CMake adding libraries for Windows/Linux - cmake

Question

Visual Studio C++ 2008 / GCC 4.4.2
I have written a program to run on Linux and now I have to port my code to run on Windows. I have decided to use CMake as I want to keep the same build system for both platforms.
However, I need to link with some libraries for both platforms. In my CMakeLists.txt I have the following:
# Compile with gcc c89 standard
IF(CMAKE_COMPILER_IS_GNUCXX)
MESSAGE(STATUS "GCC detected - Adding compiler flags")
SET(CMAKE_C_FLAGS "-pthread -ggdb -Wextra -Wall -pedantic -std=c89")
ENDIF(CMAKE_COMPILER_IS_GNUCXX)
IF(WIN32)
SET(CMAKE_C_FLAGS "ws2_32.lib")
ENDIF(WIN32)
However, when I compile on Visual Studio I get the following error:
fatal error C1083: Cannot open source file: 'ws2_32.lib': No such file or directory
What can I do to resolve this problem?
========= Edit
In the top level directory
# Project Client Server
CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
# Name of project
PROJECT(CLIENT_SERVER)
# Compile with gcc c89 standard
IF(CMAKE_COMPILER_IS_GNUCXX)
MESSAGE(STATUS "GCC detected - Adding compiler flags")
SET(CMAKE_C_FLAGS "-pthread -ggdb -Wextra -Wall -pedantic -std=c89")
ENDIF(CMAKE_COMPILER_IS_GNUCXX)
IF(WIN32)
SET(CMAKE_C_FLAGS "ws2_32")
ENDIF(WIN32)
# Includes
INCLUDE_DIRECTORIES(${CLIENT_SERVER_SOURCE_DIR}/cltsvr_ults)
INCLUDE_DIRECTORIES(${CLIENT_SERVER_SOURCE_DIR}/server)
INCLUDE_DIRECTORIES(${CLIENT_SERVER_SOURCE_DIR}/client)
# libraries
LINK_DIRECTORIES($CLIENT_SERVER/cltsvr_ults)
# Add subdirectories
ADD_SUBDIRECTORY(client)
ADD_SUBDIRECTORY(server)
ADD_SUBDIRECTORY(cltsvr_ults)
ADD_SUBDIRECTORY(test_client)
ADD_SUBDIRECTORY(test_server)
In the subdirectory of client I have this CMakeLists.txt
# libray called client from client.c
ADD_LIBRARY(client client)
And in the subdirectory of test_clt where I create and link my executable.
# Test client add executable
INCLUDE_DIRECTORIES($CLIENT_SERVER_SOURCE_DIR/client)
INCLUDE_DIRECTORIES($CLIENT_SERVER_SOURCE_DIR/cltsvr_ults)
# Link the library
LINK_DIRECTORIES($CLIENT_SERVER/client)
# Add the executable
ADD_EXECUTABLE(clt test_clt)
# Link the executable to the client library
IF(WIN32)
TARGET_LINK_LIBRARIES(clt client ws2_32)
ENDIF(WIN32)

Answer 1

Disclaimer: My answer is of philosophical nature which should encourage you to avoid touching CMAKE_C_FLAGS directly. For the direct answer that just solves your problem look what Bill ( the lead architect of the CMake btw. ) wrote.
The thing about CMake is, that it lets you describe what you want to do without referring to a specific compiler or platform. What CMake does is building the compiler and linker flags from your usage of
include_directories
add_definitions
add_library
add_executable
target_link_libraries
If there are no external dependencies, other than the compiler itself, this is all you need. For external dependencies use
find_package
It defines a set of variables, like
find_package(SDL)
defines
SDL_INCLUDE_DIR
SDL_LIBRARY
for usage with respectively include_directories and target_link_libraries. CMake ships with a bunch of so called module files, like FindSDL.cmake and many others can be googled.
The next lower level is to use
find_path
find_library
which are used in the Find???.cmake modules itself.
The CMAKE_C_FLAGS variable is composed by CMake from these commands. Modifying it means you bypass CMake. There are cases, like for special optimization flags, you want to do this, but at this point all power and thus responsibility transfered from CMake to you.

Answer 2

By adding ws2_32.lib to the C_FLAGS, you are using it at compile time, and not link time. If you look at the error message, you can see it it treating the file as if it were a c++ source file: Cannot open source file: 'ws2_32.lib'. target_link_libraries(target ws2_32) should work.

Answer 3

You need to use the Target Link Libraries command. The target would be the executable you're building.
EDIT: You shouldn't specify the libs you're linking against in C_FLAGS. You can do something like TARGET_LINK_LIBRARIES(execName, ws_32, ...). I'm not 100% sure if you need the .lib. Been a while since I used CMake.