Can you please show us some of those errors too?

Ok, yes the unit tests can only build with debug versions of libcurl since it requires that a bunch of otherwise hidden symbols are available - to get tested. As they're not part of the regular API, they're not reachable in a normal build.

I suppose the cmake build files need to skip building the unit tests unless that condition is met. That's what the autoconf based build does.

I should add that all the 16xx tests (that are in tests/unit) are such unit tests.

A cmake option to disable all tests would be nice,
When you use cmake to build for Visual Studio you end up with a single solution and you cannot pick and choose which projects build to without using the GUI (which is not good for automated builds).

-DBUILD_TESTING=OFF

I tried building the tests just now, I also ran into problems.

Right, the cmake build should only try to build the unit tests when asked to build debug builds as otherwise it fails like this...

@bagder @SparhawkSoftware @jay In fact Curl_hash_init symbols will be available if -DCURL_STATICLIB=ON because it's not exported as dllexport attribute.

@SparhawkSoftware So if you want to fix build use -DBUILD_TESTING=OFF or enable static linking with -DCURL_STATICLIB=ON (check appveyor configuration here https://ci.appveyor.com/project/curlorg/curl)

It is probably still a good idea to have the build script check if the unit tests can get built before doing that, to avoid this build error.

@bagder Linux exports all symbols by default. So all Curl_hash is in libcurl.so library.
So here you should decide if same for Windows and Curl_hash... should use dllexport.

Aha! but that's a bug - it should hide symbols by default and what's what it does in the autoconf build! That's why the "regular" build can't be used for the unit test. Also, some unit tests use symbols that are otherwise marked static and I'm not sure they're always made available even if symbols aren't hidden.

Confirmed that -DBUILD_TESTING=OFF fixes my issue.

I should have noticed that flag in the CMAKE.

Adding this near the top would be nice:
option(BUILD_TESTING "This includes the unit tests. Only valid when building the static library" OFF)

The build should simply not build the unit tests if it can't link them...

@SparhawkSoftware According to documentation this option already there and ON by default.

@bagder So what's the decision? Are Curl_hash_ functions optional? From which build option it depends?

@snikulov They're not "optional". They're internal functions that are tested with unit tests. Those functions are not public and will never be, they're not part of any official API. All functions in libcurl that start with an uppercase 'Curl' are like that. They're also subject to change at any time, but then we of course also need to update the tests.

@snikulov the option statement with a comment makes it clearer.

@bagder the issue comes from the way cmake works with VisualStudio because cmake adds all projects to a single solution and msbuild always builds all projects in the solution. It is not possible to not build the test projects without manually editing the outputs of cmake which defeats the purpose of using cmake.

I'm just saying that when cmake is invoked, it can determine if the unit tests can get built or not and only if they can it should attempt that.

The fix does not work.
Need:

if (NOT CURL_HIDDEN_SYMBOLS)
    include(CTest)
    if(BUILD_TESTING)
        add_subdirectory(tests)
    endif()
endif()

I have a similar problem, in Windows.
It's not in the curl build itself, but on a library (netcdf) that uses curl. When using

-DCURL_STATICLIB=ON

, the windows sockets library is missing from the curl library , so you get linking errors

libcurl-d.lib(select.obj) : error LNK2019: unresolved external symbol __imp__select@20 referenced in function _Curl_socket_check

one way to fix this is to manually edit the generated libcurl.vcxproj and add

<Lib>
      <AdditionalOptions>%(AdditionalOptions) /machine:X86 /machine:X86</AdditionalOptions>
      <AdditionalDependencies>ws2_32.lib</AdditionalDependencies>
    </Lib>

but this should be fixed in some way on the curl Cmake script

to reproduce the bug, do

https://github.com/pedro-vicente/nco_bld
cd nco_bld
clone.bat
bld.bat

this clones and builds curl and netcdf

for curl, it is used

cmake .. -G "Visual Studio 14 2015" ^
           -DCMAKE_BUILD_TYPE=Debug ^
           -DBUILD_SHARED_LIBS=OFF ^
           -DBUILD_TESTING=OFF ^
           -DHTTP_ONLY=ON ^
           -DENABLE_DEBUG=ON ^
           -DENABLE_CURLDEBUG=ON ^
           -DCURL_STATICLIB=ON
  msbuild curl.sln /target:build /property:configuration=debug