Many const fixes throughout the stack (although these are not all, yet)

src/include/lwip/inet_chksum.h

@@ -70,7 +70,7 @@
 extern "C" {
 #endif
 
-u16_t inet_chksum(void *dataptr, u16_t len);
+u16_t inet_chksum(const void *dataptr, u16_t len);
 u16_t inet_chksum_pbuf(struct pbuf *p);
 #if LWIP_CHKSUM_COPY_ALGORITHM
 u16_t lwip_chksum_copy(void *dst, const void *src, u16_t len);

src/include/lwip/ip4_addr.h

@@ -212,10 +212,10 @@ u8_t ip4_addr_netmask_valid(u32_t netmask);
                       ipaddr != NULL ? ip4_addr4_16(ipaddr) : 0))
 
 /* Get one byte from the 4-byte address */
-#define ip4_addr1(ipaddr) (((u8_t*)(ipaddr))[0])
-#define ip4_addr2(ipaddr) (((u8_t*)(ipaddr))[1])
-#define ip4_addr3(ipaddr) (((u8_t*)(ipaddr))[2])
-#define ip4_addr4(ipaddr) (((u8_t*)(ipaddr))[3])
+#define ip4_addr1(ipaddr) (((const u8_t*)(ipaddr))[0])
+#define ip4_addr2(ipaddr) (((const u8_t*)(ipaddr))[1])
+#define ip4_addr3(ipaddr) (((const u8_t*)(ipaddr))[2])
+#define ip4_addr4(ipaddr) (((const u8_t*)(ipaddr))[3])
 /* These are cast to u16_t, with the intent that they are often arguments
  * to printf using the U16_F format from cc.h. */
 #define ip4_addr1_16(ipaddr) ((u16_t)ip4_addr1(ipaddr))

src/include/lwip/pbuf.h

@@ -119,6 +119,19 @@ struct pbuf {
   u16_t ref;
 };
 
+
+/** Helper struct for const-correctness only.
+ * The only meaning of this one is to provide a const payload pointer
+ * for PBUF_ROM type.
+ */
+struct pbuf_rom {
+  /** next pbuf in singly linked pbuf chain */
+  struct pbuf *next;
+
+  /** pointer to the actual data in the buffer */
+  const void *payload;
+};
+
 #if LWIP_SUPPORT_CUSTOM_PBUF
 /** Prototype for a function to free a custom pbuf */
 typedef void (*pbuf_free_custom_fn)(struct pbuf *p);

src/include/lwip/snmp_asn1.h

@@ -90,7 +90,7 @@ err_t snmp_asn1_enc_length(struct pbuf *p, u16_t ofs, u16_t length);
 err_t snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u16_t octets_needed, u32_t value);
 err_t snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u16_t octets_needed, s32_t value);
 err_t snmp_asn1_enc_oid(struct pbuf *p, u16_t ofs, u8_t ident_len, const s32_t *ident);
-err_t snmp_asn1_enc_raw(struct pbuf *p, u16_t ofs, u16_t raw_len, u8_t *raw);
+err_t snmp_asn1_enc_raw(struct pbuf *p, u16_t ofs, u16_t raw_len, const u8_t *raw);
 
 #ifdef __cplusplus
 }

src/include/lwip/snmp_structs.h

@@ -138,7 +138,7 @@ struct mib_array_node
 
   /* additional struct members */
   const s32_t *objid;
-  struct mib_node* const *nptr;
+  const struct mib_node* const *nptr;
 };
 
 /** derived node, points to a fixed size mem_malloced array
@@ -254,8 +254,8 @@ s8_t snmp_mib_node_insert(struct mib_list_rootnode *rn, s32_t objid, struct mib_
 s8_t snmp_mib_node_find(struct mib_list_rootnode *rn, s32_t objid, struct mib_list_node **fn);
 struct mib_list_rootnode *snmp_mib_node_delete(struct mib_list_rootnode *rn, struct mib_list_node *n);
 
-struct mib_node* snmp_search_tree(struct mib_node *node, u8_t ident_len, s32_t *ident, struct snmp_name_ptr *np);
-struct mib_node* snmp_expand_tree(struct mib_node *node, u8_t ident_len, s32_t *ident, struct snmp_obj_id *oidret);
+struct mib_node* snmp_search_tree(const struct mib_node *node, u8_t ident_len, s32_t *ident, struct snmp_name_ptr *np);
+struct mib_node* snmp_expand_tree(const struct mib_node *node, u8_t ident_len, s32_t *ident, struct snmp_obj_id *oidret);
 u8_t snmp_iso_prefix_tst(u8_t ident_len, s32_t *ident);
 u8_t snmp_iso_prefix_expand(u8_t ident_len, s32_t *ident, struct snmp_obj_id *oidret);
 

src/include/lwip/sockets.h

@@ -125,7 +125,10 @@ struct lwip_setgetsockopt_data {
 #if LWIP_MPU_COMPATIBLE
   u8_t optval[LWIP_SETGETSOCKOPT_MAXOPTLEN];
 #else
-  void* optval;
+  union {
+     void *p;
+     const void *pc;
+  } optval;
 #endif
   /** size of *optval */
   socklen_t optlen;