Daily Archives: 9 January 2017


asn1c: How do I know how big a buffer to allocate before using ‘uper_encode_to_new_buffer’?

The following post is for the https://lionet.info/asn1c/ (repository: https://github.com/vlm/asn1c/)

There is no need to compute the space needed.

If you pass the address to a pointer that is NULL pointer as the last parameter of uper_encode_to_new_buffer(asn_TYPE_descriptor_t *td, asn_per_constraints_t *constraints, void *sptr, void **buffer_r) , then it will allocate by itself the required space.

void *buffer = NULL;
asn_per_constraints_s *constraints = NULL;
ssize_t ec = uper_encode_to_new_buffer(&asn_DEF_Image, constraints, image, &buffer);

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asn1c: What is the ‘write_stream’ parameter in the example code 1

The following post is for the https://lionet.info/asn1c/ (repository: https://github.com/vlm/asn1c/)

The asn1c usage manual (PDF), mentions an element called write_stream but it does not define what it is.

What write_stream is can be found in converter-sample.c  and in the manual under the name write_out:

/* Dump the buffer out to the specified FILE */
static int write_out(const void *buffer, size_t size, void *key) {
  FILE *fp = (FILE *)key;
  return (fwrite(buffer, 1, size, fp) == size) ? 0 : -1;
}

write_out is function that has the following signature write_out(const void *buffer, size_t size, void *app_key) and is used as a callback by der_encode() and other functions.

This callback receives as input the pointer to an element (const void *buffer), the size of that element (size_t size) and some context (void *app_key).
In this example, we can see that the user is using der_encode() which accepts a FILE * as the last parameter, which later is passed to write_out() as the context.