decoding.rst

Decoding

Another way to decode data using libcbor is to specify a callbacks that will be invoked when upon finding certain items in the input. This service is provided by

cbor_stream_decode

To get started, you might want to have a look at the simple streaming example:

#include "cbor.h"
#include <stdio.h>
#include <string.h>

/*
 * Illustrates how one might skim through a map (which is assumed to have
 * string keys and values only), looking for the value of a specific key
 *
 * Use the examples/data/map.cbor input to test this.
 */

const char * key = "a secret key";
bool key_found = false;

void find_string(void * _ctx, cbor_data buffer, size_t len)
{
    if (key_found) {
        printf("Found the value: %*s\n", (int) len, buffer);
        key_found = false;
    } else if (len == strlen(key)) {
        key_found = (memcmp(key, buffer, len) == 0);
    }
}

int main(int argc, char * argv[])
{
    FILE * f = fopen(argv[1], "rb");
    fseek(f, 0, SEEK_END);
    size_t length = (size_t)ftell(f);
    fseek(f, 0, SEEK_SET);
    unsigned char * buffer = malloc(length);
    fread(buffer, length, 1, f);

    struct cbor_callbacks callbacks = cbor_empty_callbacks;
    struct cbor_decoder_result decode_result;
    size_t bytes_read = 0;
    callbacks.string = find_string;
    while (bytes_read < length) {
        decode_result = cbor_stream_decode(buffer + bytes_read,
                                           length - bytes_read,
                                           &callbacks, NULL);
        bytes_read += decode_result.read;
    }

    free(buffer);
    fclose(f);
}

The callbacks are defined by

cbor_callbacks

When building custom sets of callbacks, feel free to start from

cbor_empty_callbacks

Related structures

cbor_decoder_status

cbor_decoder_result

Callback types definition

cbor_int8_callback

cbor_int16_callback

cbor_int32_callback

cbor_int64_callback

cbor_simple_callback

cbor_string_callback

cbor_collection_callback

cbor_float_callback

cbor_double_callback

cbor_bool_callback