Implement OS::DumpBacktrace() on FreeBSD.

src/platform-freebsd.cc

Index: src/platform-freebsd.cc
diff --git a/src/platform-freebsd.cc b/src/platform-freebsd.cc
index cebdd1ed57f43f270727751685a75b4b9b1a0bb1..62653b4ca7f73d562c17cc2241163ea2775e9512 100644
--- a/src/platform-freebsd.cc
+++ b/src/platform-freebsd.cc
@@ -198,6 +198,31 @@ void OS::DebugBreak() {
 }
 
 
+void OS::DumpBacktrace() {
+  void* trace[100];
+  int size = backtrace(trace, ARRAY_SIZE(trace));
+  char** symbols = backtrace_symbols(trace, size);
+  fprintf(stderr, "\n==== C stack trace ===============================\n\n");
+  if (size == 0) {
+    fprintf(stderr, "(empty)\n");
+  } else if (symbols == NULL) {
+    fprintf(stderr, "(no symbols)\n");
+  } else {
+    for (int i = 1; i < size; ++i) {
+      fprintf(stderr, "%2d: ", i);
+      char mangled[201];
+      if (sscanf(symbols[i], "%*[^(]%*[(]%200[^)+]", mangled) == 1) {  // NOLINT
+        fprintf(stderr, "%s\n", mangled);
+      } else {
+        fprintf(stderr, "??\n");
+      }
+    }
+  }
+  fflush(stderr);
+  free(symbols);
+}
+
+
 class PosixMemoryMappedFile : public OS::MemoryMappedFile {
  public:
   PosixMemoryMappedFile(FILE* file, void* memory, int size)