configure.ac: enable -Wwrite-strings

As discussed in bug #185 [1], const strings should be protected
from being overwritten. This change adds -Wwrite-strings to the
compiler warning flags and fixes resulting warnings by making
static strings const.

[1] https://github.com/obgm/libcoap/issues/185

configure.ac

@@ -137,6 +137,7 @@ WARNING_CFLAGS="\
 -Wswitch-default \
 -Wswitch-enum \
 -Wunused \
+-Wwrite-strings \
 "
 
 # check whether or not the compiler supports -Wlogical-op (clang does not...)

examples/client.c

@@ -559,7 +559,7 @@ usage( const char *program, const char *version) {
 
 typedef struct {
   unsigned char code;
-  char *media_type;
+  const char *media_type;
 } content_type_t;
 
 static void
@@ -968,7 +968,7 @@ cmdline_input_from_file(char *filename, str *buf) {
 
 static method_t
 cmdline_method(char *arg) {
-  static char *methods[] =
+  static const char *methods[] =
     { 0, "get", "post", "put", "delete", "fetch", "patch", "ipatch", 0};
   unsigned char i;
 

include/coap/pdu.h

@@ -132,7 +132,7 @@ struct coap_session_t;
  * @return     A zero-terminated string describing the error, or @c NULL if not
  *             found.
  */
-char *coap_response_phrase(unsigned char code);
+const char *coap_response_phrase(unsigned char code);
 
 #define COAP_ERROR_PHRASE_LENGTH   32 /**< maximum length of error phrase */
 

src/debug.c

@@ -71,7 +71,7 @@ coap_set_log_level(coap_log_t level) {
 }
 
 /* this array has the same order as the type log_t */
-static char *loglevels[] = {
+static const char *loglevels[] = {
   "EMRG", "ALRT", "CRIT", "ERR", "WARN", "NOTE", "INFO", "DEBG" 
 };
 
@@ -265,7 +265,7 @@ coap_print_addr(const struct coap_address_t *addr, unsigned char *buf, size_t le
 /** Returns a textual description of the message type @p t. */
 static const char *
 msg_type_string(uint16_t t) {
-  static char *types[] = { "CON", "NON", "ACK", "RST", "???" };
+  static const char *types[] = { "CON", "NON", "ACK", "RST", "???" };
 
   return types[min(t, sizeof(types)/sizeof(char *) - 1)];
 }

src/net.c

@@ -1426,7 +1426,7 @@ coap_new_error_response(coap_pdu_t *request, unsigned char code,
   uint16_t opt_type = 0;	/* used for calculating delta-storage */
 
 #if COAP_ERROR_PHRASE_LENGTH > 0
-  char *phrase = coap_response_phrase(code);
+  const char *phrase = coap_response_phrase(code);
 
   /* Need some more space for the error phrase and payload start marker */
   if (phrase)

src/pdu.c

@@ -344,7 +344,7 @@ coap_get_data(coap_pdu_t *pdu, size_t *len, uint8_t **data) {
 #ifndef SHORT_ERROR_RESPONSE
 typedef struct {
   unsigned char code;
-  char *phrase;
+  const char *phrase;
 } error_desc_t;
 
 /* if you change anything here, make sure, that the longest string does not 
@@ -376,7 +376,7 @@ error_desc_t coap_error[] = {
   { 0, NULL }			/* end marker */
 };
 
-char *
+const char *
 coap_response_phrase(unsigned char code) {
   int i;
   for (i = 0; coap_error[i].code; ++i) {