[CMake] How to use add_custom_command correctly

[Michael Wild]

On 09/27/2011 05:59 PM, Martin Kupke wrote:
> Hi,
> 
> in my project there is a subfolder which SHALL contain sources to
> generate a library. The problem is that at startup of the project there
> are no source files existing, because they will be generated by a code
> generator. This means within the build process the code generator needs
> to be called first, then generates the output files in the subfolder and
> then a library shall be generated from that source files (this are
> standard .c and .h files). If I start the code generator by hand to
> generate the source files and remove the custom command, then the
> compilation is successful, but I want the code generator to be started
> every time the configuration file for the code generator has changed.
> 
> In my sample below
> * the driver.c would be one of the files which the code generator would
> generate
> * the variable CodeGen is the executable tool (the code generator himself)
> * the variable CodeGenParam contains the parameters which are passed to
> be able to generate without any user interaction
> * the variable CodeGenConfig is the input file for the code generator
> 
> This subfolder contains its own CMakeLists.txt with the following settings:
> ##################### snip #####################
> project(CANstack C)
> 
> add_custom_command( OUTPUT driver.c
>                                            COMMAND ${CodeGen}
> ${CodeGenParam}
>                                            DEPENDS ${CodeGenConfig} )
> )
> 
> file(GLOB CANstack_srcs "*.c")
> file(GLOB CANstack_hdrs "*.h")
> 
> set(lib_name "CANstack")
> add_library(${lib_name} STATIC ${CANstack_srcs} ${CANstack_hdrs})
> 
> ##################### snap #####################
> 
> I don't get it work that the custom command is called and the source
> files from the code generator are produced.
> 

A few issues here:

- Never generate output in the source tree, only in the binary tree.
- Always use absolute paths with add_custom_command().
- Always list *all* outputs after the OUTPUT argument, otherwise CMake
won't know that they are generated sources.
- Never use file(GLOB ...). It is evil. And breaks in your case. Just don't.

Michael