CURL_STATIC_CRT no longer works with cmake 3.15 and above

[rogerorr]

Building curl with -DCURL_STATIC_CRT=ON

Run "x64 Native Tools Command Prompt for VS 2019"
> git clone https://github.com/curl/curl
(on commit 69d5d18)
> md build
> cd build

> cmake -DCURL_STATIC_CRT=ON -DBUILD_SHARED_LIBS=OFF ../curl
> cmake --build . --target curl

I see the following

> dumpbin /imports src\Debug\curl.exe | find ".dll"
WLDAP32.dll
WS2_32.dll
ADVAPI32.dll
KERNEL32.dll
VCRUNTIME140D.dll
ucrtbased.dll

(This is identical with the output when -DCURL_STATIC_CRT=ON is not provided)

I expected the following

> dumpbin /imports src\Debug\curl.exe | find ".dll"
WLDAP32.dll
WS2_32.dll
ADVAPI32.dll
KERNEL32.dll

curl/libcurl version

> src\Debug\curl.exe -V
curl 7.70.0-DEV (Windows) libcurl/7.70.0-DEV
Release-Date: [unreleased]
Protocols: dict file ftp gopher http imap ldap pop3 rtsp smb smtp telnet tftp
Features: AsynchDNS IPv6 Largefile NTLM

operating system

Windows 10 1909

I am using Visual Studio 16.5.1, and the cmake that is supplied with VS (cmake version 3.16.19112601-MSVC_2)

Analysis

Cmake CMP0091 ( https://cmake.org/cmake/help/git-stage/policy/CMP0091.html ) changes the way CMake on Windows handles the flags controlling the C runtime being used.

Prior to CMake 3.15 the /MT or /MTd flags could simply be added to CMAKE_C_FLAGS_DEBUG or CMAKE_C_FLAGS_RELEASE.
Starting with CMake 3.15 the CMAKE_MSVC_RUNTIME_LIBRARY abstraction should be used instead -- values provided in the other locations are ignored.

Possible Solution

Change CMakeLists.txt:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e70a6e348..a410d4955 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -232,6 +232,7 @@ set(CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS}")
 set(CMAKE_REQUIRED_FLAGS ${CMAKE_ANSI_CFLAGS})

 if(CURL_STATIC_CRT)
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
   set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE} /MT")
   set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} /MTd")
 endif()

Note: the existing settings should be left in place, for versions of cmake prior to 3.15

[bagder]

Seems primarily like a cmake bug to me... Should CMAKE_MSVC_RUNTIME_LIBRARY really be set independent of what platform/compiler that's used? Can you please provide your suggested fix as a pull request?

[rogerorr]

I agree with you that it is a cmake issue - but I think this is mostly because they're trying to improve the vexing issue of handling the Microsoft compiler's setting of the relevant options controlling the C/C++ runtime library.

The existing code handling CURL_STATIC_CRT in curl's CMakeLists.txt is already WIN32 specific, and the additional flags /MT and /MTd are Microsoft specific, so I think adding the CMAKE_MSVC_RUNTIME_LIBRARY setting here is appropriate.

P/R #5167 raised

[jzakrzewski]

The behaviour is controlled by cmake policy CMP0091, which is automatically set to NEW if your top project declares minimum version of cmake to 3.15+. It must have been #4975 that introduced the problem for curl.

If I understand the description of CMAKE_MSVC_RUNTIME_LIBRARY correctly, cmake will only apply it where it makes sense, so it can be set without thinking about other platforms/compilers, etc.

[rogerorr]

That matches my understanding.