(MFC) Fetch S3 Object Metadata

Demonstrates how to get the metadata for an S3 object using the REST API.

The HEAD operation retrieves metadata from an object without returning the object itself. This operation is useful if you are interested only in an object's metadata. To use HEAD, you must have READ access to the object.

A HEAD request has the same options as a GET operation on an object. The response is identical to the GET response except that there is no response body.

Chilkat C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

#include <CkRest.h>
#include <CkAuthAws.h>
#include <CkStringBuilder.h>

void ChilkatSample(void)
    {
    CkString strOut;

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkRest rest;

    // Connect to the Amazon AWS REST server using the correct region (in this example, "us-west-2")
    bool bTls = true;
    int port = 443;
    bool bAutoReconnect = true;
    bool success = rest.Connect("s3-us-west-2.amazonaws.com",port,bTls,bAutoReconnect);

    // Provide AWS credentials for the REST call.
    CkAuthAws authAws;
    authAws.put_AccessKey("AWS_ACCESS_KEY");
    authAws.put_SecretKey("AWS_SECRET_KEY");
    authAws.put_ServiceName("s3");
    // Make sure the Region agrees with the region in the Connect.
    authAws.put_Region("us-west-2");
    success = rest.SetAuthAws(authAws);

    // User-defined metadata are name/value pairs, and are returned in the HTTP response header.
    // Metadata header names begin with "x-amz-meta-" to distinguish them from other HTTP headers.
    // Note that Amazon S3 stores user-defined metadata keys in lowercase.

    // Set the bucket name via the HOST header.
    // In this case, the bucket name is "chilkat.ocean".
    rest.put_Host("chilkat.ocean.s3.amazonaws.com");

    // Send the HEAD request.
    success = rest.SendReqNoBody("HEAD","/seahorse.jpg");
    if (success != true) {
        strOut.append(rest.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Read the response header.
    int responseStatusCode = rest.ReadResponseHeader();
    if (responseStatusCode < 0) {
        strOut.append(rest.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    strOut.append("Response status code = ");
    strOut.appendInt(responseStatusCode);
    strOut.append("\r\n");
    if (responseStatusCode != 200) {
        strOut.append(rest.responseHeader());
        strOut.append("\r\n");
        strOut.append("Object does not exist.");
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Show the full response header that was received:
    strOut.append("Response header:");
    strOut.append("\r\n");
    strOut.append(rest.responseHeader());
    strOut.append("\r\n");
    strOut.append("--");
    strOut.append("\r\n");

    // Here is an example response header:

    // 	x-amz-id-2: uS4Flff04M8x5YWajU231TP0ClBL19mMhuyfU5ZVQd6NsUHXVhHK+H3b0sjxY98Fujet1ejhyzk=
    // 	x-amz-request-id: 27950009AA8E68AA
    // 	Date: Mon, 23 Jan 2017 20:12:58 GMT
    // 	Last-Modified: Fri, 20 Jan 2017 00:22:57 GMT
    // 	ETag: "a8551f0a5437f43a796fca7623ee9232"
    // 	x-amz-meta-species: big-belly seahorse
    // 	x-amz-meta-genus: Hippocampus
    // 	x-amz-meta-habitat: shallow tropical and temperate waters
    // 	Accept-Ranges: bytes
    // 	Content-Type: image/jpg
    // 	Content-Length: 24388
    // 	Server: AmazonS3

    // Examine particular response headers (the object metadata headers..)
    strOut.append("x-amz-meta-species: ");
    strOut.append(rest.responseHdrByName("x-amz-meta-species"));
    strOut.append("\r\n");
    strOut.append("x-amz-meta-genus: ");
    strOut.append(rest.responseHdrByName("x-amz-meta-genus"));
    strOut.append("\r\n");
    strOut.append("x-amz-meta-habitat: ");
    strOut.append(rest.responseHdrByName("x-amz-meta-habitat"));
    strOut.append("\r\n");
    strOut.append("--");
    strOut.append("\r\n");

    // It is possible to iterate over the header fields to find all x-amz-meta* headers
    int i = 0;
    int numHeaders = rest.get_NumResponseHeaders();
    CkStringBuilder sbName;
    while (i < numHeaders) {
        sbName.SetString(rest.responseHdrName(i));
        if (sbName.StartsWith("x-amz-meta",false) == true) {
            strOut.append(sbName.getAsString());
            strOut.append(": ");
            strOut.append(rest.responseHdrValue(i));
            strOut.append("\r\n");
        }

        i = i + 1;
    }

    // The output:

    // 	x-amz-meta-species: big-belly seahorse
    // 	x-amz-meta-genus: Hippocampus
    // 	x-amz-meta-habitat: shallow tropical and temperate waters


    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }