Add new function string_set_n to extract a sub-string from a string.

m-string.h

@@ -242,6 +242,23 @@ string_set (string_t v1, const string_t v2)
   STRING_CONTRACT (v1);
 }
 
+static inline void
+string_set_n(string_t v, const string_t ref, size_t offset, size_t length)
+{
+  STRING_CONTRACT (v);
+  STRING_CONTRACT (ref);
+  assert (v != ref); // Not supported yet
+  if (M_UNLIKELY (ref->size == 0)) {
+    v->size = 0;
+    if (v->ptr) {
+      v->ptr[0] = 0;
+    }
+  } else {
+    assert (ref->ptr != NULL);
+    string_set_strn(v, ref->ptr + offset, length);
+  }
+}
+
 static inline void
 string_init_set(string_t v1, const string_t v2)
 {

tests/test-mstring.c

@@ -50,6 +50,11 @@ int main(void)
   assert (strcmp(string_get_cstr(s1), "Hello") == 0);
   string_clean(s1);
   
+  string_set_str (s1, "Hello");
+  string_set_str (s2, "Hello, world!");
+  string_set_n (s1, s2, 7, 5);
+  assert (strcmp(string_get_cstr(s1), "world") == 0);
+  
   string_set_str (s1, "Hello beautiful world!");
   assert (string_size(s1) == 22);
   assert (strcmp(string_get_cstr(s1), "Hello beautiful world!") == 0);