Question

I am using asn1c to de/encode DER encoded data. That’s working fine. However, for logging it would be nice to be able to write the sent/received data in XER format (XML-like). The asn1c generated files contain routines to write XER encoded data to a FILE*, such as stdout. However, I would like to encode into a buffer, so that I can process the data before I print it. Can anyone tell me how to do that?

I would like a similar interface as encode_to_buffer(), which encodes a sructure in DER data.

Answer 1

I just found out how it can be done: The xer_encode() function generated by asn1c takes a callback, which gets a XER snippet and it’s size in bytes as well as a userdata pointer. By writing an appropriate callback, we can put the XER encoded data in a buffer.

Below is how I solved my problem. I implemented a xer_buffer_t which holds a buffer, it’s size and tracks how full the buffer is so that the callback can realloc if necessary.

void init_xer_buffer(xer_buffer_t* xer_buffer) {
    xer_buffer->buffer = malloc(1024);
    assert(xer_buffer->buffer != NULL);
    xer_buffer->buffer_size = 1024;
    xer_buffer->buffer_filled = 0;
}

void free_xer_buffer(xer_buffer_t* xer_buffer) {
    free(xer_buffer->buffer);
    xer_buffer->buffer_size = 0;
    xer_buffer->buffer_filled = 0;
}

static int xer_print2xerbuf_cb(const void *buffer, size_t size, void *app_key) {
    xer_buffer_t* xb = (xer_buffer_t*) app_key;
    while (xb->buffer_size - xb->buffer_filled <= size+1) {
        xb->buffer_size *= 2;
        xb->buffer_size += 1;
        xb->buffer = realloc(xb->buffer, xb->buffer_size);
        assert(xb->buffer != NULL);
    }
    memcpy(xb->buffer+xb->buffer_filled, buffer, size);
    xb->buffer_filled += size;
    *(xb->buffer+xb->buffer_filled) = '\0';
    return 1;
}

int xer_encode_to_buffer(xer_buffer_t* xb, asn_TYPE_descriptor_t *td, void *sptr) {
    asn_enc_rval_t er;
    if (!td || !sptr) return -1;
    er = xer_encode(td, sptr, XER_F_BASIC, xer_print2xerbuf_cb, xb);
    if (er.encoded == -1) return -1;
    return 0;
}