diff --git a/app/Makefile b/app/Makefile
index 962d0c42..db441588 100644
--- a/app/Makefile
+++ b/app/Makefile
@@ -316,7 +316,7 @@ ${ZSV_UTIL_A}: ${BUILD_DIR}/objs/utils/util.a
 	@mkdir -p `dirname $@`
 	cp -p $< $@
 
-UTIL_A_OBJ:=writer file dirs-no-jq os ${UTIL_A_OBJ_WIN}
+UTIL_A_OBJ:=index writer file dirs-no-jq os ${UTIL_A_OBJ_WIN}
 UTIL_A_OBJ:=$(addprefix ${BUILD_DIR}/objs/utils/,$(addsuffix .o,${UTIL_A_OBJ}))
 
 ${BUILD_DIR}/objs/utils/util.a: ${UTIL_A_OBJ}
diff --git a/app/ext_example/Makefile b/app/ext_example/Makefile
index c8cd1a67..d37d8f93 100644
--- a/app/ext_example/Makefile
+++ b/app/ext_example/Makefile
@@ -77,7 +77,6 @@ TEST_PASS=printf "${COLOR_BLUE}$@: ${COLOR_GREEN}Passed${COLOR_NONE}\n"
 TEST_FAIL=(printf "${COLOR_BLUE}$@: ${COLOR_RED}Failed!${COLOR_NONE}\n" && exit 1)
 TEST_INIT=printf "${COLOR_PINK}$@: ${COLOR_NONE}\n"
 
-UTILS1=
 CFLAGS_SHARED=-shared
 ifneq ($(findstring emcc,$(CC)),) # emcc
   CFLAGS_SHARED=-s SIDE_MODULE=1 -s LINKABLE=1
@@ -86,9 +85,6 @@ else
   INSTALLED_EXTENSION=
 endif
 
-UTILS1+=writer
-UTILS=$(addprefix ${BUILD_DIR}/objs/utils/,$(addsuffix .o,${UTILS1}))
-
 CFLAGS+= -I${THIS_LIB_BASE}/include -I${PREFIX}/include
 
 all: ${TARGET} ${TARGET_SHEET}
@@ -218,8 +214,9 @@ YAJL_SRC_DIR=${THIS_MAKEFILE_DIR}/../external/yajl
 YAJL_INCLUDE=-I${YAJL_SRC_DIR}/build/yajl-2.1.1/include
 YAJL_HELPER_INCLUDE=-I${THIS_MAKEFILE_DIR}/../external/yajl_helper
 ${TARGET_SHEET}: LIBS="../external/sqlite3/sqlite3.c" -lzsv -lzsvutil -L${PREFIX}/lib -ljq -lutf8proc
+${TARGET}: LIBS=-lzsvutil -lzsv -L${PREFIX}/lib
 ${TARGET} ${TARGET_SHEET}: ${BUILD_DIR}/bin/zsvext%.${SO} : %_extension.c ${UTILS}
 	@mkdir -p `dirname "$@"`
-	${CC} ${CFLAGS} ${CFLAGS_SHARED} $< ${UTILS} -o $@ ${LIBS} ${YAJL_INCLUDE} ${YAJL_HELPER_INCLUDE}
+	${CC} ${CFLAGS} ${CFLAGS_SHARED} $< -o $@ ${LIBS} ${YAJL_INCLUDE} ${YAJL_HELPER_INCLUDE}
 
 .PHONY: all test test-% clean install
diff --git a/app/sheet/index.c b/app/sheet/index.c
index 6f1822a5..56f5bc7a 100644
--- a/app/sheet/index.c
+++ b/app/sheet/index.c
@@ -12,8 +12,9 @@ static void build_memory_index_row_handler(void *ctx) {
   struct zsvsheet_indexer *ixr = ctx;
   struct zsv_index *ix = ixr->ix;
   zsv_parser parser = ixr->parser;
+  uint64_t line_end = zsv_cum_scanned_length(parser);
 
-  if (zsv_index_add_row(ix, parser) != zsv_index_status_ok)
+  if (zsv_index_add_row(ix, line_end) != zsv_index_status_ok)
     zsv_abort(parser);
 }
 
diff --git a/app/sheet/transformation.c b/app/sheet/transformation.c
index c876223c..1ebcbe0c 100644
--- a/app/sheet/transformation.c
+++ b/app/sheet/transformation.c
@@ -5,6 +5,7 @@
 #include "transformation.h"
 #include "pthread.h"
 #include "zsv/utils/file.h"
+#include "zsv/utils/index.h"
 #include "zsv/utils/prop.h"
 
 struct zsvsheet_transformation {
@@ -60,6 +61,7 @@ enum zsv_status zsvsheet_transformation_new(struct zsvsheet_transformation_opts
 
   struct zsv_csv_writer_options writer_opts = {
     .with_bom = 0,
+    .index = opts.index,
     .write = transformation_write,
     .stream = trn,
     .table_init = NULL,
@@ -153,6 +155,7 @@ static void *zsvsheet_run_buffer_transformation(void *arg) {
   char *buff_status_old = uib->status;
   uib->write_progressed = 1;
   uib->write_done = 1;
+  uib->index_ready = 1;
   if (buff_status_old == trn->default_status)
     uib->status = NULL;
   pthread_mutex_unlock(mutex);
@@ -172,6 +175,7 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   const char *filename = zsvsheet_buffer_data_filename(buff);
   enum zsvsheet_status stat = zsvsheet_status_error;
   struct zsvsheet_buffer_info info = zsvsheet_buffer_get_info(buff);
+  struct zsv_index *index = NULL;
 
   // TODO: Starting a second transformation before the first ends works, but if the second is faster
   //       than the first then it can end prematurely and read a partially written row.
@@ -179,12 +183,16 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   if (info.write_in_progress && !info.write_done)
     return zsvsheet_status_busy;
 
+  if (!(index = zsv_index_new()))
+    return zsvsheet_status_memory;
+
   // TODO: custom_prop_handler is not passed to extensions?
   struct zsvsheet_transformation_opts trn_opts = {
     .custom_prop_handler = NULL,
     .input_filename = filename,
     .on_done = opts.on_done,
     .ui_buffer = NULL,
+    .index = index,
   };
   zsvsheet_transformation trn = NULL;
   struct zsv_opts zopts = zsvsheet_buffer_get_zsv_opts(buff);
@@ -194,13 +202,13 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   zopts.stream = fopen(filename, "rb");
 
   if (!zopts.stream)
-    goto out;
+    goto error;
 
   trn_opts.zsv_opts = zopts;
 
   enum zsv_status zst = zsvsheet_transformation_new(trn_opts, &trn);
   if (zst != zsv_status_ok)
-    return stat;
+    goto error;
 
   // Transform part of the file to initially populate the UI buffer
   // TODO: If the transformation is a reduction that doesn't output for some time this will caus a pause
@@ -214,13 +222,13 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   case zsv_status_no_more_input:
   case zsv_status_cancelled:
     if (zsv_finish(parser) != zsv_status_ok)
-      goto out;
+      goto error;
     zsv_writer_flush(trn->writer);
     break;
   case zsv_status_ok:
     break;
   default:
-    goto out;
+    goto error;
   }
 
   struct zsvsheet_ui_buffer_opts uibopts = {0};
@@ -230,14 +238,17 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
 
   stat = zsvsheet_open_file_opts(ctx, &uibopts);
   if (stat != zsvsheet_status_ok)
-    goto out;
+    goto error;
 
   struct zsvsheet_ui_buffer *nbuff = zsvsheet_buffer_current(ctx);
   trn->ui_buffer = nbuff;
   nbuff->write_progressed = 1;
+  nbuff->index_started = 1;
+  nbuff->index = index;
 
   if (zst != zsv_status_ok) {
     nbuff->write_done = 1;
+    nbuff->index_ready = 1;
     goto out;
   }
 
@@ -247,6 +258,9 @@ enum zsvsheet_status zsvsheet_push_transformation(zsvsheet_proc_context_t ctx,
   zsvsheet_ui_buffer_create_worker(nbuff, zsvsheet_run_buffer_transformation, trn);
   return stat;
 
+error:
+  free(index);
+
 out:
   if (trn && trn->on_done)
     opts.on_done(trn);
diff --git a/app/sheet/transformation.h b/app/sheet/transformation.h
index 87ea7226..f5af1a8f 100644
--- a/app/sheet/transformation.h
+++ b/app/sheet/transformation.h
@@ -15,6 +15,7 @@ struct zsvsheet_transformation_opts {
   const char *input_filename;
   void (*on_done)(zsvsheet_transformation trn);
   struct zsvsheet_ui_buffer *ui_buffer;
+  struct zsv_index *index;
 };
 
 enum zsv_status zsvsheet_transformation_new(struct zsvsheet_transformation_opts, zsvsheet_transformation *);
diff --git a/app/utils/index.c b/app/utils/index.c
index 5cd56635..96720969 100644
--- a/app/utils/index.c
+++ b/app/utils/index.c
@@ -1,5 +1,4 @@
 #include <assert.h>
-#include <ctype.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
@@ -30,10 +29,9 @@ void zsv_index_delete(struct zsv_index *ix) {
   }
 }
 
-enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, zsv_parser parser) {
+enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, uint64_t line_end) {
   struct zsv_index_array *arr = ix->array;
   size_t len = arr->len, cap = arr->capacity;
-  uint64_t line_end = zsv_cum_scanned_length(parser);
 
   if (!ix->header_line_end) {
     ix->header_line_end = line_end;
diff --git a/app/utils/writer.c b/app/utils/writer.c
index 39be2c30..80b912e5 100644
--- a/app/utils/writer.c
+++ b/app/utils/writer.c
@@ -6,6 +6,7 @@
  * https://opensource.org/licenses/MIT
  */
 
+#include "zsv/utils/index.h"
 #include <zsv/utils/writer.h>
 #include <zsv/utils/compiler.h>
 #include <stdio.h>
@@ -108,6 +109,7 @@ struct zsv_output_buff {
   size_t (*write)(const void *restrict, size_t size, size_t nitems, void *restrict stream);
   void *stream;
   size_t used;
+  uint64_t written;
   unsigned char close_on_delete : 1;
   unsigned char _ : 7;
 };
@@ -122,6 +124,7 @@ struct zsv_writer_data {
   void *table_init_ctx;
 
   const char *cell_prepend;
+  struct zsv_index *index;
 
   unsigned char with_bom : 1;
   unsigned char started : 1;
@@ -132,6 +135,7 @@ struct zsv_writer_data {
 
 static inline void zsv_output_buff_flush(struct zsv_output_buff *b) {
   b->write(b->buff, b->used, 1, b->stream);
+  b->written += b->used;
   b->used = 0;
 }
 
@@ -141,6 +145,7 @@ static inline void zsv_output_buff_write(struct zsv_output_buff *b, const unsign
       zsv_output_buff_flush(b);
       if (n > ZSV_OUTPUT_BUFF_SIZE) { // n too big, so write directly
         b->write(s, n, 1, b->stream);
+        b->written += n;
         return;
       }
     }
@@ -199,6 +204,7 @@ zsv_csv_writer zsv_writer_new(struct zsv_csv_writer_options *opts) {
       w->with_bom = opts->with_bom;
       w->table_init = opts->table_init;
       w->table_init_ctx = opts->table_init_ctx;
+      w->index = opts->index;
     }
   }
   return w;
@@ -223,17 +229,21 @@ enum zsv_writer_status zsv_writer_delete(zsv_csv_writer w) {
   if (!w)
     return zsv_writer_status_missing_handle;
 
+  if (w->started)
+    zsv_output_buff_write(&w->out, (const unsigned char *)"\n", 1);
+
   if (w->out.stream && w->out.write && w->out.buff)
     zsv_output_buff_flush(&w->out);
 
-  if (w->started)
-    w->out.write("\n", 1, 1, w->out.stream);
+  if (w->started && w->index)
+    zsv_index_add_row(w->index, w->out.written);
 
   if (w->out.buff)
     free(w->out.buff);
 
   if (w->out.close_on_delete && w->out.stream)
     fclose(w->out.stream);
+
   free(w);
   return zsv_writer_status_ok;
 }
@@ -266,9 +276,11 @@ enum zsv_writer_status zsv_writer_cell(zsv_csv_writer w, char new_row, const uns
     if (w->with_bom)
       zsv_output_buff_write(&w->out, (const unsigned char *)"\xef\xbb\xbf", 3);
     w->started = 1;
-  } else if (new_row)
+  } else if (new_row) {
+    if (w->index)
+      zsv_index_add_row(w->index, (uint64_t)(w->out.used + w->out.written));
     zsv_output_buff_write(&w->out, (const unsigned char *)"\n", 1);
-  else
+  } else
     zsv_output_buff_write(&w->out, (const unsigned char *)",", 1);
 
   if (VERY_UNLIKELY(w->cell_prepend && *w->cell_prepend)) {
diff --git a/include/zsv/utils/index.h b/include/zsv/utils/index.h
index b9466a7d..f5805c94 100644
--- a/include/zsv/utils/index.h
+++ b/include/zsv/utils/index.h
@@ -2,8 +2,7 @@
 #define ZSV_UTILS_INDEX_H
 
 #include <stdint.h>
-#include <stdio.h>
-#include <pthread.h>
+#include <stddef.h>
 
 #include "zsv/common.h"
 
@@ -37,7 +36,7 @@ struct zsv_index {
 
 struct zsv_index *zsv_index_new(void);
 void zsv_index_delete(struct zsv_index *ix);
-enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, zsv_parser parser);
+enum zsv_index_status zsv_index_add_row(struct zsv_index *ix, uint64_t line_end);
 enum zsv_index_status zsv_index_row_end_offset(const struct zsv_index *ix, uint64_t row, uint64_t *offset_out,
                                                uint64_t *remaining_rows_out);
 enum zsv_index_status zsv_index_seek_row(const struct zsv_index *ix, struct zsv_opts *opts, uint64_t row);
diff --git a/include/zsv/utils/writer.h b/include/zsv/utils/writer.h
index e9a0a9d0..7727522e 100644
--- a/include/zsv/utils/writer.h
+++ b/include/zsv/utils/writer.h
@@ -26,6 +26,7 @@ struct zsv_csv_writer_options {
   void (*table_init)(void *);
   void *table_init_ctx;
   const char *output_path; // if provided, will be created by zsv_writer_new() and closed by zsv_writer_delete()
+  struct zsv_index *index;
 };
 
 void zsv_writer_set_default_opts(struct zsv_csv_writer_options opts);