Object Storage Service:Download objects as files using OSS SDK for C

Prerequisites

Before you begin, ensure that you have:

• An OSS bucket that contains the object you want to download

• The oss:GetObject permission on the object (and kms:Decrypt if the object uses KMS server-side encryption)

• The OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables set to your Alibaba Cloud access key ID and access key secret

Usage notes

• The examples in this topic use the public endpoint for the China (Hangzhou) region. If your application runs inside an Alibaba Cloud service in the same region as your bucket, use an internal endpoint instead to reduce latency and data transfer costs. For a full list of endpoints, see Regions and endpoints.

• The examples create an OSSClient instance using an OSS endpoint. To initialize OSSClient with a custom domain name or Security Token Service (STS), see Initialization.

Permissions

By default, an Alibaba Cloud account has full permissions. RAM users and RAM roles have no permissions by default. To grant access, use RAM Policy or Bucket policies.

Download an object to a local file

oss_get_object_to_file() downloads an object from OSS and writes it to a local file path. It returns an aos_status_t pointer that indicates whether the download succeeded. If a file at the specified path already exists, the download overwrites it; otherwise, the function creates a new file.

Function signature

aos_status_t *oss_get_object_to_file(
    oss_request_options_t *options,
    const aos_string_t    *bucket,
    const aos_string_t    *object,
    aos_table_t           *headers,
    aos_table_t           *params,
    aos_string_t          *filename,
    aos_table_t           **resp_headers
);

Parameters

Return value: An aos_status_t * where aos_status_is_ok(resp_status) returns true on success.

Example

#include "oss_api.h"
#include "aos_http_io.h"

/* Replace the following placeholders with your actual values. */
const char *endpoint    = "yourEndpoint";     /* e.g., https://oss-cn-hangzhou.aliyuncs.com */
const char *bucket_name = "examplebucket";
const char *object_name = "exampledir/exampleobject.txt";
const char *local_filename = "yourLocalFilename";
const char *region      = "yourRegion";       /* e.g., cn-hangzhou */

void init_options(oss_request_options_t *options)
{
    options->config = oss_config_create(options->pool);

    /* Initialize the endpoint, credentials, region, and signature version. */
    aos_str_set(&options->config->endpoint, endpoint);
    aos_str_set(&options->config->access_key_id,     getenv("OSS_ACCESS_KEY_ID"));
    aos_str_set(&options->config->access_key_secret, getenv("OSS_ACCESS_KEY_SECRET"));
    aos_str_set(&options->config->region, region);
    options->config->signature_version = 4;

    /* Set is_cname to 0 to use the standard OSS endpoint (not a custom domain). */
    options->config->is_cname = 0;

    /* Configure network settings such as timeout. */
    options->ctl = aos_http_controller_create(options->pool, 0);
}

int main(int argc, char *argv[])
{
    /* Initialize global resources (network, memory). Must be called once before any SDK operation. */
    if (aos_http_io_initialize(NULL, 0) != AOSE_OK) {
        exit(1);
    }

    /* Create a memory pool to manage all allocations for this request. */
    aos_pool_t *pool;
    aos_pool_create(&pool, NULL);

    /* Create and initialize the client options. */
    oss_request_options_t *oss_client_options;
    oss_client_options = oss_request_options_create(pool);
    init_options(oss_client_options);

    /* Set up the bucket, object, and local file path. */
    aos_string_t bucket;
    aos_string_t object;
    aos_string_t file;
    aos_table_t *params;
    aos_table_t *headers      = NULL;
    aos_table_t *resp_headers = NULL;
    aos_status_t *resp_status = NULL;

    aos_str_set(&bucket, bucket_name);
    aos_str_set(&object, object_name);
    aos_str_set(&file,   local_filename);
    params = aos_table_make(pool, 0);

    /* Download the object to the local file.
       If the file already exists, it is overwritten. Otherwise, a new file is created. */
    resp_status = oss_get_object_to_file(
        oss_client_options, &bucket, &object,
        headers, params, &file, &resp_headers
    );

    if (aos_status_is_ok(resp_status)) {
        printf("Get object from file succeeded\n");
    } else {
        printf("Get object from file failed\n");
    }

    /* Release the memory pool and global resources. */
    aos_pool_destroy(pool);
    aos_http_io_deinitialize();
    return 0;
}

Placeholders

Replace the following placeholders with your actual values:

What's next

• For the complete sample code, see oss_get_object_sample.c on GitHub.

• For the API reference of the underlying GetObject operation, see GetObject.