A CIL Tutorial - Department of Computer Science - ETH Zürich

CHAPTER 6. OVERRIDING FUNCTIONS 48
open Cil
let tut6 (f : file) : unit = ()
6.3 tut6.c
In tut6.c, we demonstrate how to use the dynamic linker to override library functions. In particular,
we wrap calls to pthread mutex lock and pthread mutex unlock. We'll do this by calling dlsym
found in dlfcn.h with the ag RTLD NEXT, which fetches a pointer to the original function we are
overriding. Below are the includes we'll need.
../ciltut-lib/src/tut6.c
# define _GNU_SOURCE // Needed for RTLD_NEXT
# include // For printf
# include // for RTLD_NEXT
# include // for pthread_*
# include // for checked_dlsym
First, we set up function pointers at global scope to point at the original versions of the functions,
which we'll call from inside of our wrappers.
../ciltut-lib/src/tut6.c
static int (*pthread_mutex_lock_orig) (pthread_mutex_t *m) = NULL;
static int (*pthread_mutex_unlock_orig)(pthread_mutex_t *m) = NULL;
We'll also declare a ag so that we can enable and disable lock tracing.
../ciltut-lib/src/tut6.c
static int enable_lock_tracking = 0;
With the pointers to the original functions declared, we can now write the wrappers. The wrappers
have the same name as the original functions. This way, calls to the functions will be routed to these
versions. The rst thing that the wrapper functions do is set up the pointers to the original calls
using checked dlsym. checked dlsym will abort the program if the function named by the string
does not exist. After ensuring that the original functions exist, the wrappers may then execute
whatever actions are needed to implement their purpose, and to call the original functions.