Object Storage Service:Manage object metadata (C SDK)

Prerequisites

Before you begin, ensure that you have:

• The OSS C SDK installed with the build environment configured

• The OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables set with valid credentials

• The oss:PutObject permission to set object metadata

• The oss:GetObject permission to retrieve object metadata

For permission setup, see Attach a custom policy to a RAM user.

Usage notes

• The examples use the public endpoint for the China (Hangzhou) region. To access OSS from other Alibaba Cloud services in the same region, use the internal endpoint instead. For details, see Regions and endpoints.

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

Set and retrieve object metadata

The following example uploads an object with standard HTTP headers and user-defined metadata, then retrieves the metadata using oss_get_object_meta.

You can call oss_get_object_meta after upload or before reading the object to retrieve the object's metadata, such as its length and type.

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

/* Replace with the endpoint for your bucket's region.
   Example for China (Hangzhou): https://oss-cn-hangzhou.aliyuncs.com */
const char *endpoint = "yourEndpoint";

/* Bucket and object path */
const char *bucket_name = "examplebucket";
const char *object_name = "exampledir/exampleobject.txt";
const char *object_content = "hello world";

/* Replace with your bucket's region ID. Example: cn-hangzhou */
const char *region = "yourRegion";

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

    aos_str_set(&options->config->endpoint, endpoint);

    /* Load credentials from environment variables.
       Set OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET before running. */
    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"));

    /* Required for Signature V4 */
    aos_str_set(&options->config->region, region);
    options->config->signature_version = 4;

    /* Set to 0: no CNAME */
    options->config->is_cname = 0;

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

int main(int argc, char *argv[])
{
    /* Initialize global network and memory resources */
    if (aos_http_io_initialize(NULL, 0) != AOSE_OK) {
        exit(1);
    }

    aos_pool_t *pool;
    aos_pool_create(&pool, NULL);

    oss_request_options_t *oss_client_options;
    oss_client_options = oss_request_options_create(pool);
    init_options(oss_client_options);

    /* Initialize request parameters */
    aos_string_t bucket;
    aos_string_t object;
    aos_table_t *headers;
    aos_list_t buffer;
    aos_table_t *resp_headers = NULL;
    aos_status_t *resp_status = NULL;
    aos_buf_t *content = NULL;
    char *content_length_str = NULL;
    char *object_type = NULL;
    char *object_author = NULL;
    int64_t content_length = 0;

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

    /* Set metadata headers before upload */
    headers = aos_table_make(pool, 2);
    apr_table_set(headers, "Expires", "Fri, 28 Feb 2032 05:38:42 GMT");  /* Standard HTTP header */
    apr_table_set(headers, "x-oss-meta-author", "oss");                  /* User-defined metadata */

    /* Build the request body from a buffer */
    aos_list_init(&buffer);
    content = aos_buf_pack(oss_client_options->pool, object_content, strlen(object_content));
    aos_list_add_tail(&content->node, &buffer);

    /* Upload the object with metadata */
    resp_status = oss_put_object_from_buffer(oss_client_options, &bucket, &object,
                   &buffer, headers, &resp_headers);
    if (aos_status_is_ok(resp_status)) {
        printf("put object from buffer with md5 succeeded\n");
    } else {
        printf("put object from buffer with md5 failed\n");
    }

    /* Retrieve object metadata (no object body is downloaded) */
    resp_status = oss_get_object_meta(oss_client_options, &bucket, &object, &resp_headers);
    if (aos_status_is_ok(resp_status)) {
        content_length_str = (char*)apr_table_get(resp_headers, OSS_CONTENT_LENGTH);
        if (content_length_str != NULL) {
            content_length = atol(content_length_str);
        }
        object_author = (char*)apr_table_get(resp_headers, OSS_AUTHORIZATION);
        object_type = (char*)apr_table_get(resp_headers, OSS_OBJECT_TYPE);
        printf("get object meta succeeded, object author:%s, object type:%s, content_length:%ld\n",
               object_author, object_type, content_length);
    } else {
        printf("req:%s, get object meta failed\n", resp_status->req_id);
    }

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

References

• Complete sample code: GitHub

• API reference: GetObjectMeta