add a str_adopt constructor for sbstring.

src/util/strings.c

@@ -266,6 +266,16 @@ void sbs_init(struct sbstring *sbs, char *buffer, size_t size)
     buffer[0] = '\0';
 }
 
+void sbs_adopt(struct sbstring *sbs, char *buffer, size_t size)
+{
+    size_t len = strlen(buffer);
+    assert(sbs);
+    assert(size > len);
+    sbs->begin = buffer;
+    sbs->size = size;
+    sbs->end = buffer + len;
+}
+
 void sbs_strncat(struct sbstring *sbs, const char *str, size_t size)
 {
     size_t len;

src/util/strings.h

@@ -49,6 +49,7 @@ extern "C" {
     } sbstring;
 
     void sbs_init(struct sbstring *sbs, char *buffer, size_t size);
+    void sbs_adopt(struct sbstring *sbs, char *buffer, size_t size);
     void sbs_strcat(struct sbstring *sbs, const char *str);
     void sbs_strncat(struct sbstring *sbs, const char *str, size_t size);
     void sbs_strcpy(struct sbstring *sbs, const char *str);

src/util/strings.test.c

@@ -165,6 +165,10 @@ static void test_sbstring(CuTest * tc)
     CuAssertStrEquals(tc, "12345678901234", sbs.begin);
     sbs_strncat(&sbs, "567890", 6);
     CuAssertStrEquals(tc, "123456789012345", sbs.begin);
+
+    sbs_adopt(&sbs, buffer + 2, sizeof(buffer) - 2);
+    CuAssertStrEquals(tc, "3456789012345", sbs.begin);
+    CuAssertIntEquals(tc, 13, sbs_length(&sbs));
 }
 
 static void test_sbs_strcat(CuTest * tc)