NanoPB: How to handle string types in C

syntax = "proto3";

package example;

message StringMessage {
  string name = 1;
  string description = 2;
}

protoc --nanopb_out=. strings.proto

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include "strings.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

int main() {
    // Buffer for encoded message
    uint8_t buffer[256];
    size_t message_length;
    
    // --- ENCODING ---
    example_StringMessage message = example_StringMessage_init_zero;
    
    // Set string values (must fit in buffer size)
    const char* name = "NanoPB";
    const char* description = "Protocol Buffers for embedded systems";
    
    strncpy(message.name, name, sizeof(message.name) - 1);
    strncpy(message.description, description, sizeof(message.description) - 1);
    
    // Create stream for encoding
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    // Encode the message
    if (!pb_encode(&ostream, example_StringMessage_fields, &message)) {
        printf("Encoding failed: %s\n", PB_GET_ERROR(&ostream));
        return 1;
    }
    
    message_length = ostream.bytes_written;
    printf("Encoded %zu bytes\n", message_length);
    
    // Print hex dump of encoded data
    printf("Encoded data: ");
    for (size_t i = 0; i < message_length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");
    
    // --- DECODING ---
    example_StringMessage decoded = example_StringMessage_init_zero;
    
    // Create stream for decoding
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    
    // Decode the message
    if (!pb_decode(&istream, example_StringMessage_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    // Print decoded values
    printf("Decoded values:\n");
    printf("  name: %s\n", decoded.name);
    printf("  description: %s\n", decoded.description);
    
    return 0;
}

gcc -o strings_example strings_example.c strings.pb.c pb_common.c pb_encode.c pb_decode.c -I.

import strings_pb2

# Read the binary data
with open('encoded.bin', 'rb') as f:
    data = f.read()

# Decode
msg = strings_pb2.StringMessage()
msg.ParseFromString(data)

print("Python decoded values:")
print(f"  name: {msg.name}")
print(f"  description: {msg.description}")

protoc --python_out=. strings.proto

// After encoding, add this:
FILE *f = fopen("encoded.bin", "wb");
fwrite(buffer, 1, message_length, f);
fclose(f);

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include "strings.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

typedef struct {
    char* data;
    size_t size;
} dynamic_string_t;

// Encoder callback for strings
bool string_encode_callback(pb_ostream_t *stream, const pb_field_t *field, void * const *arg) {
    const dynamic_string_t* str = (const dynamic_string_t*)*arg;
    
    if (!pb_encode_tag_for_field(stream, field))
        return false;
    
    return pb_encode_string(stream, (const pb_byte_t*)str->data, str->size);
}

// Decoder callback for strings
bool string_decode_callback(pb_istream_t *stream, const pb_field_t *field, void **arg) {
    dynamic_string_t* str = (dynamic_string_t*)*arg;
    
    // Allocate buffer
    size_t size = stream->bytes_left;
    str->data = (char*)malloc(size + 1);
    
    if (!pb_read(stream, (pb_byte_t*)str->data, size)) {
        free(str->data);
        str->data = NULL;
        return false;
    }
    
    str->data[size] = '\0'; // Null-terminate
    str->size = size;
    return true;
}

int main() {
    uint8_t buffer[256];
    size_t message_length;
    
    dynamic_string_t name = {"NanoPB", 6};
    dynamic_string_t description = {"Protocol Buffers for embedded systems", 38};
    
    // --- ENCODING ---
    example_StringMessage message = example_StringMessage_init_zero;
    
    message.name.funcs.encode = string_encode_callback;
    message.name.arg = &name;
    message.description.funcs.encode = string_encode_callback;
    message.description.arg = &description;
    
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    if (!pb_encode(&ostream, example_StringMessage_fields, &message)) {
        printf("Encoding failed: %s\n", PB_GET_ERROR(&ostream));
        return 1;
    }
    
    message_length = ostream.bytes_written;
    printf("Encoded %zu bytes\n", message_length);
    
    // --- DECODING ---
    example_StringMessage decoded = example_StringMessage_init_zero;
    dynamic_string_t decoded_name = {NULL, 0};
    dynamic_string_t decoded_description = {NULL, 0};
    
    decoded.name.funcs.decode = string_decode_callback;
    decoded.name.arg = &decoded_name;
    decoded.description.funcs.decode = string_decode_callback;
    decoded.description.arg = &decoded_description;
    
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    
    if (!pb_decode(&istream, example_StringMessage_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    printf("Decoded values:\n");
    printf("  name: %s\n", decoded_name.data);
    printf("  description: %s\n", decoded_description.data);
    
    // Free allocated memory
    free(decoded_name.data);
    free(decoded_description.data);
    
    return 0;
}