Use data() instead of c_str() where appropriate

src/libutil/hash.cc

@@ -136,7 +136,7 @@ string printHash32(const Hash & hash)
     Hash hash2(hash);
     unsigned int len = hashLength32(hash);
 
-    const char * chars = base32Chars.c_str();
+    const char * chars = base32Chars.data();
     
     string s(len, '0');
 
@@ -186,7 +186,7 @@ Hash parseHash32(HashType ht, const string & s)
 {
     Hash hash(ht);
 
-    const char * chars = base32Chars.c_str();
+    const char * chars = base32Chars.data();
 
     for (unsigned int i = 0; i < s.length(); ++i) {
         char c = s[i];
@@ -271,7 +271,7 @@ Hash hashString(HashType ht, const string & s)
     Ctx ctx;
     Hash hash(ht);
     start(ht, ctx);
-    update(ht, ctx, (const unsigned char *) s.c_str(), s.length());
+    update(ht, ctx, (const unsigned char *) s.data(), s.length());
     finish(ht, ctx, hash.hash);
     return hash;
 }

src/libutil/serialise.cc

@@ -159,7 +159,7 @@ void writeString(const unsigned char * buf, size_t len, Sink & sink)
 
 void writeString(const string & s, Sink & sink)
 {
-    writeString((const unsigned char *) s.c_str(), s.size(), sink);
+    writeString((const unsigned char *) s.data(), s.size(), sink);
 }
 
 

src/libutil/util.cc

@@ -236,7 +236,7 @@ void writeFile(const Path & path, const string & s)
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT, 0666);
     if (fd == -1)
         throw SysError(format("opening file `%1%'") % path);
-    writeFull(fd, (unsigned char *) s.c_str(), s.size());
+    writeFull(fd, (unsigned char *) s.data(), s.size());
 }
 
 
@@ -263,7 +263,7 @@ string readLine(int fd)
 void writeLine(int fd, string s)
 {
     s += '\n';
-    writeFull(fd, (const unsigned char *) s.c_str(), s.size());
+    writeFull(fd, (const unsigned char *) s.data(), s.size());
 }
 
 
@@ -485,7 +485,7 @@ void printMsg_(Verbosity level, const format & f)
         prefix = "\033[" + escVerbosity(level) + "s";
     string s = (format("%1%%2%\n") % prefix % f.str()).str();
     try {
-        writeToStderr((const unsigned char *) s.c_str(), s.size());
+        writeToStderr((const unsigned char *) s.data(), s.size());
     } catch (SysError & e) {
         /* Ignore failing writes to stderr if we're in an exception
            handler, otherwise throw an exception.  We need to ignore