Object Storage Service:Download an object to memory (C SDK)

Prerequisites

Before you begin, make sure you have:

• An OSS bucket with at least one object

• The oss:GetObject RAM permission (see Permissions for conditional requirements)

• AccessKey ID and AccessKey Secret set as environment variables OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET

Permissions

By default, an Alibaba Cloud account has full permissions. RAM users or RAM roles have no permissions by default and must be granted access through RAM Policy or Bucket policies.

How it works

Call oss_get_object_to_buffer() with the bucket name and object key. The function downloads the object content into a linked list of aos_buf_t buffers (aos_list_t) and returns an aos_status_t that indicates success or failure. To work with the content as a contiguous byte array, iterate the list and copy each chunk into a single allocated buffer using memcpy.

Sample code

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

/* Replace yourEndpoint with the endpoint of the region where your bucket is located.
   Example: https://oss-cn-hangzhou.aliyuncs.com */
const char *endpoint = "yourEndpoint";

/* Replace examplebucket with your bucket name. */
const char *bucket_name = "examplebucket";

/* Specify the full object path, excluding the bucket name.
   Example: exampledir/exampleobject.txt */
const char *object_name = "exampledir/exampleobject.txt";

/* Replace yourRegion with the region ID where your bucket is located.
   Example: cn-hangzhou */
const char *region = "yourRegion";

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

    /* Initialize config fields from char* strings. */
    aos_str_set(&options->config->endpoint, endpoint);

    /* Load credentials from environment variables.
       Set OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET before running this sample. */
    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 to 0 to use the OSS endpoint directly (no CNAME). */
    options->config->is_cname = 0;

    options->ctl = aos_http_controller_create(options->pool, 0);
}

int main(int argc, char *argv[])
{
    /* Initialize global resources (network, memory). Call once at program startup. */
    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 request options (endpoint, credentials, region). */
    oss_request_options_t *oss_client_options;
    oss_client_options = oss_request_options_create(pool);
    init_options(oss_client_options);

    /* Declare request and response variables. */
    aos_string_t bucket;
    aos_string_t object;
    aos_list_t buffer;
    aos_buf_t *content = NULL;
    aos_table_t *params = NULL;
    aos_table_t *headers = NULL;
    aos_table_t *resp_headers = NULL;
    aos_status_t *resp_status = NULL;
    char *buf = NULL;
    int64_t len = 0;
    int64_t size = 0;
    int64_t pos = 0;

    aos_str_set(&bucket, bucket_name);
    aos_str_set(&object, object_name);
    aos_list_init(&buffer);

    /* Step 1: Download the object into the linked-list buffer. */
    resp_status = oss_get_object_to_buffer(oss_client_options, &bucket, &object,
                                 headers, params, &buffer, &resp_headers);

    if (aos_status_is_ok(resp_status)) {
        printf("get object to buffer succeeded\n");

        /* Step 2: Copy the buffer list into a single contiguous byte array.
           aos_buf_list_len() returns the total content length across all chunks. */
        len = aos_buf_list_len(&buffer);
        buf = aos_pcalloc(pool, len + 1);
        buf[len] = '\0';
        aos_list_for_each_entry(aos_buf_t, content, &buffer, node) {
            size = aos_buf_size(content);
            memcpy(buf + pos, content->pos, size);
            pos += size;
        }
        /* buf now contains the full object content as a null-terminated byte array. */
    } else {
        printf("get object to buffer failed\n");
    }

    /* Release the memory pool. This frees all memory allocated during the request. */
    aos_pool_destroy(pool);

    /* Release global resources initialized at startup. */
    aos_http_io_deinitialize();
    return 0;
}

Placeholder reference

The examples in this topic use the public endpoint of the China (Hangzhou) region. To access OSS from another Alibaba Cloud service in the same region, use the internal endpoint instead. For endpoint details, see Regions and endpoints. To create an OSSClient instance using a custom domain name or Security Token Service (STS), see Initialization.

What's next

• For the complete sample, see the GitHub example.

• For the GetObject API reference, see GetObject.