Example 1:

$ cat t.c
#include <unistd.h>
int main() {
  execl("/bin/bash", "/bin/bash", NULL);
}
$ bin/clang -fsanitize=address t.c -o t -g && ./t
dyld: Symbol not found: ___asan_mapping_offset
  Referenced from: /Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
  Expected in: flat namespace
 in /Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
Trace/BPT trap: 5

- i.e. we're still trying to preload the runtime library after exec().

Example 2:

$ cat f.c 
void foo() {}
$ bin/clang f.c -dynamiclib -o f.dylib
==13074== Parsed ASAN_OPTIONS: verbosity=1
==13074== exec()-ing the program with
==13074== DYLD_INSERT_LIBRARIES=/Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
==13074== to enable ASan wrappers.
==13074== Set ASAN_OPTIONS=allow_reexec=0 to disable this.
==13074== Parsed ASAN_OPTIONS: verbosity=1
==13074== exec()-ing the program with
==13074== DYLD_INSERT_LIBRARIES=/Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
==13074== to enable ASan wrappers.
==13074== Set ASAN_OPTIONS=allow_reexec=0 to disable this.
==13074== Parsed ASAN_OPTIONS: verbosity=1
==13074== exec()-ing the program with
==13074== DYLD_INSERT_LIBRARIES=/Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
==13074== to enable ASan wrappers.
==13074== Set ASAN_OPTIONS=allow_reexec=0 to disable this.
==13074== Parsed ASAN_OPTIONS: verbosity=1
==13074== exec()-ing the program with
==13074== DYLD_INSERT_LIBRARIES=/Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
==13074== to enable ASan wrappers.
==13074== Set ASAN_OPTIONS=allow_reexec=0 to disable this.
==13074== Parsed ASAN_OPTIONS: verbosity=1
==13074== exec()-ing the program with
==13074== DYLD_INSERT_LIBRARIES=/Users/glider/src/asan/llvm/llvm_cmake_build/lib/clang/3.3/lib/darwin/libclang_rt.asan_osx_dynamic.dylib
==13074== to enable ASan wrappers.
==13074== Set ASAN_OPTIONS=allow_reexec=0 to disable this.
==13074== Parsed ASAN_OPTIONS: verbosity=1
==13074== exec()-ing the program with
...

- i.e. the application falls into an exec() loop.