(MFC) Read S3 Object Metadata of File Already Uploaded to S3

Demonstrates how to retrieve the metadata from an S3 object.

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 <CkHttp.h>
#include <CkMime.h>
#include <CkStringBuilder.h>

void ChilkatSample(void)
    {
    CkString strOut;

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

    CkHttp http;

    // Insert your AWS keys here:
    http.put_AwsAccessKey("AWS_ACCESS_KEY");
    http.put_AwsSecretKey("AWS_SECRET_KEY");

    const char *bucketName = "chilkat.ocean";
    const char *objectName = "seahorse.jpg";

    // 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.

    // A HEAD request can be sent to return the response header without the response body.
    // The S3_FileExists method sends a HEAD request.
    // It can be used to get the response header.
    int retval = http.S3_FileExists(bucketName,objectName);
    if (retval < 0) {
        strOut.append("Failed to check for the S3 object existence");
        strOut.append("\r\n");
        strOut.append(http.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    if (retval == 0) {
        strOut.append("The S3 object does not exist.");
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // The response header is available in the LastResponseHeader property.
    const char *responseHeader = http.lastResponseHeader();
    strOut.append("Response header:");
    strOut.append("\r\n");
    strOut.append(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

    // HTTP requests and responses are MIME.  For easy parsing, the response header
    // can be loaded into a Chilkat MIME object
    CkMime mime;

    mime.LoadMime(responseHeader);

    // Examine the metadata values:
    strOut.append("x-amz-meta-species: ");
    strOut.append(mime.getHeaderField("x-amz-meta-species"));
    strOut.append("\r\n");
    strOut.append("x-amz-meta-genus: ");
    strOut.append(mime.getHeaderField("x-amz-meta-genus"));
    strOut.append("\r\n");
    strOut.append("x-amz-meta-habitat: ");
    strOut.append(mime.getHeaderField("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 = mime.get_NumHeaderFields();
    CkStringBuilder sbName;
    while (i < numHeaders) {
        sbName.SetString(mime.getHeaderFieldName(i));
        if (sbName.StartsWith("x-amz-meta",false) == true) {
            strOut.append(sbName.getAsString());
            strOut.append(": ");
            strOut.append(mime.getHeaderFieldValue(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());

    }