doc: guide: include sync unallow example

Author: bradjc

doc/guide.md

@@ -410,3 +410,47 @@ returncode_t libtocksync_sensor_read(int* val) {
   return err;
 }
 ```
+
+### More Complicated Example:
+
+Drivers that use allow buffers must un-allow the buffers after the operation
+finishes.
+
+```c
+#include "digest.h"
+#include "syscalls/digest_syscalls.h"
+
+returncode_t libtocksync_digest_compute(uint8_t* input_buffer,
+                                        uint32_t input_buffer_len,
+                                        uint8_t* output_buffer,
+                                        uint32_t output_buffer_len) {
+  returncode_t ret;
+
+  // First allow for input. If it fails, return.
+  ret = libtock_digest_set_readonly_allow(input_buffer, input_buffer_len);
+  if (ret != RETURNCODE_SUCCESS) return ret;
+
+  // Second allow for output. If it fails, unallow first buffer then return.
+  ret = libtock_digest_set_readwrite_allow(output_buffer, output_buffer_len);
+  if (ret != RETURNCODE_SUCCESS) goto exit1;
+
+  // Attempt the command. If it fails, unallow both buffers.
+  ret = libtock_digest_command_compute_digest();
+  if (err != RETURNCODE_SUCCESS) goto exit2;
+
+  // Wait for the digest to compute.
+  ret = libtock_digest_yield_wait_for(val);
+
+exit2:
+  // Do the input unallow. We have to ignore the return value to 1) return the
+  // correct error of the actual failing operation if something happened, and
+  // 2) do the second unallow unconditionally.
+  libtock_digest_set_readonly_allow(NULL, 0);
+
+exit1:
+  // Do the output unallow.
+  libtock_digest_set_readonly_allow(NULL, 0);
+
+  return ret;
+}
+```