This is a text-only version of the following page on https://raymii.org:
---
Title       : 	C++ set up cpp-httplib with SSL support with cMake
Author      : 	Remy van Elst
Date        : 	14-12-2020
URL         : 	https://raymii.org/s/tutorials/Cpp_set_up_cpp-httplib_with_ssl_support_with_cmake.md.html
Format      : 	Markdown/HTML
---



For a small personal project that talks to a few JSON API's and does some
data parsing I needed a header only C++ HTTP library. Header only because
that is the simplest way to include it in my project, just copy and paste
the file. I came across the project [cpp-httplib][1], which fits my needs,
does all the http methods, small, a few examples and it looks modern and 
has recent development commits. 

Setup and getting it working was easy, but as soon as I tried an https url, I
got an exception (` what():  'https' scheme is not supported.`). This guide
shows you how to setup cpp-httplib for SSL support with cmake. It took me 
a bit longer than I wanted to set it up correctly, so why not save you the 
effort.

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This is the example code we're compiling, as is in the readme, for HTTP:

    httplib::Client cli("http://jsonplaceholder.typicode.com");

    if (auto res = cli.Get("/todos/1")) {
        if (res->status == 200) {
          std::cout << res->body << std::endl;
        }
    }

Example output for HTTP:

    /home/remy/CLionProjects/example1/cmake-build-debug/example1
    {
      "userId": 1,
      "id": 1,
      "title": "delectus aut autem",
      "completed": false
    }

For HTTPS you must read the entire article, at the bottom is the correct
code and CmakeLists.txt, since just changing the URL will not work.


### OpenSSL support via cmake

The [README][2] has one paragraph on SSL support, if you strip out the example,
it's more like one line:

> SSL support is available with CPPHTTPLIB OPENSSL SUPPORT. libssl and libcrypto should be linked.

On Ubuntu you should install the OpenSSL development libraries:

    apt install libssl-dev

For the rest I assume you have your development libraries installed and cmake 
set up.

In your `CmakeLists.txt` you should add the following to add OpenSSL:


    find_package(OpenSSL REQUIRED)
    if(OPENSSL_FOUND)
        set(HTTPLIB_IS_USING_OPENSSL TRUE)
    endif()


The `cpp-httplib` library also needs a few linker flags and compile options,
which you can add to cmake with the below lines:

    target_link_libraries(${PROJECT_NAME} PUBLIC
            $<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:OpenSSL::SSL>
            $<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:OpenSSL::Crypto>)

    target_compile_definitions(${PROJECT_NAME} PUBLIC
            $<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:CPPHTTPLIB_OPENSSL_SUPPORT>
            )

The compile definitons are a complicated way of doing `-DCPPHTTPLIB_OPENSSL_SUPPORT`, based on an earlier variable. 

These two blocks should be everything you need for your code to be able to 
link agains OpenSSL.

### Hostname without the https part

This took me another few minutes to find out. Each and every request I tried to do
failed, with a `httplib::Connection` error:

![error][5]

Nothing more, no specific SSL error, nothing. Turns out, this code doesn't work if you include the
scheme part inside the URL (`https://`):

    httplib::SSLClient cli("https://jsonplaceholder.typicode.com");

If you omit that part, it will work:

    httplib::SSLClient cli("jsonplaceholder.typicode.com");

It doesn't help that there is no error message and that the `.Error()` method
just says `Connection`. What is happening here, more than just "something
wrong". Took me a few more tries and looking at the unit tests to figure out
what the intended behaviour should be.

You can use the preprocessor macro to check if you can use SSL, example below:

    #ifdef CPPHTTPLIB_OPENSSL_SUPPORT
            httplib::SSLClient cli(_domain);
            cli.enable_server_certificate_verification(true);
    #else
            httplib::Client cli(_domain);
    #endif

Make sure to catch any exceptions and check for nullpointers.

### Conclusion & Full Code

Setting up the correct cMake flags and troubleshooting why code that should work doesn't work 
took me more time than I wanted, so I hope this guide saves you some time. Below you'll find
the full example code.

CmakeLists.txt:

    cmake_minimum_required(VERSION 3.17)
    project(Example1)

    set(CMAKE_CXX_STANDARD 17)

    find_package(OpenSSL REQUIRED)
    if(OPENSSL_FOUND)
        set(HTTPLIB_IS_USING_OPENSSL TRUE)
    endif()

    add_executable(${PROJECT_NAME} main.cpp)

    target_link_libraries(${PROJECT_NAME} PUBLIC
            $<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:OpenSSL::SSL>
            $<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:OpenSSL::Crypto>)

    target_compile_definitions(${PROJECT_NAME} PUBLIC
            $<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:CPPHTTPLIB_OPENSSL_SUPPORT>
            )

main.cpp

    int main() {
        #ifdef CPPHTTPLIB_OPENSSL_SUPPORT
            httplib::SSLClient cli("jsonplaceholder.typicode.com");
            cli.enable_server_certificate_verification(true);
        #else
            httplib::Client cli(http://jsonplaceholder.typicode.com);
        #endif

        if (auto res = cli.Get("/todos/1")) {
            if (res->status == 200) {
              std::cout << res->body << std::endl;
            }
        }
        return 0;
    }

Output:

    /home/remy/CLionProjects/example1/cmake-build-debug/example1
    {
      "userId": 1,
      "id": 1,
      "title": "delectus aut autem",
      "completed": false
    }


[1]: https://github.com/yhirose/cpp-httplib
[2]: https://github.com/yhirose/cpp-httplib#openssl-support
[5]: /s/inc/img/httplib-ssl.png

---

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