Wrap string into tensor in tensorflow C API (v2.4+)

TensorFlow General Discussion
1

Before Tensorflow V2.4, string tensors were defined as (tensorflow/tensorflow/c/c_api.h at r1.13 · tensorflow/tensorflow · GitHub) a list of uint64 offsets to varint prefixed char strings (where the varint defines the length of the string).

we have the following code to pass a string tensor as argument.

TF_Tensor* ScalarStringTensor(const char* str, TF_Status* status) {
  size_t nbytes = 8 + TF_StringEncodedSize(strlen(str));
  TF_Tensor* t = TF_AllocateTensor(TF_STRING, NULL, 0, nbytes);
  void* data = TF_TensorData(t);
  memset(data, 0, 8);  // 8-byte offset of first string.
  TF_StringEncode(str, strlen(str), data + 8, nbytes - 8, status);
  return t;
}

void foo() {
  TF_Tensor* t = ScalarStringTensor(checkpoint_prefix, model->status);
  if (!Okay(model->status)) {
    TF_DeleteTensor(t);
    return 0;
  }
  TF_Output inputs[1] = {model->checkpoint_file};
  TF_Tensor* input_values[1] = {t};
  const TF_Operation* op[1] = {type == SAVE ? model->save_op
                                            : model->restore_op};
  TF_SessionRun(model->session, NULL, inputs, input_values, 1,
                /* No outputs */
                NULL, NULL, 0,
                /* The operation */
                op, 1, NULL, model->status);
  TF_DeleteTensor(t);
}

Since tensorflow V2.4, the string representation in C/C++/TFCore is unified.

  • The byte layout for string tensors across the C-API has been updated to match TF Core/C++; i.e., a contiguous array of tensorflow::tstring/TF_TStrings.
  • C-API functions TF_StringDecode, TF_StringEncode, and TF_StringEncodedSize are no longer relevant and have been removed; see core/platform/ctstring.h for string access/modification in C.

And this document describes how a string is represented in memory.

How should I update the code above to send a string as a tensor? just copy the memory layout into a 1-d tensor? What about memory padding? I am unclear about this part.

Thank you in advance

3

Have you figured out the solution? I am having the same issue: http://discuss.ai.google.dev/t/how-to-create-a-tf-string-type-tensor-from-c-c/14263/3.
Would be great if you could share it (assuming you have it).
Thanks,

5

An assumed solution is posted in my thread linked above.