diff --git a/src/HttpServer.cc b/src/HttpServer.cc
index e3735140..3e662e93 100644
--- a/src/HttpServer.cc
+++ b/src/HttpServer.cc
@@ -1585,8 +1585,9 @@ int start_listen(event_base *evbase, Sessions *sessions, const Config *config)
 {
   addrinfo hints;
   int r;
-  char service[10];
-  snprintf(service, sizeof(service), "%u", config->port);
+
+  auto service = util::utos(config->port);
+
   memset(&hints, 0, sizeof(addrinfo));
   hints.ai_family = AF_UNSPEC;
   hints.ai_socktype = SOCK_STREAM;
@@ -1600,7 +1601,7 @@ int start_listen(event_base *evbase, Sessions *sessions, const Config *config)
   auto listen_handler = listen_handler_store.get();
 
   addrinfo *res, *rp;
-  r = getaddrinfo(nullptr, service, &hints, &res);
+  r = getaddrinfo(nullptr, service.c_str(), &hints, &res);
   if(r != 0) {
     std::cerr << "getaddrinfo() failed: " << gai_strerror(r) << std::endl;
     return -1;
diff --git a/src/shrpx-unittest.cc b/src/shrpx-unittest.cc
index e6e14968..0a6c16fc 100644
--- a/src/shrpx-unittest.cc
+++ b/src/shrpx-unittest.cc
@@ -118,6 +118,7 @@ int main(int argc, char* argv[])
                    shrpx::test_util_to_base64) ||
       !CU_add_test(pSuite, "util_percent_encode_token",
                    shrpx::test_util_percent_encode_token) ||
+      !CU_add_test(pSuite, "util_utox", shrpx::test_util_utox) ||
       !CU_add_test(pSuite, "gzip_inflate", test_nghttp2_gzip_inflate)) {
      CU_cleanup_registry();
      return CU_get_error();
diff --git a/src/shrpx.cc b/src/shrpx.cc
index c283d288..b9a910f0 100644
--- a/src/shrpx.cc
+++ b/src/shrpx.cc
@@ -83,9 +83,8 @@ int resolve_hostname(sockaddr_union *addr, size_t *addrlen,
 {
   addrinfo hints;
   int rv;
-  char service[10];
 
-  snprintf(service, sizeof(service), "%u", port);
+  auto service = util::utos(port);
   memset(&hints, 0, sizeof(addrinfo));
 
   hints.ai_family = family;
@@ -95,7 +94,7 @@ int resolve_hostname(sockaddr_union *addr, size_t *addrlen,
 #endif // AI_ADDRCONFIG
   addrinfo *res;
 
-  rv = getaddrinfo(hostname, service, &hints, &res);
+  rv = getaddrinfo(hostname, service.c_str(), &hints, &res);
   if(rv != 0) {
     LOG(FATAL) << "Unable to resolve address for " << hostname
                << ": " << gai_strerror(rv);
@@ -136,8 +135,8 @@ evconnlistener* create_evlistener(ListenHandler *handler, int family)
   addrinfo hints;
   int fd = -1;
   int r;
-  char service[10];
-  snprintf(service, sizeof(service), "%u", get_config()->port);
+
+  auto service = util::utos(get_config()->port);
   memset(&hints, 0, sizeof(addrinfo));
   hints.ai_family = family;
   hints.ai_socktype = SOCK_STREAM;
@@ -147,7 +146,7 @@ evconnlistener* create_evlistener(ListenHandler *handler, int family)
 #endif // AI_ADDRCONFIG
 
   addrinfo *res, *rp;
-  r = getaddrinfo(get_config()->host, service, &hints, &res);
+  r = getaddrinfo(get_config()->host, service.c_str(), &hints, &res);
   if(r != 0) {
     if(LOG_ENABLED(INFO)) {
       LOG(INFO) << "Unable to get IPv" << (family == AF_INET ? "4" : "6")
@@ -1253,15 +1252,25 @@ int main(int argc, char **argv)
     }
   }
 
-  char hostport[NI_MAXHOST+16];
   bool downstream_ipv6_addr =
     is_ipv6_numeric_addr(get_config()->downstream_host);
-  snprintf(hostport, sizeof(hostport), "%s%s%s:%u",
-           downstream_ipv6_addr ? "[" : "",
-           get_config()->downstream_host,
-           downstream_ipv6_addr ? "]" : "",
-           get_config()->downstream_port);
-  set_config_str(&mod_config()->downstream_hostport, hostport);
+
+  std::string hostport;
+
+  if(downstream_ipv6_addr) {
+    hostport += "[";
+  }
+
+  hostport += get_config()->downstream_host;
+
+  if(downstream_ipv6_addr) {
+    hostport += "]";
+  }
+
+  hostport += ":";
+  hostport += util::utos(get_config()->downstream_port);
+
+  set_config_str(&mod_config()->downstream_hostport, hostport.c_str());
 
   if(LOG_ENABLED(INFO)) {
     LOG(INFO) << "Resolving backend address";
diff --git a/src/shrpx_http_downstream_connection.cc b/src/shrpx_http_downstream_connection.cc
index a4d35b8c..4a895bff 100644
--- a/src/shrpx_http_downstream_connection.cc
+++ b/src/shrpx_http_downstream_connection.cc
@@ -258,36 +258,37 @@ int HttpDownstreamConnection::push_request_headers()
 int HttpDownstreamConnection::push_upload_data_chunk
 (const uint8_t *data, size_t datalen)
 {
-  ssize_t res = 0;
   int rv;
   int chunked = downstream_->get_chunked_request();
   auto output = bufferevent_get_output(bev_);
+
   if(chunked) {
-    char chunk_size_hex[16];
-    rv = snprintf(chunk_size_hex, sizeof(chunk_size_hex), "%X\r\n",
-                  static_cast<unsigned int>(datalen));
-    res += rv;
-    rv = evbuffer_add(output, chunk_size_hex, rv);
+    auto chunk_size_hex = util::utox(datalen);
+    chunk_size_hex += "\r\n";
+
+    rv = evbuffer_add(output, chunk_size_hex.c_str(), chunk_size_hex.size());
     if(rv == -1) {
       DCLOG(FATAL, this) << "evbuffer_add() failed";
       return -1;
     }
   }
+
   rv = evbuffer_add(output, data, datalen);
+
   if(rv == -1) {
     DCLOG(FATAL, this) << "evbuffer_add() failed";
     return -1;
   }
-  res += rv;
+
   if(chunked) {
     rv = evbuffer_add(output, "\r\n", 2);
     if(rv == -1) {
       DCLOG(FATAL, this) << "evbuffer_add() failed";
       return -1;
     }
-    res += 2;
   }
-  return res;
+
+  return 0;
 }
 
 int HttpDownstreamConnection::end_upload_data()
diff --git a/src/shrpx_https_upstream.cc b/src/shrpx_https_upstream.cc
index 45e01e73..6a00f3be 100644
--- a/src/shrpx_https_upstream.cc
+++ b/src/shrpx_https_upstream.cc
@@ -650,11 +650,12 @@ int HttpsUpstream::on_downstream_header_complete(Downstream *downstream)
   if(LOG_ENABLED(INFO)) {
     DLOG(INFO, downstream) << "HTTP response header completed";
   }
-  char temp[16];
-  snprintf(temp, sizeof(temp), "HTTP/%d.%d ",
-           downstream->get_request_major(),
-           downstream->get_request_minor());
-  std::string hdrs = temp;
+
+  std::string hdrs = "HTTP/";
+  hdrs += util::utos(downstream->get_request_major());
+  hdrs += ".";
+  hdrs += util::utos(downstream->get_request_minor());
+  hdrs += " ";
   hdrs += http2::get_status_string(downstream->get_response_http_status());
   hdrs += "\r\n";
   downstream->normalize_response_headers();
@@ -757,10 +758,12 @@ int HttpsUpstream::on_downstream_body(Downstream *downstream,
   }
   auto output = bufferevent_get_output(handler_->get_bev());
   if(downstream->get_chunked_response()) {
-    char chunk_size_hex[16];
-    rv = snprintf(chunk_size_hex, sizeof(chunk_size_hex), "%X\r\n",
-                  static_cast<unsigned int>(len));
-    if(evbuffer_add(output, chunk_size_hex, rv) != 0) {
+    auto chunk_size_hex = util::utox(len);
+    chunk_size_hex += "\r\n";
+
+    rv = evbuffer_add(output, chunk_size_hex.c_str(), chunk_size_hex.size());
+
+    if(rv != 0) {
       ULOG(FATAL, this) << "evbuffer_add() failed";
       return -1;
     }
diff --git a/src/util.cc b/src/util.cc
index e5440836..8b5f418a 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -42,6 +42,8 @@ namespace util {
 
 const char DEFAULT_STRIP_CHARSET[] = "\r\n\t ";
 
+const char UPPER_XDIGITS[] = "0123456789ABCDEF";
+
 bool isAlpha(const char c)
 {
   return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z');
@@ -68,13 +70,14 @@ std::string percentEncode(const unsigned char* target, size_t len)
 {
   std::string dest;
   for(size_t i = 0; i < len; ++i) {
-    if(inRFC3986UnreservedChars(target[i])) {
-      dest += target[i];
+    unsigned char c = target[i];
+
+    if(inRFC3986UnreservedChars(c)) {
+      dest += c;
     } else {
-      char temp[4];
-      snprintf(temp, sizeof(temp), "%%%02X", target[i]);
-      dest.append(temp);
-      //dest.append(fmt("%%%02X", target[i]));
+      dest += "%";
+      dest += UPPER_XDIGITS[c >> 4];
+      dest += UPPER_XDIGITS[(c & 0x0f)];
     }
   }
   return dest;
@@ -102,13 +105,14 @@ std::string percent_encode_token(const std::string& target)
   std::string dest;
 
   for(size_t i = 0; i < len; ++i) {
-    char c = target[i];
+    unsigned char c = target[i];
+
     if(c != '%' && in_token(c)) {
       dest += c;
     } else {
-      char temp[4];
-      snprintf(temp, sizeof(temp), "%%%02X", c);
-      dest += temp;
+      dest += "%";
+      dest += UPPER_XDIGITS[c >> 4];
+      dest += UPPER_XDIGITS[(c & 0x0f)];
     }
   }
   return dest;
diff --git a/src/util.h b/src/util.h
index bdd39402..2c5f9362 100644
--- a/src/util.h
+++ b/src/util.h
@@ -399,6 +399,27 @@ std::string utos(T n)
   return res;
 }
 
+extern const char UPPER_XDIGITS[];
+
+template<typename T>
+std::string utox(T n)
+{
+  std::string res;
+  if(n == 0) {
+    res = "0";
+    return res;
+  }
+  int i = 0;
+  T t = n;
+  for(; t; t /= 16, ++i);
+  res.resize(i);
+  --i;
+  for(; n; --i, n /= 16) {
+    res[i] = UPPER_XDIGITS[(n & 0x0f)];
+  }
+  return res;
+}
+
 template<typename T, typename... U>
 typename std::enable_if<!std::is_array<T>::value, std::unique_ptr<T>>::type
 make_unique(U&&... u)
diff --git a/src/util_test.cc b/src/util_test.cc
index c18f9ebf..3556f164 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -101,4 +101,14 @@ void test_util_percent_encode_token(void)
   CU_ASSERT("http%202" == util::percent_encode_token("http 2"));
 }
 
+void test_util_utox(void)
+{
+  CU_ASSERT("0" == util::utox(0));
+  CU_ASSERT("1" == util::utox(1));
+  CU_ASSERT("F" == util::utox(15));
+  CU_ASSERT("10" == util::utox(16));
+  CU_ASSERT("3B9ACA07" == util::utox(1000000007));
+  CU_ASSERT("100000000" == util::utox(1LL << 32));
+}
+
 } // namespace shrpx
diff --git a/src/util_test.h b/src/util_test.h
index 7fd1686a..8a85ee89 100644
--- a/src/util_test.h
+++ b/src/util_test.h
@@ -32,6 +32,7 @@ void test_util_strieq(void);
 void test_util_inp_strlower(void);
 void test_util_to_base64(void);
 void test_util_percent_encode_token(void);
+void test_util_utox(void);
 
 } // namespace shrpx