diff --git a/rtc_base/checks.cc b/rtc_base/checks.cc
index 32ac8d6e8b..b8b5c9da9b 100644
--- a/rtc_base/checks.cc
+++ b/rtc_base/checks.cc
@@ -35,53 +35,72 @@
 
 #include "rtc_base/checks.h"
 
+namespace {
+#if defined(__GNUC__)
+__attribute__((__format__(__printf__, 2, 3)))
+#endif
+  void AppendFormat(std::string* s, const char* fmt, ...) {
+  va_list args, copy;
+  va_start(args, fmt);
+  va_copy(copy, args);
+  const int predicted_length = std::vsnprintf(nullptr, 0, fmt, copy);
+  va_end(copy);
+
+  if (predicted_length > 0) {
+    const size_t size = s->size();
+    s->resize(size + predicted_length);
+    // Pass "+ 1" to vsnprintf to include space for the '\0'.
+    std::vsnprintf(&((*s)[size]), predicted_length + 1, fmt, args);
+  }
+  va_end(args);
+}
+}
+
 namespace rtc {
 namespace webrtc_checks_impl {
 
 // Reads one argument from args, appends it to s and advances fmt.
 // Returns true iff an argument was sucessfully parsed.
-bool ParseArg(va_list* args,
-              const CheckArgType** fmt,
-              std::ostream& s) {  // no-presubmit-check TODO(webrtc:8982)
+bool ParseArg(va_list* args, const CheckArgType** fmt, std::string* s) {
   if (**fmt == CheckArgType::kEnd)
     return false;
 
   switch (**fmt) {
     case CheckArgType::kInt:
-      s << va_arg(*args, int);
+      AppendFormat(s, "%d", va_arg(*args, int));
       break;
     case CheckArgType::kLong:
-      s << va_arg(*args, long);
+      AppendFormat(s, "%ld", va_arg(*args, long));
       break;
     case CheckArgType::kLongLong:
-      s << va_arg(*args, long long);
+      AppendFormat(s, "%lld", va_arg(*args, long long));
       break;
     case CheckArgType::kUInt:
-      s << va_arg(*args, unsigned);
+      AppendFormat(s, "%u", va_arg(*args, unsigned));
       break;
     case CheckArgType::kULong:
-      s << va_arg(*args, unsigned long);
+      AppendFormat(s, "%lu", va_arg(*args, unsigned long));
       break;
     case CheckArgType::kULongLong:
-      s << va_arg(*args, unsigned long long);
+      AppendFormat(s, "%llu", va_arg(*args, unsigned long long));
       break;
     case CheckArgType::kDouble:
-      s << va_arg(*args, double);
+      AppendFormat(s, "%g", va_arg(*args, double));
       break;
     case CheckArgType::kLongDouble:
-      s << va_arg(*args, long double);
+      AppendFormat(s, "%Lg", va_arg(*args, long double));
       break;
     case CheckArgType::kCharP:
-      s << va_arg(*args, const char*);
+      s->append(va_arg(*args, const char*));
       break;
     case CheckArgType::kStdString:
-      s << *va_arg(*args, const std::string*);
+      s->append(*va_arg(*args, const std::string*));
       break;
     case CheckArgType::kVoidP:
-      s << reinterpret_cast<std::uintptr_t>(va_arg(*args, const void*));
+      AppendFormat(s, "%p", va_arg(*args, const void*));
       break;
     default:
-      s << "[Invalid CheckArgType:" << static_cast<int8_t>(**fmt) << "]";
+      s->append("[Invalid CheckArgType]");
       return false;
   }
   (*fmt)++;
@@ -96,9 +115,14 @@ RTC_NORETURN void FatalLog(const char* file,
   va_list args;
   va_start(args, fmt);
 
-  std::ostringstream ss;  // no-presubmit-check TODO(webrtc:8982)
-  ss << "\n\n#\n# Fatal error in: " << file << ", line " << line
-     << "\n# last system error: " << LAST_SYSTEM_ERROR << "\n# Check failed: ";
+  std::string s;
+  AppendFormat(&s,
+               "\n\n"
+               "#\n"
+               "# Fatal error in: %s, line %d\n"
+               "# last system error: %u\n"
+               "# Check failed: %s",
+               file, line, LAST_SYSTEM_ERROR, message);
 
   if (*fmt == CheckArgType::kCheckOp) {
     // This log message was generated by RTC_CHECK_OP, so we have to complete
@@ -106,20 +130,19 @@ RTC_NORETURN void FatalLog(const char* file,
     // two arguments.
     fmt++;
 
-    std::ostringstream s1, s2;  // no-presubmit-check TODO(webrtc:8982)
-    if (ParseArg(&args, &fmt, s1) && ParseArg(&args, &fmt, s2))
-      ss << message << " (" << s1.str() << " vs. " << s2.str() << ")\n# ";
+    std::string s1, s2;
+    if (ParseArg(&args, &fmt, &s1) && ParseArg(&args, &fmt, &s2))
+      AppendFormat(&s, " (%s vs. %s)\n# ", s1.c_str(), s2.c_str());
   } else {
-    ss << message << "\n# ";
+    s.append("\n# ");
   }
 
   // Append all the user-supplied arguments to the message.
-  while (ParseArg(&args, &fmt, ss))
+  while (ParseArg(&args, &fmt, &s))
     ;
 
   va_end(args);
 
-  std::string s = ss.str();
   const char* output = s.c_str();
 
 #if defined(WEBRTC_ANDROID)