Add global variable example (#51)

Signed-off-by: Alan Jowett <[email protected]>

CMakeLists.txt

@@ -25,6 +25,7 @@ add_custom_target(samples ALL
                           ${CMAKE_CURRENT_SOURCE_DIR}/build/dependent_read.o
                           ${CMAKE_CURRENT_SOURCE_DIR}/build/exposeptr.o
                           ${CMAKE_CURRENT_SOURCE_DIR}/build/exposeptr2.o
+                          ${CMAKE_CURRENT_SOURCE_DIR}/build/global_variable.o
                           ${CMAKE_CURRENT_SOURCE_DIR}/build/loop.o
                           ${CMAKE_CURRENT_SOURCE_DIR}/build/mapoverflow.o
                           ${CMAKE_CURRENT_SOURCE_DIR}/build/mapunderflow.o
@@ -93,6 +94,10 @@ add_custom_command(OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/build/exposeptr2.o
                    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/src/exposeptr2.c
                    COMMAND clang -g -target bpf -Wall -O2 -I ${libbpf_SOURCE_DIR}/src -c ${CMAKE_CURRENT_SOURCE_DIR}/src/exposeptr2.c -o ${CMAKE_CURRENT_SOURCE_DIR}/build/exposeptr2.o)
 
+add_custom_command(OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/build/global_variable.o
+                   DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/src/global_variable.c
+                   COMMAND clang -g -target bpf -Wall -O2 -I ${libbpf_SOURCE_DIR}/src -c ${CMAKE_CURRENT_SOURCE_DIR}/src/global_variable.c -o ${CMAKE_CURRENT_SOURCE_DIR}/build/global_variable.o)
+
 add_custom_command(OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/build/loop.o
                    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/src/loop.c
                    COMMAND clang -g -target bpf -Wall -O2 -I ${libbpf_SOURCE_DIR}/src -c ${CMAKE_CURRENT_SOURCE_DIR}/src/loop.c -o ${CMAKE_CURRENT_SOURCE_DIR}/build/loop.o)

src/global_variable.c

@@ -0,0 +1,19 @@
+// Copyright (c) Prevail Verifier contributors.
+// SPDX-License-Identifier: MIT
+
+#include "bpf.h"
+
+// This sample demonstrates how to use global variables in BPF programs.
+// Global variables are shared among all threads in the same BPF program and
+// can be used to store global states. They must be declared as `volatile`
+// to prevent the compiler from optimizing them away and must accessed
+// using atomic operations to avoid race conditions.
+
+static volatile uint32_t global_var = 0;
+static volatile uint32_t global_var_2 = 0;
+
+int func(void* ctx) {
+    __sync_fetch_and_add(&global_var, 1);
+    __sync_fetch_and_add(&global_var_2, 2);
+    return 0;
+}