ADDED   testtg.c
Index: testtg.c
==================================================================
--- /dev/null
+++ testtg.c
@@ -0,0 +1,26 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "aocutils.h"
+
+int main(void) {
+    char *input = NULL;
+    size_t ilen = slurp(&input, "test.txt");
+
+    struct TextGrid tg;
+    tg.data = input;
+    tg.cols = strchr(input, '\n') - input + 1;
+    tg.rows = tg.cols - 1; // rows includes the '\n'
+
+    if (*TGcharptr(&tg, 11, 11) == '.') puts("1");
+    if (TGcharptr(&tg, 12, 11) == NULL) puts("2");
+    if (TGcharptr(&tg, 11, 12) == NULL) puts("3");
+    if (*TGcharptr(&tg, 11, 12) == '\n') puts("33");
+    if (TGcharptr(&tg, -1, -2) == NULL) puts("4");
+
+    for (int k = 0; k < 12; k++) {
+        *TGcharptr(&tg, k, k) = '#';
+    }
+    printf("\n\n%s\n", tg.data);
+}