aws s3 download pre allocate disk space

alloc.cpp

#include <iostream>
#include <fstream>
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>

#include <aws/core/Aws.h>
#include <aws/s3/S3Client.h>
#include <aws/s3/model/HeadObjectRequest.h>
#include <aws/s3/model/GetObjectRequest.h>

int main(int argc, char** argv)
{
    if (argc < 4) {
        std::cerr << "Usage: s3_download <bucket> <key> <local_file>" << std::endl;
        return 1;
    }

    const Aws::String bucket_name = argv[1];
    const Aws::String key = argv[2];
    const std::string local_file_path = argv[3];

    // Initialize the SDK
    Aws::SDKOptions options;
    Aws::InitAPI(options);

    {
        // Create an S3 client
        Aws::S3::S3Client s3_client;

        // Head the object to get its size
        Aws::S3::Model::HeadObjectRequest head_request;
        head_request.SetBucket(bucket_name);
        head_request.SetKey(key);
        auto head_outcome = s3_client.HeadObject(head_request);
        if (!head_outcome.IsSuccess()) {
            std::cerr << "Failed to head object: " << head_outcome.GetError().GetMessage() << std::endl;
            return 1;
        }

        const auto file_size = head_outcome.GetResult().GetSize();

        // Open the local file for writing
        std::ofstream ofs(local_file_path, std::ios::out | std::ios::binary);
        if (!ofs.good()) {
            std::cerr << "Failed to open file: " << local_file_path << std::endl;
            return 1;
        }

        // Pre-allocate disk space for the file
        const auto fd = ofs.rdbuf()->fd();
        if (posix_fallocate(fd, 0, file_size) != 0) {
            std::cerr << "Failed to pre-allocate disk space: " << strerror(errno) << std::endl;
        }

        // Download the object and write it to the local file
        Aws::S3::Model::GetObjectRequest get_request;
        get_request.SetBucket(bucket_name);
        get_request.SetKey(key);
        auto get_outcome = s3_client.GetObject(get_request);
        if (!get_outcome.IsSuccess()) {
            std::cerr << "Failed to get object: " << get_outcome.GetError().GetMessage() << std::endl;
            return 1;
        }

        auto& body_stream = get_outcome.GetResultWithOwnership().GetBody();
        char buffer[4096];
        while (body_stream.good()) {
            body_stream.read(buffer, sizeof(buffer));
            ofs.write(buffer, body_stream.gcount());
        }
    }

    // Shutdown the SDK
    Aws::ShutdownAPI(options);

    return 0;
}