Hide the loadable extension's internal symbols

Makefile

@@ -95,14 +95,15 @@ $(BUILD_DIR): $(prefix)
 $(TARGET_LOADABLE): sqlite-vec.c sqlite-vec.h $(prefix)
 	$(CC) \
 		-fPIC -shared \
+		-fvisibility=hidden \
 		-Wall -Wextra \
 		-Ivendor/ \
 		-O3 \
 		$(CFLAGS) \
 		$< -o $@
 
 $(TARGET_STATIC): sqlite-vec.c sqlite-vec.h $(prefix) $(OBJS_DIR)
-	$(CC) -Ivendor/ $(CFLAGS) -DSQLITE_CORE -DSQLITE_VEC_STATIC \
+	$(CC) -Ivendor/ -fvisibility=hidden $(CFLAGS) -DSQLITE_CORE -DSQLITE_VEC_STATIC \
 	-O3 -c  $< -o $(OBJS_DIR)/vec.o
 	$(AR) rcs $@ $(OBJS_DIR)/vec.o
 
@@ -130,14 +131,15 @@ $(LIBS_DIR)/shell.a: $(OBJS_DIR)/shell.o $(LIBS_DIR)
 	$(AR) rcs $@ $<
 
 $(OBJS_DIR)/sqlite-vec.o: sqlite-vec.c $(OBJS_DIR)
-	$(CC) -c -g3 -Ivendor/ -I./ $(CFLAGS) $< -o $@
+	$(CC) -c -g3 -fvisibility=hidden -Ivendor/ -I./ $(CFLAGS) $< -o $@
 
 $(LIBS_DIR)/sqlite-vec.a: $(OBJS_DIR)/sqlite-vec.o $(LIBS_DIR)
 	$(AR) rcs $@ $<
 
 
 $(TARGET_CLI): sqlite-vec.h $(LIBS_DIR)/sqlite-vec.a $(LIBS_DIR)/shell.a $(LIBS_DIR)/sqlite3.a examples/sqlite3-cli/core_init.c $(prefix)
 	$(CC) -g3  \
+	-fvisibility=hidden \
 	-Ivendor/ -I./ \
 	-DSQLITE_CORE \
 	-DSQLITE_VEC_STATIC \

site/compiling.md

@@ -37,10 +37,10 @@ There will now be `sqlite-vec.c` and `sqlite-vec.h` available in your current di
 
 ```bash
 # Linux 
-gcc -g -fPIC -shared sqlite-vec.c -o vec0.so
+gcc -g -fPIC -fvisibility=hidden -shared sqlite-vec.c -o vec0.so
 
 # MacOS
-gcc -g -fPIC -dynamiclib sqlite-vec.c -o vec0.dylib
+gcc -g -fPIC -fvisibility=hidden -dynamiclib sqlite-vec.c -o vec0.dylib
 
 # Windows, MSVC compiler
 cl sqlite-vec.c -link -dll -out:sqlite-vec.dll
@@ -49,6 +49,8 @@ cl sqlite-vec.c -link -dll -out:sqlite-vec.dll
 gcc -g -shared sqlite-vec.c -o vec0.dll
 ```
 
+`-fvisibility=hidden` keeps the library from exporting any symbol but its entry point, `sqlite3_vec_init`. SQLite loads extensions with `RTLD_GLOBAL`, so an exported internal function can otherwise be called in place of a same-named function of another library in the process, another build of `sqlite-vec` included.
+
 Different platforms, compiler, or architectures may require different compilation flags.
 
 ## Compile-time options

sqlite-vec.h.tmpl

@@ -7,13 +7,19 @@
 #include "sqlite3.h"
 #endif
 
+// The Makefile compiles with -fvisibility=hidden, so the entry point is the
+// one symbol a shared library built from sqlite-vec exports.
 #ifdef SQLITE_VEC_STATIC
-  #define SQLITE_VEC_API
+  #if defined(__GNUC__) && !defined(_WIN32)
+    #define SQLITE_VEC_API __attribute__((visibility("default")))
+  #else
+    #define SQLITE_VEC_API
+  #endif
 #else
   #ifdef _WIN32
     #define SQLITE_VEC_API __declspec(dllexport)
   #else
-    #define SQLITE_VEC_API
+    #define SQLITE_VEC_API __attribute__((visibility("default")))
   #endif
 #endif
 

tests/test-loadable.py

@@ -1,6 +1,9 @@
 # ruff: noqa: E731
 
+import ctypes
+import os
 import re
+import sys
 from typing import List
 import sqlite3
 import unittest
@@ -153,6 +156,17 @@ def test_modules():
     assert modules == MODULES
 
 
+def test_exports_only_entry_point():
+    # SQLite opens an extension with RTLD_GLOBAL, so calls inside one library
+    # can bind to another library's exported function of the same name, as they
+    # do between two builds of sqlite-vec loaded into one process.
+    suffix = {"darwin": ".dylib", "win32": ".dll"}.get(sys.platform, ".so")
+    lib = ctypes.CDLL(os.path.abspath(EXT_PATH + suffix))
+    assert hasattr(lib, "sqlite3_vec_init")
+    for name in ["sqlite3_api", "bitmap_new", "vec0Filter_fullscan"]:
+        assert not hasattr(lib, name), name
+
+
 def test_vec_version():
     vec_version = lambda *args: db.execute("select vec_version()", args).fetchone()[0]
     assert vec_version()[0] == "v"