libheaptrace: Fix segfaults when directly using LD_PRELOAD

The current usage of 'heaptrace' works fine as follows.

  $ heaptrace samples/sample.out
  [heaptrace] initialized for /proc/4084/maps (sample.out)
  [heaptrace]   finalized for /proc/4084/maps (sample.out)

It should be fine when running it with 'LD_PRELOAD' without using
'heaptrace' helper binary, but it gets crashed as follows.

  $ LD_PRELOAD=./libheaptrace.so samples/sample.out
  Segmentation falut (core dumped)

The problem is because 'getenv()' returns NULL in this case, but it
misses to check whether the returned values are NULL or not.

This patch fixes this problem and it makes LD_PRELOAD usage works fine.

  $ LD_PRELOAD=./libheaptrace.so samples/sample.out
  [heaptrace] initialized for /proc/4167/maps (sample.out)
  [heaptrace]   finalized for /proc/4167/maps (sample.out)

Fixed: #18

Reported-by: Bojun Seo <[email protected]>
Signed-off-by: Honggyu Kim <[email protected]>

heaptrace.cc

@@ -101,6 +101,7 @@ static void init_options(int argc, char *argv[])
 	};
 
 	// set default option values
+	// TODO: create constexpr variables instead of default magic values.
 	opts.top = 10;
 	opts.sort_keys = "size";
 	opts.flamegraph = false;

libheaptrace.cc

@@ -73,6 +73,7 @@ static void heaptrace_init()
 	int pid = getpid();
 	std::stringstream ss;
 	std::string comm = utils::get_comm_name();
+	char *env;
 
 	real_malloc = (MallocFunction)dlsym(RTLD_NEXT, "malloc");
 	real_free = (FreeFunction)dlsym(RTLD_NEXT, "free");
@@ -89,9 +90,15 @@ static void heaptrace_init()
 	sighandler_init();
 
 	// setup option values
-	opts.top = strtol(getenv("HEAPTRACE_NUM_TOP_BACKTRACE"), NULL, 0);
-	opts.sort_keys = getenv("HEAPTRACE_SORT_KEYS");
-	opts.flamegraph = strtol(getenv("HEAPTRACE_FLAME_GRAPH"), NULL, 0);
+	// TODO: create constexpr variables instead of default magic values.
+	env = getenv("HEAPTRACE_NUM_TOP_BACKTRACE");
+	opts.top = env ? strtol(env, NULL, 0) : 10;
+
+	env = getenv("HEAPTRACE_SORT_KEYS");
+	opts.sort_keys = env ? env : "size";
+
+	env = getenv("HEAPTRACE_FLAME_GRAPH");
+	opts.flamegraph = env ? strtol(env, NULL, 0) : false;
 
 	opts.outfile = getenv("HEAPTRACE_OUTFILE");
 	if (opts.outfile) {