally, this flag can be added to any type:
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=over 4
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=item C<OSSL_PARAM_POINTER_FLAG>
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With this flag, C<buffer> doesn't point directly at the data, but at a
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pointer that points at the data.
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This can be used to indicate that constant data is or will be passed,
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and there is therefore no need to copy the data that is passed, just
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the pointer to it.
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If an C<OSSL_PARAM> with this flag set is used to set a parameter,
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C<buffer_size> must be set to the size of the data, not the size of
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the pointer to the data.
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If this C<OSSL_PARAM> is used in a parameter request, C<buffer_size>
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is not relevant.
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However, the I<responder> will set C<*return_size> to the size of the
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data (again, not the size of the pointer to the data).
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Note that the use of this flag is B<fragile> and can only be safely
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used for data that remains constant and in a constant location for a
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long enough duration (such as the life-time of the entity that
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offers these parameters).
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=back
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For convenience, these types are provided:
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=over 4
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=item C<OSSL_PARAM_UTF8_STRING_PTR>
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=item C<OSSL_PARAM_OCTET_STRING_PTR>
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These are combinations of C<OSSL_PARAM_UTF8_STRING> as well as
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C<OSSL_PARAM_OCTET_STRING> with C<OSSL_PARAM_POINTER_FLAG>.
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=back
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=head1 NOTES
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Both when setting and requesting parameters, the functions that are
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called will have to decide what is and what is not an error.
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The recommended behaviour is:
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=over 4
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=item *
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Keys that a I<setter> or I<responder> doesn't recognise should simply
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be ignored.
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That in itself isn't an error.
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=item *
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If the keys that a called I<setter> recognises form a consistent
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enough set of data, that call should succeed.
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=item *
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A I<responder> must never change the fields of an C<OSSL_PARAM>, it
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may only change the contents of the buffers that C<buffer> and
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C<return_size> point at.
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=item *
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If the data type for a key that it's associated with is incorrect,
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the called function may return an error.
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The called function may also try to convert the data to a suitable
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form (for example, it's plausible to pass a large number as an octet
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string, so even though a given key is defined as an
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C<OSSL_PARAM_UNSIGNED_INTEGER>, is plausible to pass the value as an
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C<OSSL_PARAM_OCTET_STRING>), but this is in no way mandatory.
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=item *
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If a I<responder> finds that some buffers are too small for the
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requested data, it must set C<*return_size> for each such
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C<OSSL_PARAM> item to the required size, and eventually return an
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error.
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=back
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=begin comment RETURN VALUES doesn't make sense for a manual that only
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describes a type, but document checkers still want that section, and
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to have more than just the section title.
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=head1 RETURN VALUES
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txt
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=end comment
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=head1 EXAMPLES
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A couple of examples to just show how C<OSSL_PARAM> arrays could be
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set up.
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=head3 Example 1
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This example is for setting parameters on some object:
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#include <openssl/core.h>
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const char *foo = "some string";
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size_t foo_l = strlen(foo) + 1;
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const char bar[] = "some other string";
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const OSSL_PARAM set[] = {
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{ "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, foo_l, NULL },
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{ "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), NULL },
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{ NULL, 0, NULL, 0, NULL }
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};
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=head3 Example 2
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This example is for requesting parameters on some object:
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const char *foo = NULL;
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size_t foo_l;
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char bar[1024];
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size_t bar_l;
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const OSSL_PARAM request[] = {
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{ "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, 0 /*irrelevant*/, &foo_l },
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{ "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), &bar_l },
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{ NULL, 0, NULL, 0, NULL }
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};
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A I<responder> that receives this array (as C<params> in this example)
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could fill in the parameters like this:
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/* const OSSL_PARAM *params */
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int i;
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for (i = 0; params[i].key != NULL; i++) {
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if (strcmp(params[i].key, "foo") == 0) {
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*(char **)params[i].buffer = "foo value";
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*params[i].return_size = 10; /* size of "foo value" */
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} else if (strcmp(params[i].key, "bar") == 0) {
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memcpy(params[1].buffer, "bar value", 10);
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*params[1].return_size = 10; /* size of "bar value" */
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}
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/* Ignore stuff we don't know */
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}
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=head1 SEE ALSO
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L<openssl-core.h(7)>
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=head1 HISTORY
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C<OSSL_PARAM> was added in OpenSSL 3.0.
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=head1 COPYRIGHT
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Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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