compat: add macro to map symbol in static build

It was pointed out in my examples that doing shared library symbol versioning by
partitioning symbols to version specific functions (as opposed to leaving the
latest symol version at the base symbol name), neglects to take into account
static builds.  Add a macro to handle that.  If you choose a versioning approach
that uniquely names every version of the symbol, then this macro lets you map
your symbol choice to the base name when building a static library

Also, while I'm at it, since we're documenting this in the guide, take the
abbreviated example out of the header

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
[Thomas: remove remaining part of old example]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>

diff --git a/lib/librte_compat/rte_compat.h b/lib/librte_compat/rte_compat.h
index 45ab88f..929a918 100644 (file)
--- a/lib/librte_compat/rte_compat.h
+++ b/lib/librte_compat/rte_compat.h
@@ -45,26 +45,10 @@
  * VERSION_SYMBOL macros are created.  They, in conjunction with the
  * <library>_version.map file for a given library allow for multiple versions of
  * a symbol to exist in a shared library so that older binaries need not be
- * immediately recompiled. Their use is outlined in the following example:
- * Assumptions: DPDK 2.(X) contains a function int foo(char *string)
- *              DPDK 2.(X+1) needs to change foo to be int foo(int index)
- *
- * To accomplish this:
- * 1) Edit lib/<library>/library_version.map to add a DPDK_2.(X+1) node, in which
- * foo is exported as a global symbol.
- *
- * 2) rename the existing function int foo(char *string) to
- *     int foo_v20(char *string)
- *
- * 3) Add this macro immediately below the function
- *     VERSION_SYMBOL(foo, _v20, 2.0);
- *
- * 4) Implement a new version of foo.
- *     char foo(int value, int otherval) { ...}
- *
- * 5) Mark the newest version as the default version
- *     BIND_DEFAULT_SYMBOL(foo, _v21, 2.1);
+ * immediately recompiled.
  *
+ * Refer to the guidelines document in the docs subdirectory for details on the
+ * use of these macros
  */
 
 /*
@@ -72,6 +56,8 @@
  * b - function base name
  * e - function version extension, to be concatenated with base name
  * n - function symbol version string to be applied
+ * f - function prototype
+ * p - full function symbol name
  */
 
 /*
@@ -96,6 +82,19 @@
 #define BIND_DEFAULT_SYMBOL(b, e, n) __asm__(".symver " RTE_STR(b) RTE_STR(e) ", " RTE_STR(b) "@@DPDK_" RTE_STR(n))
 #define __vsym __attribute__((used))
 
+/*
+ * MAP_STATIC_SYMBOL
+ * If a function has been bifurcated into multiple versions, none of which
+ * are defined as the exported symbol name in the map file, this macro can be
+ * used to alias a specific version of the symbol to its exported name.  For
+ * example, if you have 2 versions of a function foo_v1 and foo_v2, where the
+ * former is mapped to foo@DPDK_1 and the latter is mapped to foo@DPDK_2 when
+ * building a shared library, this macro can be used to map either foo_v1 or
+ * foo_v2 to the symbol foo when building a static library, e.g.:
+ * MAP_STATIC_SYMBOL(void foo(), foo_v2);
+ */
+#define MAP_STATIC_SYMBOL(f, p)
+
 #else
 /*
  * No symbol versioning in use
@@ -104,7 +103,7 @@
 #define __vsym
 #define BASE_SYMBOL(b, e)
 #define BIND_DEFAULT_SYMBOL(b, e, n)
-
+#define MAP_STATIC_SYMBOL(f, p) f __attribute__((alias(RTE_STR(p))))
 /*
  * RTE_BUILD_SHARED_LIB=n
  */