@SGaist
You can link external .dll files for your point You don't link to .dll files. . I was able to do that for an external library linking with QT using sqlite3.dll and it worked fine. Not sure why you referred that.

@tham
recently I found the same issue with those deep learning libraries written by python, the truth is, many developers do not care as long as their libs work on linux.
True. I found that. But QT is not like that. QT is a commercial product for sale. Thats what makes a product/concept mature and marketable.

I definitely believe there needs to be better documentation on minor issues and tweaks for product like QT. Frankly, its one of the best I found for C++ UI Controls and the performance is... good for the features it provides.

One of the biggest issues of C++ community and adoption is the project set up documentation and project deployment documentation is not good.

I share your pain, but this is not just the problem of c++, recently I found the same issue with those deep learning libraries written by python, the truth is, many developers do not care as long as their libs work on linux.

May I ask you why you refuse to use the network modules provided by Qt5?

Here is what resolved my issue for developers who face the issue of External Library not linking correctly to a QT project in windows:

1. I added the .lib / .dll.a file with full path (from C:) in .pro
2. I added the include full path (from C:) in .pro
3. I added the curl.exe and libcurl.exe in the release folders along with other dependency .dlls

Detailed steps:

1. Instead of the relative path, I gave the actual full path to library file in the .pro file and it resolved my issue. Here are the file paths I added.
   C:\git\QT\APITester\vendor\curl\lib\libcurl.dll.a or C:\git\QT\APITester\vendor\curl\lib\libcurl.lib and

2. I added the include full path: C:\git\QT\APITester\vendor\curl\include

3. I added the curl.exe and libcurl.dll in the release folder (main folder) with other dependency .dlls of QT and created a release build.

The above steps work as well for debug but you have to have dlld files there for curl there

I do not know what config settings are getting messed up for this issue to happen. And most of all such linking issues with third party libraries are not documented well. One of the biggest issues of C++ community and adoption is the project set up documentation and project deployment documentation is not good.

This community saved me here. Thanks all for the effort and support. Thanks to @mrjj and @SGaist who spent time to answer my questions.

You don't link to .dll files.

Note that you haven't answered @mrjj'question: which compiler are you using ?

try with
win32:CONFIG(release, debug|release): LIBS += -L$$PWD/vendor/curl/lib/ -lcurl.dll
( that is remove the lib start of the name. here.)
make release build.

Sorry its hard to offer more input without the compiler output :)

I think its nearly correct. path seems fine etc.

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = APITester
TEMPLATE = app

DEFINES += QT_DEPRECATED_WARNINGS

#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000

SOURCES += main.cpp
mainwindow.cpp
apitab.cpp \

HEADERS += mainwindow.h
apitab.h \

FORMS += mainwindow.ui
apitab.ui \

DISTFILES +=

RESOURCES +=

INCLUDEPATH +=

LIBS +=

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/vendor/curl/lib/ -llibcurl.dll
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/vendor/curl/lib/ -llibcurl.dlld
else:unix: LIBS += -L$$PWD/vendor/curl/lib/ -llibcurl.dll

INCLUDEPATH += $$PWD/vendor/curl/include \

DEPENDPATH += $$PWD/vendor/curl/include

win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/vendor/curl/lib/libcurl.dll.a
else:win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/vendor/curl/lib/libcurl.dlld.a
else:win32:!win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/vendor/curl/lib/libcurl.dll.lib
else:win32:!win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/vendor/curl/lib/libcurl.dlld.lib
else:unix: PRE_TARGETDEPS += $$PWD/vendor/curl/lib/libcurl.dll.a

==========================

The folder structure is :

projectfolder/
file.cpp (and others)
file.h
main.cpp
vendor/
vendor/curl/
vendor/curl/include/
vendor/curl/include/curl.h (and others)
vendor/curl/lib/
vendor/curl/lib/libcurl.dll.a

Errors are follow (one of them is. Its one of the functions curl exposes) :

undefined reference to _imp__curl_easy_init()'

This is my import :
#include <curl/curl.h>

This is my code within my widget constructor

CURL *curl;
CURLcode res;

  curl = curl_easy_init();
  if(curl) {
      curl_easy_setopt(curl, CURLOPT_URL, "http://google.com");
      curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
      res = curl_easy_perform(curl);
      if(res != CURLE_OK){
          qWarning("curl_easy_perform() failed: \n");
      }
    curl_easy_cleanup(curl);

Second, I have downloaded all the source. Not the built ones so I am able to use that in both windows and linux after compilation. I have not built the libCurl first but I have tried putting the binary in the lib folder and tried but it did not work.

When they are not full as in
c:/vendor/xxx
then often they are relative
../../vendor/xxxx

Yours seems to be neither ?

Also, its unclear if you build it all your self or you try to use PRECOMPILED dlls ?
Did you fist build the actual libCurl ?