diff --git a/src/curl.cpp b/src/curl.cpp
index 40f7b8a..c5c6bb8 100644
--- a/src/curl.cpp
+++ b/src/curl.cpp
@@ -40,8 +40,8 @@
 //-------------------------------------------------------------------
 // Symbols
 //-------------------------------------------------------------------
-static const std::string empty_payload_hash         = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855";
-static const std::string empty_md5_base64_hash      = "1B2M2Y8AsgTpgAmY7PhCfg==";
+static const char EMPTY_PAYLOAD_HASH[]              = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855";
+static const char EMPTY_MD5_BASE64_HASH[]           = "1B2M2Y8AsgTpgAmY7PhCfg==";
 
 //-------------------------------------------------------------------
 // Class S3fsCurl
@@ -50,10 +50,10 @@ static const int MULTIPART_SIZE                     = 10 * 1024 * 1024;
 static const int GET_OBJECT_RESPONSE_LIMIT          = 1024;
 
 static const int IAM_EXPIRE_MERGIN                  = 20 * 60;  // update timing
-static const std::string ECS_IAM_ENV_VAR            = "AWS_CONTAINER_CREDENTIALS_RELATIVE_URI";
-static const std::string IAMCRED_ACCESSKEYID        = "AccessKeyId";
-static const std::string IAMCRED_SECRETACCESSKEY    = "SecretAccessKey";
-static const std::string IAMCRED_ROLEARN            = "RoleArn";
+static const char ECS_IAM_ENV_VAR[]                 = "AWS_CONTAINER_CREDENTIALS_RELATIVE_URI";
+static const char IAMCRED_ACCESSKEYID[]             = "AccessKeyId";
+static const char IAMCRED_SECRETACCESSKEY[]         = "SecretAccessKey";
+static const char IAMCRED_ROLEARN[]                 = "RoleArn";
 
 // [NOTE] about default mime.types file
 // If no mime.types file is specified in the mime option, s3fs
@@ -2682,7 +2682,7 @@ void S3fsCurl::insertV4Headers()
     std::string date8601;
     get_date_sigv3(strdate, date8601);
 
-    std::string contentSHA256 = payload_hash.empty() ? empty_payload_hash : payload_hash;
+    std::string contentSHA256 = payload_hash.empty() ? EMPTY_PAYLOAD_HASH : payload_hash;
     const std::string realpath = pathrequeststyle ? "/" + bucket + server_path : server_path;
 
     //string canonical_headers, signed_headers;
@@ -2836,9 +2836,9 @@ int S3fsCurl::GetIAMCredentials()
 
     // url
     if(is_ecs){
-        const char *env = std::getenv(ECS_IAM_ENV_VAR.c_str());
+        const char *env = std::getenv(ECS_IAM_ENV_VAR);
         if(env == NULL){
-            S3FS_PRN_ERR("%s is not set.", ECS_IAM_ENV_VAR.c_str());
+            S3FS_PRN_ERR("%s is not set.", ECS_IAM_ENV_VAR);
             return -EIO;
         }
         url = std::string(S3fsCurl::IAM_cred_url) + env;
@@ -3232,7 +3232,7 @@ int S3fsCurl::PutRequest(const char* tpath, headers_t& meta, int fd)
                 return -EIO;
             }
         }else{
-            strMD5 = empty_md5_base64_hash;
+            strMD5 = EMPTY_MD5_BASE64_HASH;
         }
         requestHeaders = curl_slist_sort_insert(requestHeaders, "Content-MD5", strMD5.c_str());
     }
diff --git a/src/curl_util.cpp b/src/curl_util.cpp
index f5b4345..82dd7ef 100644
--- a/src/curl_util.cpp
+++ b/src/curl_util.cpp
@@ -330,14 +330,14 @@ std::string url_to_host(const std::string &url)
 {
     S3FS_PRN_INFO3("url is %s", url.c_str());
 
-    static const std::string http = "http://";
-    static const std::string https = "https://";
+    static const char HTTP[] = "http://";
+    static const char HTTPS[] = "https://";
     std::string hostname;
 
-    if (is_prefix(url.c_str(), http.c_str())) {
-        hostname = url.substr(http.size());
-    } else if (is_prefix(url.c_str(), https.c_str())) {
-        hostname = url.substr(https.size());
+    if (is_prefix(url.c_str(), HTTP)) {
+        hostname = url.substr(sizeof(HTTP) - 1);
+    } else if (is_prefix(url.c_str(), HTTPS)) {
+        hostname = url.substr(sizeof(HTTPS) - 1);
     } else {
         S3FS_PRN_EXIT("url does not begin with http:// or https://");
         abort();
diff --git a/src/s3fs.cpp b/src/s3fs.cpp
index af1be97..dac4e2d 100644
--- a/src/s3fs.cpp
+++ b/src/s3fs.cpp
@@ -100,10 +100,10 @@ static off_t max_dirty_data       = 5LL * 1024LL * 1024LL * 1024LL;
 static bool use_wtf8              = false;
 static off_t fake_diskfree_size   = -1; // default is not set(-1)
 
-static const std::string allbucket_fields_type;              // special key for mapping(This name is absolutely not used as a bucket name)
-static const std::string keyval_fields_type    = "\t";       // special key for mapping(This name is absolutely not used as a bucket name)
-static const std::string aws_accesskeyid       = "AWSAccessKeyId";
-static const std::string aws_secretkey         = "AWSSecretKey";
+static const char ALLBUCKET_FIELDS_TYPE[] = "";         // special key for mapping(This name is absolutely not used as a bucket name)
+static const char KEYVAL_FIELDS_TYPE[]    = "\t";       // special key for mapping(This name is absolutely not used as a bucket name)
+static const char AWS_ACCESSKEYID[]       = "AWSAccessKeyId";
+static const char AWS_SECRETKEY[]         = "AWSSecretKey";
 
 //-------------------------------------------------------------------
 // Global functions : prototype
@@ -3682,7 +3682,7 @@ static int parse_passwd_file(bucketkvmap_t& resmap)
         kv[key] = val;
     }
     // set special key name
-    resmap[std::string(keyval_fields_type)] = kv;
+    resmap[KEYVAL_FIELDS_TYPE] = kv;
 
     // read ':' type
     for(iter = linelist.begin(); iter != linelist.end(); ++iter){
@@ -3701,7 +3701,7 @@ static int parse_passwd_file(bucketkvmap_t& resmap)
             secret    = trim(iter->substr(last_pos + 1, std::string::npos));
         }else{
             // formatted by "accesskey:secretkey"
-            bucketname    = allbucket_fields_type;
+            bucketname    = ALLBUCKET_FIELDS_TYPE;
             accesskey = trim(iter->substr(0, first_pos));
             secret    = trim(iter->substr(first_pos + 1, std::string::npos));
         }
@@ -3710,8 +3710,8 @@ static int parse_passwd_file(bucketkvmap_t& resmap)
             return -1;
         }
         kv.clear();
-        kv[std::string(aws_accesskeyid)] = accesskey;
-        kv[std::string(aws_secretkey)] = secret;
+        kv[AWS_ACCESSKEYID] = accesskey;
+        kv[AWS_SECRETKEY] = secret;
         resmap[bucketname] = kv;
     }
     return (resmap.empty() ? 0 : 1);
@@ -3724,8 +3724,8 @@ static int parse_passwd_file(bucketkvmap_t& resmap)
 //
 static int check_for_aws_format(const kvmap_t& kvmap)
 {
-    std::string str1(aws_accesskeyid);
-    std::string str2(aws_secretkey);
+    std::string str1(AWS_ACCESSKEYID);
+    std::string str2(AWS_SECRETKEY);
 
     if(kvmap.empty()){
         return 0;
@@ -3909,7 +3909,7 @@ static int read_passwd_file()
     //
     // check key=value type format.
     //
-    bucketkvmap_t::iterator it = bucketmap.find(keyval_fields_type);
+    bucketkvmap_t::iterator it = bucketmap.find(KEYVAL_FIELDS_TYPE);
     if(bucketmap.end() != it){
         // aws format
         result = check_for_aws_format(it->second);
@@ -3921,7 +3921,7 @@ static int read_passwd_file()
         }
     }
 
-    std::string bucket_key = allbucket_fields_type;
+    std::string bucket_key = ALLBUCKET_FIELDS_TYPE;
     if(!bucket.empty() && bucketmap.end() != bucketmap.find(bucket)){
         bucket_key = bucket;
     }
@@ -3931,8 +3931,8 @@ static int read_passwd_file()
         return EXIT_FAILURE;
     }
     keyval = it->second;
-    kvmap_t::iterator aws_accesskeyid_it = keyval.find(aws_accesskeyid);
-    kvmap_t::iterator aws_secretkey_it = keyval.find(aws_secretkey);
+    kvmap_t::iterator aws_accesskeyid_it = keyval.find(AWS_ACCESSKEYID);
+    kvmap_t::iterator aws_secretkey_it = keyval.find(AWS_SECRETKEY);
     if(keyval.end() == aws_accesskeyid_it || keyval.end() == aws_secretkey_it){
         S3FS_PRN_EXIT("Not found access key/secret key in passwd file.");
         return EXIT_FAILURE;
diff --git a/src/string_util.cpp b/src/string_util.cpp
index ff08290..a679a24 100644
--- a/src/string_util.cpp
+++ b/src/string_util.cpp
@@ -35,7 +35,7 @@
 //-------------------------------------------------------------------
 // Global variables
 //-------------------------------------------------------------------
-const std::string SPACES = " \t\r\n";
+const char SPACES[] = " \t\r\n";
 
 //-------------------------------------------------------------------
 // Templates
@@ -108,13 +108,13 @@ std::string lower(std::string s)
     return s;
 }
 
-std::string trim_left(const std::string &s, const std::string &t /* = SPACES */)
+std::string trim_left(const std::string &s, const char *t /* = SPACES */)
 {
     std::string d(s);
     return d.erase(0, s.find_first_not_of(t));
 }
 
-std::string trim_right(const std::string &s, const std::string &t /* = SPACES */)
+std::string trim_right(const std::string &s, const char *t /* = SPACES */)
 {
     std::string d(s);
     std::string::size_type i(d.find_last_not_of(t));
@@ -125,7 +125,7 @@ std::string trim_right(const std::string &s, const std::string &t /* = SPACES */
     }
 }
 
-std::string trim(const std::string &s, const std::string &t /* = SPACES */)
+std::string trim(const std::string &s, const char *t /* = SPACES */)
 {
     return trim_left(trim_right(s, t), t);
 }
diff --git a/src/string_util.h b/src/string_util.h
index 8975a13..275aa56 100644
--- a/src/string_util.h
+++ b/src/string_util.h
@@ -27,7 +27,7 @@
 //-------------------------------------------------------------------
 // Global variables
 //-------------------------------------------------------------------
-extern const std::string SPACES;
+extern const char SPACES[];
 
 //-------------------------------------------------------------------
 // Inline functions
@@ -68,9 +68,9 @@ off_t cvt_strtoofft(const char* str, int base);
 //
 // String Manipulation
 //
-std::string trim_left(const std::string &s, const std::string &t = SPACES);
-std::string trim_right(const std::string &s, const std::string &t = SPACES);
-std::string trim(const std::string &s, const std::string &t = SPACES);
+std::string trim_left(const std::string &s, const char *t = SPACES);
+std::string trim_right(const std::string &s, const char *t = SPACES);
+std::string trim(const std::string &s, const char *t = SPACES);
 std::string lower(std::string s);
 
 //