Add test_wrap.c

doc/README.md

@@ -8,8 +8,9 @@ Option 1: `LD_PRELOAD`
 * No need to re-link
 * Works for *all* functions
 * Works only on dynamically linked executables
+* Intercepts all calls (including stack allocations etc.)
 
-Example:
+Example (`preload.c`):
 ```c
 #include <stdlib.h>
 #include <dlfcn.h>
@@ -17,7 +18,7 @@ Example:
 
 void *malloc(size_t size) {
     // before call to malloc
-    void *(* _malloc)(size_t);
+    void *(*_malloc)(size_t);
     if ((_malloc = dlsym(RTLD_NEXT, "malloc")) == NULL) {
         errno = ENOSYS;
         return NULL;
@@ -28,21 +29,30 @@ void *malloc(size_t size) {
 }
 ```
 
+```shell
+# ./main is already compiled and ready
+gcc -shared -fPIC -o preload.so preload.c
+LD_PRELOAD="$(pwd)/preload.so" ./main
+```
+
 
 Option 2: `gcc --wrap`
 ----------------------
 
-* Need to re-link
+* Need to re-link(/-comiple)
 * Relatively simple code:
   * Function name: `__wrap_<symbol>`
   * Call to real function inside wrapper: `__real_<symbol>`
 * Works for *all* functions
 * Works only on dynamically linked executables
+* Intercepts only calls inside the given source file
 
-Example:
+Example (`wrap.c`):
 ```c
 #include <stdlib.h>
 
+extern void *__real_malloc(size_t size);
+
 void *__wrap_malloc(size_t size) {
     // before call to malloc
     void *ret = __real_malloc(size);
@@ -51,6 +61,11 @@ void *__wrap_malloc(size_t size) {
 }
 ```
 
+```shell
+gcc -o main_wrapped main.c wrap.c -Wl,--wrap=malloc
+./main_wrapped
+```
+
 
 Option 3: Linux kernel
 ----------------------