diff --git a/icu4c/source/common/ucnv_cnv.c b/icu4c/source/common/ucnv_cnv.c
index c995a28afd..118c36cf29 100644
--- a/icu4c/source/common/ucnv_cnv.c
+++ b/icu4c/source/common/ucnv_cnv.c
@@ -168,3 +168,28 @@ ucnv_getUChar32KeepOverflow(UConverter *cnv, const UChar *buffer, int32_t length
     }
     return c;
 }
+
+/* update target offsets after a callback call */
+U_CFUNC int32_t *
+ucnv_updateCallbackOffsets(int32_t *offsets, int32_t length, int32_t sourceIndex) {
+    if(offsets!=NULL) {
+        if(sourceIndex>=0) {
+            /* add the sourceIndex to the relative offsets that the callback wrote */
+            while(length>0) {
+                *offsets+=sourceIndex;
+                ++offsets;
+                --length;
+            }
+        } else {
+            /* sourceIndex==-1, set -1 offsets */
+            while(length>0) {
+                *offsets=-1;
+                ++offsets;
+                --length;
+            }
+        }
+        return offsets;
+    } else {
+        return NULL;
+    }
+}
diff --git a/icu4c/source/common/ucnv_cnv.h b/icu4c/source/common/ucnv_cnv.h
index f1cafc66b8..bf87b61427 100644
--- a/icu4c/source/common/ucnv_cnv.h
+++ b/icu4c/source/common/ucnv_cnv.h
@@ -231,4 +231,27 @@ U_CDECL_END
 U_CFUNC UChar32
 ucnv_getUChar32KeepOverflow(UConverter *cnv, const UChar *buffer, int32_t length);
 
+/**
+ * This helper function updates the offsets array after a callback function call.
+ * It adds the sourceIndex to each offsets item, or sets each of them to -1 if
+ * sourceIndex==-1.
+ *
+ * @param offsets The pointer to offsets entry that corresponds to the first target
+ *                unit that the callback wrote.
+ * @param length  The number of output units that the callback wrote.
+ * @param sourceIndex The sourceIndex of the input sequence that the callback
+ *                    function was called for.
+ * @return offsets+length if offsets!=NULL, otherwise NULL
+ */
+U_CFUNC int32_t *
+ucnv_updateCallbackOffsets(int32_t *offsets, int32_t length, int32_t sourceIndex);
+
+/** Always use fallbacks from codepage to Unicode */
+#define TO_U_USE_FALLBACK(useFallback) TRUE
+#define UCNV_TO_U_USE_FALLBACK(cnv) TRUE
+
+/** Use fallbacks from Unicode to codepage when cnv->useFallback or for private-use code points */
+#define FROM_U_USE_FALLBACK(useFallback, c) ((useFallback) || (uint32_t)((c)-0xe000)<0x1900 || (uint32_t)((c)-0xf0000)<0x20000)
+#define UCNV_FROM_U_USE_FALLBACK(cnv, c) FROM_U_USE_FALLBACK((cnv)->useFallback, c)
+
 #endif /* UCNV_CNV */