Add example of loading at runtime from C (#248)

* Add example of loading at runtime from C

* Try windows

* Silence windows warnings

* Typo fix

* README tweaks

.github/workflows/main.yaml

@@ -8,7 +8,7 @@ on:
   workflow_dispatch: {}
 
 env:
-  CACHE_VERSION: 0
+  CACHE_VERSION: 1
 
 # only run one copy per PR
 concurrency:
@@ -37,17 +37,39 @@ jobs:
           path: ./stan/
           key: ${{ runner.os }}-stan-${{ hashFiles('stan/src/stan/version.hpp') }}-v${{ env.CACHE_VERSION }}
 
+
+      - name: Set up PATH for C example
+        if: matrix.os == 'windows-latest'
+        run: |
+          Add-Content $env:GITHUB_PATH "$(pwd)/stan/lib/stan_math/lib/tbb"
+          Add-Content $env:GITHUB_PATH "$(pwd)/test_models/full"
+
+
+      - name: Build C example (Windows)
+        if: matrix.os == 'windows-latest'
+        run: |
+          cd c-example/
+          make -j4 example.exe example_runtime.exe
+          rm ../src/bridgestan.o
+
+          echo "Dynamically linked example"
+          ./example.exe
+          echo "Runtime loading example"
+          ./example_runtime.exe ../test_models/full/full_model.so
+
       - name: Build C example (Unix)
         if: matrix.os != 'windows-latest'
         run: |
           cd c-example/
-          make example -j4
-          make example_static
-          rm ../src/bridgestan.o
-          rm ../test_models/full/full_model.a
+          make -j4 example example_static example_runtime
+          rm ../src/bridgestan.o ../test_models/full/*.a
 
+          echo "Dynamically linked example"
           ./example
+          echo "Statically linked example"
           ./example_static
+          echo "Runtime loading example"
+          ./example_runtime ../test_models/full/full_model.so
         shell: bash
 
       # we use the cache here to build the Stan models once for multiple interfaces

.gitignore

@@ -7,6 +7,7 @@
 *.a
 c-example/example
 c-example/example_static
+c-example/example_runtime
 *.exe
 make/local
 

c-example/Makefile

@@ -3,12 +3,18 @@ BS_ROOT=..
 include ../Makefile
 
 MODEL?=full
-CC ?= gcc
+CC = gcc
+# .dll on Windows, .so on Linux
+ifeq ($(OS_TAG),windows)
+	DLL = dll
+else
+	DLL = so
+endif
 
-../test_models/$(MODEL)/lib$(MODEL)_model.so: ../test_models/$(MODEL)/$(MODEL)_model.so
-	cp ../test_models/$(MODEL)/$(MODEL)_model.so ../test_models/$(MODEL)/lib$(MODEL)_model.so
+../test_models/$(MODEL)/lib$(MODEL)_model.$(DLL): ../test_models/$(MODEL)/$(MODEL)_model.so
+	cp ../test_models/$(MODEL)/$(MODEL)_model.so ../test_models/$(MODEL)/lib$(MODEL)_model.$(DLL)
 
-example$(EXE): example.c ../test_models/$(MODEL)/lib$(MODEL)_model.so
+example$(EXE): example.c ../test_models/$(MODEL)/lib$(MODEL)_model.$(DLL)
 	$(CC) -c -I ../src example.c -o example.o
 	$(LINK.c) -o example$(EXE) example.o -Wl,-rpath ../test_models/$(MODEL) -L ../test_models/$(MODEL) -l$(MODEL)_model
 	$(RM) example.o
@@ -28,3 +34,6 @@ example_static$(EXE):  example.c ../test_models/$(MODEL)/$(MODEL)_model.a
 	$(CC) -c -I ../src example.c -o example.o
 	$(LINK.cpp) -o example_static$(EXE) example.o ../test_models/$(MODEL)/$(MODEL)_model.a $(LDLIBS) $(LIBSUNDIALS) $(MPI_TARGETS) $(TBB_TARGETS)
 	$(RM) example.o
+
+example_runtime$(EXE): runtime_loading.c
+	$(CC) -I ../src runtime_loading.c -o example_runtime$(EXE)

c-example/README.md

@@ -7,7 +7,9 @@ This shows how one could write a C program which calls BridgeStan.
 Any compiled language with a C foreign function interface and
 the ability to link against C libraries should be able to work similarly.
 
-## Usage with dynamic linking
+## Binding a specific model at build time
+
+### Dynamic linking
 
 It is possible to link against the same `name_model.so` object used by the other
 BridgeStan interfaces. This creates a dynamic link.
@@ -26,17 +28,20 @@ It has 1 parameters.
 
 You can change the test model by specifying `MODEL` on the command line.
 Models which require data can have a path passed in as the first argument.
+
 ```shell
 make MODEL=multi example
 ./example ../test_models/multi/multi.data.json
 ```
+
 This will output
+
 ```
 This model's name is multi_model.
 It has 10 parameters.
 ```
 
-### Notes
+#### Notes
 
 The basic steps for using with a generic BridgeStan model are
 
@@ -48,13 +53,13 @@ The basic steps for using with a generic BridgeStan model are
    The Makefile in this folder does that by making a copy.
 
 This dynamic linking will work on Windows, but Windows does not record the paths
-of shared libraries in executables. As such, `libNAME_model.so` will need to be
+of shared libraries in executables. As such, `libNAME_model.dll` will need to be
 in the same folder as the executable, or on your `PATH`.
 
-On all platforms, dynamic linking requires that the original `NAME_model.so` object
+On all platforms, dynamic linking requires that the original `libNAME_model.(so|dll)` object
 still exist when the executable is run.
 
-## Usage with static linking
+### Static linking
 
 The makefile here also shows how to create a `.a` static library using the BridgeStan
 source, and then compiling an executable which is independent of the location of the model.
@@ -70,3 +75,27 @@ Will output the same as the above. Note that some Stan libraries such as TBB
 are still dynamically linked.
 
 `MODEL` can also be used to specify which model to statically link.
+
+## Loading a model at runtime
+
+The `runtime_loading.c` file shows how to use `dlfcn.h` on Unix and
+`libloaderapi.h` on Windows to load a model at runtime.
+This is useful if you want to load a model based on user input, or if you want to
+load different models in the same executable.
+
+```shell
+make example_runtime
+# unlike above, this did not automatically build a model, since it
+# was not needed to _build_, but we still need one to _run_ the program
+make ../test_models/full/full_model.so
+./example_runtime ../test_models/full/full_model.so
+```
+
+will output
+
+```
+This model's name is full_model.
+It has 1 parameters.
+```
+
+The same executable can be passed different models without recompiling.

c-example/runtime_loading.c

@@ -0,0 +1,90 @@
+#include "bridgestan.h"
+#include <stdio.h>
+
+#ifdef _WIN32
+// hacky way to get dlopen and friends on Windows
+
+#include <libloaderapi.h>
+#include <errhandlingapi.h>
+#define dlopen(lib, flags) LoadLibraryA(lib)
+#define dlsym(handle, sym) (void*)GetProcAddress(handle, sym)
+
+char* dlerror() {
+  DWORD err = GetLastError();
+  int length = snprintf(NULL, 0, "%d", err);
+  char* str = malloc(length + 1);
+  snprintf(str, length + 1, "%d", err);
+  return str;
+}
+#else
+#include <dlfcn.h>
+#endif
+
+#if __STDC_VERSION__ < 202000
+#define typeof __typeof__
+#endif
+
+int main(int argc, char** argv) {
+  char* lib;
+  char* data;
+
+  // require at least the library name
+  if (argc > 2) {
+    lib = argv[1];
+    data = argv[2];
+  } else if (argc > 1) {
+    lib = argv[1];
+    data = NULL;
+  } else {
+    fprintf(stderr, "Usage: %s <library> [data]\n", argv[0]);
+    return 1;
+  }
+
+  // load the shared library
+  void* handle = dlopen(lib, RTLD_LAZY);
+  if (!handle) {
+    fprintf(stderr, "Error: %s\n", dlerror());
+    return 1;
+  }
+
+  int major = *(int*)dlsym(handle, "bs_major_version");
+  int minor = *(int*)dlsym(handle, "bs_minor_version");
+  int patch = *(int*)dlsym(handle, "bs_patch_version");
+  fprintf(stderr, "Using BridgeStan version %d.%d.%d\n", major, minor, patch);
+
+  // Get function pointers. Uses C23's typeof to re-use bridgestan.h
+  // definitions. We could also write out the types and not include bridgestan.h
+  typeof(&bs_model_construct) bs_model_construct
+      = dlsym(handle, "bs_model_construct");
+  typeof(&bs_free_error_msg) bs_free_error_msg
+      = dlsym(handle, "bs_free_error_msg");
+  typeof(&bs_model_destruct) bs_model_destruct
+      = dlsym(handle, "bs_model_destruct");
+  typeof(&bs_name) bs_name = dlsym(handle, "bs_name");
+  typeof(&bs_param_num) bs_param_num = dlsym(handle, "bs_param_num");
+
+  if (!bs_model_construct || !bs_free_error_msg || !bs_model_destruct
+      || !bs_name || !bs_param_num) {
+    fprintf(stderr, "Error: %s\n", dlerror());
+    return 1;
+  }
+
+  // from here on, the code is exactly the same as example.c
+
+  // this could potentially error, and we may get information back about why.
+  char* err;
+  bs_model* model = bs_model_construct(data, 123, &err);
+  if (!model) {
+    if (err) {
+      printf("Error: %s", err);
+      bs_free_error_msg(err);
+    }
+    return 1;
+  }
+
+  printf("This model's name is %s.\n", bs_name(model));
+  printf("It has %d parameters.\n", bs_param_num(model, 0, 0));
+
+  bs_model_destruct(model);
+  return 0;
+}

docs/languages/c-api.rst

@@ -13,9 +13,12 @@ BridgeStan's pre-requisites and downloaded a copy of the BridgeStan source code.
 Example Program
 ---------------
 
-An example program is provided alongside the BridgeStan source in :file:`c-example/`.
+Two example programs are provided alongside the BridgeStan source in :file:`c-example/`.
 Details for building the example can be found in :file:`c-example/Makefile`.
 
+The first assumes you wish to link a specific model into the program,
+and the second demonstrates how to load a model at runtime
+
 .. raw:: html
 
    <details>
@@ -30,6 +33,20 @@ Details for building the example can be found in :file:`c-example/Makefile`.
    </details>
 
 
+.. raw:: html
+
+   <details>
+   <summary><a>Show runtime_loading.c</a></summary>
+
+
+.. literalinclude:: ../../c-example/runtime_loading.c
+   :language: c
+
+.. raw:: html
+
+   </details>
+
+
 API Reference
 -------------