CORE: Add a generic callback function type

This offers a very generic way to define a callback as well as calling
it.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10412)

1 files changed, 15 insertions, 0 deletions

diff --git a/include/openssl/core.h b/include/openssl/core.h
index 74a3bfb91e..bed580c811 100644
--- a/include/openssl/core.h
+++ b/include/openssl/core.h
@@ -187,6 +187,21 @@ extern OSSL_provider_init_fn OSSL_provider_init;
 #  pragma names restore
 # endif
 
+/*
+ * Generic callback function signature.
+ *
+ * The expectation is that any provider function that wants to offer
+ * a callback / hook can do so by taking an argument with this type,
+ * as well as a pointer to caller-specific data.  When calling the
+ * callback, the provider function can populate an OSSL_PARAM array
+ * with data of its choice and pass that in the callback call, along
+ * with the caller data argument.
+ *
+ * libcrypto may use the OSSL_PARAM array to create arguments for an
+ * application callback it knows about.
+ */
+typedef int (OSSL_CALLBACK)(const OSSL_PARAM params[], void *arg);
+
 # ifdef __cplusplus
 }
 # endif