WinHTTP on kenji.blog

How to post a message to Slack using Slack App + OAuth2

Wed, 16 Jul 2025 23:36:27 +0900

✅ Posting Method Using an Access Token (Slack Web API)

Slack’s “OAuth Token (such as xoxb-…)” is a key used to post using Slack’s Web API. In this case, unlike Webhooks, you POST to the Slack API Endpoint with an Authorization: Bearer header.

🔑 Required Prerequisites

You need to include chat:write in the OAuth Scopes of your Slack app:

Setup Steps

Go to https://api.slack.com/apps
Create an app or select an existing one
Under “OAuth & Permissions” > Scopes → Add chat:write
“Install to Workspace” or “Reinstall” to get the Access Token (e.g., xoxb-xxxxxxxxxx)

💻 C++ Code (Posting to Slack API using WinHTTP)

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#include <windows.h>
#include <winhttp.h>
#include <iostream>

#pragma comment(lib, "winhttp.lib")

bool PostSlackMessage(const std::wstring& accessToken, const std::string& channel, const std::string& text) {
 const wchar_t* host = L"slack.com";
 const wchar_t* path = L"/api/chat.postMessage";

 HINTERNET hSession = WinHttpOpen(L"SlackPoster/1.0",
 WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
 WINHTTP_NO_PROXY_NAME,
 WINHTTP_NO_PROXY_BYPASS, 0);
 if (!hSession) return false;

 HINTERNET hConnect = WinHttpConnect(hSession, host, INTERNET_DEFAULT_HTTPS_PORT, 0);
 if (!hConnect) return false;

 HINTERNET hRequest = WinHttpOpenRequest(hConnect, L"POST", path,
 NULL, WINHTTP_NO_REFERER,
 WINHTTP_DEFAULT_ACCEPT_TYPES,
 WINHTTP_FLAG_SECURE);

 // Authorization header and Content-Type
 std::wstring headers = L"Content-Type: application/json\r\n";
 headers += L"Authorization: Bearer " + accessToken + L"\r\n";

 // JSON body
 std::string body = R"({"channel":")" + channel + R"(","text":")" + text + R"("})";

 BOOL result = WinHttpSendRequest(hRequest,
 headers.c_str(),
 (DWORD)-1,
 (LPVOID)body.c_str(),
 body.length(),
 body.length(),
 0);

 if (!result || !WinHttpReceiveResponse(hRequest, NULL)) {
 std::cerr << "Send error\n";
 return false;
 }

 // Check status code
 DWORD statusCode = 0;
 DWORD size = sizeof(statusCode);
 WinHttpQueryHeaders(hRequest,
 WINHTTP_QUERY_STATUS_CODE | WINHTTP_QUERY_FLAG_NUMBER,
 WINHTTP_HEADER_NAME_BY_INDEX,
 &statusCode, &size, WINHTTP_NO_HEADER_INDEX);

 WinHttpCloseHandle(hRequest);
 WinHttpCloseHandle(hConnect);
 WinHttpCloseHandle(hSession);

 return (statusCode == 200);
}

int main() {
 std::wstring token = L"xoxb-your-access-token"; // Access Token
 std::string channel = "channel-ID-or-#general"; // Example: "#general" or "C0123456789"
 std::string message = "I tried posting to Slack from C++!";

 if (PostSlackMessage(token, channel, message)) {
 std::cout << "Post successful!\n";
 } else {
 std::cerr << "Post failed.\n";
 }

 return 0;
}

📌 How to Get the Channel ID

Using just the channel name (e.g., #general) may sometimes result in an error. A more reliable method is to use the ID found in the URL of the Slack channel page.

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https://app.slack.com/client/Txxxxx/C0123456789
 ↑ This part is the Channel ID

✅ About Token Types (Supplementary)

Token Format	Purpose	Example Use Case
`xoxb-...`	Bot Token (Recommended)	Post, edit, delete
`xoxp-...`	User Token (Not Recommended)	Non-bot operations
Refresh Token	Long-term token renewal	Usually not used

📝 Summary

To post to Slack from C++, the Slack Web API + Bearer Token method is reliable.
Use the chat.postMessage endpoint with HTTPS POST.
The token must be sent in the Bearer header.
Specifying the channel by ID is recommended.

How to Post Messages to Slack using C++ (Win32 API + WinHTTP) [Webhook Supported]

Wed, 16 Jul 2025 19:42:56 +0900

How to Post Messages to Slack using C++ (Win32 API + WinHTTP) [Webhook Supported]

I want to post messages to Slack from C++. It’s common in Node.js or Python, but doing it with “C++ × Win32 API × WinHTTP” is quite rare, isn’t it?

In this article, I will explain how to send messages from C++ to Slack using a Webhook URL, step-by-step and in an easy-to-understand manner.

✅ Overall Flow

To post to Slack, follow these steps:

Obtain a Slack Webhook URL (API key)
Send a POST request using WinHTTP
Assemble the message body in JSON format
Check the result and you’re done!

🔑 Step 1: How to Obtain a Slack Webhook URL

Slack allows you to easily post messages from external services using a feature called Incoming Webhooks.

Steps to Obtain

Access https://api.slack.com/apps
Click Create New App
Choose From scratch, then select the app name and the workspace to post to
Select “Incoming Webhooks” from the left menu and enable it
Click “Add New Webhook to Workspace” and select a channel
Copy the generated URL (e.g., https://hooks.slack.com/services/xxx/yyy/zzz)

This URL functions like an API key.

💻 Step 2: Send a Message to Slack with C++ Code

Technologies Used

Win32 API
WinHTTP (Standard Library)
JSON formatted messages

Sample Code (Posting to Slack)

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#include <windows.h>
#include <winhttp.h>
#include <iostream>

#pragma comment(lib, "winhttp.lib")

bool PostToSlack(const std::wstring& webhookUrl, const std::string& messageJson) {
 // Parse URL
 URL_COMPONENTS urlComp{};
 wchar_t hostName[256];
 wchar_t urlPath[1024];

 urlComp.dwStructSize = sizeof(urlComp);
 urlComp.lpszHostName = hostName;
 urlComp.dwHostNameLength = _countof(hostName);
 urlComp.lpszUrlPath = urlPath;
 urlComp.dwUrlPathLength = _countof(urlPath);

 if (!WinHttpCrackUrl(webhookUrl.c_str(), 0, 0, &urlComp)) {
 std::wcerr << L"Failed to parse URL\n";
 return false;
 }

 // Connect and start HTTP session
 HINTERNET hSession = WinHttpOpen(L"SlackPoster/1.0",
 WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
 WINHTTP_NO_PROXY_NAME,
 WINHTTP_NO_PROXY_BYPASS, 0);
 HINTERNET hConnect = WinHttpConnect(hSession, hostName, urlComp.nPort, 0);
 HINTERNET hRequest = WinHttpOpenRequest(hConnect, L"POST", urlPath,
 NULL, WINHTTP_NO_REFERER,
 WINHTTP_DEFAULT_ACCEPT_TYPES,
 WINHTTP_FLAG_SECURE);

 std::wstring headers = L"Content-Type: application/json\r\n";
 BOOL result = WinHttpSendRequest(hRequest,
 headers.c_str(),
 -1,
 (LPVOID)messageJson.c_str(),
 messageJson.length(),
 messageJson.length(),
 0);

 if (!result) {
 std::cerr << "Send request failed\n";
 return false;
 }

 WinHttpReceiveResponse(hRequest, NULL);

 DWORD statusCode = 0;
 DWORD size = sizeof(statusCode);
 WinHttpQueryHeaders(hRequest,
 WINHTTP_QUERY_STATUS_CODE | WINHTTP_QUERY_FLAG_NUMBER,
 WINHTTP_HEADER_NAME_BY_INDEX,
 &statusCode, &size, WINHTTP_NO_HEADER_INDEX);

 // Release resources
 WinHttpCloseHandle(hRequest);
 WinHttpCloseHandle(hConnect);
 WinHttpCloseHandle(hSession);

 return (statusCode == 200);
}

int main() {
 std::wstring webhookUrl = L"https://hooks.slack.com/services/xxx/yyy/zzz"; // Replace with your Webhook URL

 std::string message = R"({
 "text": "Hello from C++ :rocket:",
 "username": "C++ Bot",
 "icon_emoji": ":robot_face:"
 })";

 if (PostToSlack(webhookUrl, message)) {
 std::cout << "Posted successfully!\n";
 } else {
 std::cerr << "Failed to post.\n";
 }

 return 0;
}

🧪 Customizing JSON Messages

With Slack Webhooks, you can include parameters like the following:

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{
 "text": "Notification content",
 "username": "Bot name",
 "icon_emoji": ":rocket:",
 "channel": "#desired_channel_name"
}

📌 Supplementary Notes

Content-Type must be specified as "application/json"
Pass the Webhook URL as a wstring without any changes (URL encoding is unnecessary)
Since it’s HTTPS communication, don’t forget WINHTTP_FLAG_SECURE

🎉 Bonus: Example of Post Confirmation in Slack

It will be displayed in Slack like this:

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[C++ Bot]
Hello from C++ :rocket:

✍️ Summary

Item	Details
Posting Method	Webhook (Incoming Webhooks)
Communication Library	WinHTTP
Data Format	JSON
Usable Parameters	text, username, icon_emoji, channel, etc.

Even if you thought “Integrating C++ with Slack? No way…”, you can start embedding a notification bot today!

🚀 Teaser for Next Time?

If you’re interested, next time:

File attachments
UI with buttons
Flexible API operations with Slack App + OAuth2

I can introduce you to more advanced Slack integrations like these!

How to Post Messages to Microsoft Teams from C++ (WinHTTP + Graph API)

Mon, 14 Jul 2025 23:40:15 +0900

How to Post Messages to Microsoft Teams from C++ (WinHTTP + Graph API)

I want to automatically post to a Microsoft Teams chat –
Microsoft Graph API is exactly what you can use in such cases.
In this article, I will introduce a C++ code example using WinHTTP ** and the ** necessary API authentication steps step by step.

🔧 Necessary Preparations (Microsoft Graph API Authentication Settings)

1. Register App in Azure Portal

First, to use the Microsoft Graph API, you need to register an app in Azure.

Access the Azure Portal
“Microsoft Entra ID ” > “+ Add ” > “ App registrations ” > “ New registration”
Enter any app name and click “Register”

2. Add API Permissions

Go to the “API permissions” left menu
Under “Microsoft Graph ” > “ Add a permission ”, search for the following scopes and click “ Update permissions”

Chat.ReadWrite
User.Read

If you want to post to a channel, ChannelMessage.Send is also required

3. Note Client ID and Tenant ID

Make a note of the following two items displayed on the “Overview” tab:

Application (client) ID
Directory (tenant) ID

4. Create Client Secret

Go to the “Certificates & secrets” tab
“New client secret” > set the expiration date, and click “Add”
Be sure to note ** the displayed value (secret) ** on the spot

🔐 Obtaining an Access Token (OAuth2)

Use the client_credentials flow to obtain it.
Run the following command with curl to obtain an access token.

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curl -X POST ^
 https://login.microsoftonline.com/{tenant_id}/oauth2/v2.0/token ^
 -H "Content-Type: application/x-www-form-urlencoded" ^
 -d "client_id={client_id}" ^
 -d "scope=https%3A%2F%2Fgraph.microsoft.com%2F.default" ^
 -d "client_secret={client_secret}" ^
 -d "grant_type=client_credentials"

Example Response

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{
 "token_type":"Bearer",
 "expires_in":3599,
 "ext_expires_in":3599,
 "access_token": "eyJ0eXAiOiJKV1QiLCJub... (omitted)"
}

Call the Microsoft Graph API using this access_token.

💬 C++ Sample for Posting to Teams Chat

Here is a C++ example of posting to a chat using WinHTTP.

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#include <windows.h>
#include <winhttp.h>
#include <iostream>
#include <string>

#pragma comment(lib, "winhttp.lib")

void PostToTeamsChat(const std::wstring& accessToken, const std::wstring& chatId, const std::wstring& message) {
 HINTERNET hSession = WinHttpOpen(L"TeamsPoster/1.0", WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
 WINHTTP_NO_PROXY_NAME, WINHTTP_NO_PROXY_BYPASS, 0);

 HINTERNET hConnect = WinHttpConnect(hSession, L"graph.microsoft.com", INTERNET_DEFAULT_HTTPS_PORT, 0);

 std::wstring endpoint = L"/v1.0/chats/" + chatId + L"/messages";
 HINTERNET hRequest = WinHttpOpenRequest(hConnect, L"POST", endpoint.c_str(), NULL,
 WINHTTP_NO_REFERER, WINHTTP_DEFAULT_ACCEPT_TYPES, WINHTTP_FLAG_SECURE);

 std::wstring jsonBody = L"{\"body\": {\"content\": \"" + message + L"\"}}";
 std::wstring headers = L"Authorization: Bearer " + accessToken + L"\r\nContent-Type: application/json\r\n";

 BOOL bResult = WinHttpSendRequest(hRequest,
 headers.c_str(), (DWORD)-1L,
 (LPVOID)jsonBody.c_str(), (DWORD)(jsonBody.length() * sizeof(wchar_t)),
 (DWORD)(jsonBody.length() * sizeof(wchar_t)), 0);

 if (bResult)
 WinHttpReceiveResponse(hRequest, NULL);

 DWORD dwSize = 0;
 WinHttpQueryDataAvailable(hRequest, &dwSize);

 if (dwSize > 0) {
 std::wstring response(dwSize / sizeof(wchar_t), 0);
 DWORD dwDownloaded = 0;
 WinHttpReadData(hRequest, &response[0], dwSize, &dwDownloaded);
 std::wcout << L"Response: " << response << std::endl;
 }

 WinHttpCloseHandle(hRequest);
 WinHttpCloseHandle(hConnect);
 WinHttpCloseHandle(hSession);
}

int main() {
 std::wstring access;
 std::wstring chatId = L"19:";
 std::wstring message = L"Hello from C++!";
 std::wcout << L"Enter your access token: ";
 std::getline(std::wcin, access);
 std::wcout << L"Enter chat ID: ";
 std::getline(std::wcin, chatId);
 PostToTeamsChat(access, chatId, message);
 return 0;
}

🔍 How to Get Chat ID

You can check the Chat ID with GET /v1.0/me/chats.

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curl -X GET ^
 https://graph.microsoft.com/v1.0/me/chats ^
 -H "Authorization: Bearer {access_token}" ^
 -H "Content-Type: application/json"

Example Response

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{
 "value": [
 {
 "id": "19:abc123xyz@thread.v2",
 "topic": null,
 "chatType": "oneOnOne"
 }
 ]
}

📌 Points to Note

This sample is a minimal implementation. In actual operation:
- Token expiration handling
- HTTPS certificate validation
- Enhanced error handling
For channel posting, use teams/{team-id}/channels/{channel-id}/messages.
Sending attachments requires multipart processing or the Graph Drive API.

Summary

📎 Summary

Feature	Overview
Graph API	Official API to interact with Teams
App Registration	Required authentication procedures on Azure
Access Token	Obtained via OAuth2, used for requests
C++ Implementation	Call Graph API using WinHTTP

🚀 Next Steps

As more advanced samples, the following are also possible:

Team channel posting (/teams/{team-id}/channels/...)
Posting with attachments
Switching between bot accounts and user accounts
Full C++ implementation including token acquisition

If you have any requests, please feel free to comment!