I'm learning how to work with DLLs in Windows using C++, specifically with the LoadLibrary and GetProcAddress functions.

For practice, I've chosen libcurl as an example to experiment with dynamic linking. I managed to use libcurl as a DLL, but I had to define several constants in my own .cpp file, such as:

#define CURLOPT_URL                 10002   
#define CURLOPT_POST                47      
#define CURLOPT_POSTFIELDS          10015           
#define CURLOPT_WRITEFUNCTION       20011
#define CURLOPT_WRITEDATA           10001
#define CURLE_OK                    0

These are used in my code like this:

    curlFunctions->easy_setopt(curl, CURLOPT_URL, request.url.c_str());
    curlFunctions->easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeCallback);
    curlFunctions->easy_setopt(curl, CURLOPT_WRITEDATA, response);

My question is:

Since I'm dynamically linking to libcurl.dll and not statically linking to libcurl.lib, do I still need to include curl.h in my project? Or, can I just use LoadLibrary and GetProcAddress to load and call functions from the DLL without the need for including the header?

I apologize if this is a basic question - I'm still learning the WinAPI and C++, and am trying to build a deeper understanding of these concepts.

EDIT

For clarification:

I added manually the #defines in my project so I could test the LoadLibrary and GetProcAddress functions. It worked, and I could make HTTP requests.

Then, I tried to add curl.h (so I don't have to declare manually the #defines needed to perform the curl operations), but I get a bunch of warnings, because the compiler looks inside curl.h and sees a lot of definitions, but of course I didn't implement them. I am importing the functions from the DLL using GetProcAddress.

Warnings example (posting just 4, but there are 35 in total, all the same "function definition xxxxx not found"):

1. Function definition for 'curl_strequal' not found.

2. Function definition for 'curl_strnequal' not found.

3. Function definition for 'curl_mime_init' not found.

4. Function definition for 'curl_mime_free' not found.*

(...)

Example of how I am importing from libcurl.dll (I cleaned it up a litlt bit just to show the "main" functions usage):

hModuleLibCurl = LoadLibrary(TEXT("libcurl-x64.dll"));
if (!hModuleLibCurl) {
    return;
}

// Load function pointers
easy_init = (CurlEasyInit)GetProcAddress(hModuleLibCurl, "curl_easy_init");
easy_cleanup = (CurlEasyCleanup)GetProcAddress(hModuleLibCurl, "curl_easy_cleanup");
easy_setopt = (CurlEasySetopt)GetProcAddress(hModuleLibCurl, "curl_easy_setopt");
easy_perform = (CurlEasyPerform)GetProcAddress(hModuleLibCurl, "curl_easy_perform");
easy_getinfo = (CurlEasyGetinfo)GetProcAddress(hModuleLibCurl, "curl_easy_getinfo");
slist_append = (CurlSlistAppend)GetProcAddress(hModuleLibCurl, "curl_slist_append");

// Check if any of the function pointers failed to load
if (!easy_init || !easy_cleanup || !easy_setopt || !easy_perform || !easy_getinfo || !slist_append) {
    FreeLibrary(hModuleLibCurl);
}

That's how I am using functions from inside the DLL. But, in order the perform operations, I need the CURLOPT_... values (the ones that, initially I defined manually to test the code). So, after adding curl.h, I get the warnings.

I think the main question is:

When dynamically loading a .dll and using its functions, how to deal with the values that have been DEFINED (with #define) inside the library? Should I statically link the header file, even though I'm trying to only load it dynamically? If so, how to deal with the warnings? A .h contains way more functions than needs to be defined, but I don't need them all in my project.

I'm learning how to work with DLLs in Windows using C++, specifically with the LoadLibrary and GetProcAddress functions.

For practice, I've chosen libcurl as an example to experiment with dynamic linking. I managed to use libcurl as a DLL, but I had to define several constants in my own .cpp file, such as:

#define CURLOPT_URL                 10002   
#define CURLOPT_POST                47      
#define CURLOPT_POSTFIELDS          10015           
#define CURLOPT_WRITEFUNCTION       20011
#define CURLOPT_WRITEDATA           10001
#define CURLE_OK                    0

These are used in my code like this:

    curlFunctions->easy_setopt(curl, CURLOPT_URL, request.url.c_str());
    curlFunctions->easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeCallback);
    curlFunctions->easy_setopt(curl, CURLOPT_WRITEDATA, response);

My question is:

Since I'm dynamically linking to libcurl.dll and not statically linking to libcurl.lib, do I still need to include curl.h in my project? Or, can I just use LoadLibrary and GetProcAddress to load and call functions from the DLL without the need for including the header?

I apologize if this is a basic question - I'm still learning the WinAPI and C++, and am trying to build a deeper understanding of these concepts.

EDIT

For clarification:

I added manually the #defines in my project so I could test the LoadLibrary and GetProcAddress functions. It worked, and I could make HTTP requests.

Then, I tried to add curl.h (so I don't have to declare manually the #defines needed to perform the curl operations), but I get a bunch of warnings, because the compiler looks inside curl.h and sees a lot of definitions, but of course I didn't implement them. I am importing the functions from the DLL using GetProcAddress.

Warnings example (posting just 4, but there are 35 in total, all the same "function definition xxxxx not found"):

1. Function definition for 'curl_strequal' not found.

2. Function definition for 'curl_strnequal' not found.

3. Function definition for 'curl_mime_init' not found.

4. Function definition for 'curl_mime_free' not found.*

(...)

Example of how I am importing from libcurl.dll (I cleaned it up a litlt bit just to show the "main" functions usage):

hModuleLibCurl = LoadLibrary(TEXT("libcurl-x64.dll"));
if (!hModuleLibCurl) {
    return;
}

// Load function pointers
easy_init = (CurlEasyInit)GetProcAddress(hModuleLibCurl, "curl_easy_init");
easy_cleanup = (CurlEasyCleanup)GetProcAddress(hModuleLibCurl, "curl_easy_cleanup");
easy_setopt = (CurlEasySetopt)GetProcAddress(hModuleLibCurl, "curl_easy_setopt");
easy_perform = (CurlEasyPerform)GetProcAddress(hModuleLibCurl, "curl_easy_perform");
easy_getinfo = (CurlEasyGetinfo)GetProcAddress(hModuleLibCurl, "curl_easy_getinfo");
slist_append = (CurlSlistAppend)GetProcAddress(hModuleLibCurl, "curl_slist_append");

// Check if any of the function pointers failed to load
if (!easy_init || !easy_cleanup || !easy_setopt || !easy_perform || !easy_getinfo || !slist_append) {
    FreeLibrary(hModuleLibCurl);
}

That's how I am using functions from inside the DLL. But, in order the perform operations, I need the CURLOPT_... values (the ones that, initially I defined manually to test the code). So, after adding curl.h, I get the warnings.

I think the main question is:

When dynamically loading a .dll and using its functions, how to deal with the values that have been DEFINED (with #define) inside the library? Should I statically link the header file, even though I'm trying to only load it dynamically? If so, how to deal with the warnings? A .h contains way more functions than needs to be defined, but I don't need them all in my project.

• c++
• windows
• dll
• loadlibrary

• 3 "do I still need to include libcurl.h in my project" -- Does libcurl.h define the CURLOPT_* macros for you? Does it declare easy_setopt()? Did you try compiling without including the header? – JaMiT Commented Jan 18 at 1:05
• The curl.h (sorry i said libcurl.h, i am going to fix in the question) defines the CURLOPT_* macros (what i need). The functions, like easy_setopt(), don't have their implementation in the curl.h file ( i got them using GetProcAddress - windows) – Adriano Pinheiro Commented Jan 18 at 1:20
• 1 "The curl.h [...] defines the CURLOPT_ macros"* -- So why did you define these constants in your own .cpp file instead of including the header? – JaMiT Commented Jan 18 at 1:30
• i didnt import the .h yet, because it generates a lot of warnings (a lot of things without definition) and because i was trying to understand the general flow... Actually i thought it should have another way to make it. So, in order to work i created them manually. Again.. just learning – Adriano Pinheiro Commented Jan 18 at 1:38
• 1 "i didnt import the .h yet, because it generates a lot of warnings" -- In other words, when the right way to do something did not work, you decided to improvise your own approach. It's only when your improvised approach failed that you asked for help. Have you heard of "the XY problem"? – JaMiT Commented Jan 18 at 1:53

1 Answer 1

You should #include <curl.h> if you want to use facilities provided by libcurl, regardless of the way you want to use them: static linking, dynamic linking, or via LoadLibrary and GetProcAddress.

You should not manually copy any #define directives or anything else from curl.h to your own code. There are many reasons why such manual copying is a bad idea. #include <curl.h> literally copies the content of curl.h to your source file each time you compile it. That's the way header files are meant to be used.

If you have a problem with building your program when having #include <curl.h> in your code, you should ask a question about that problem.