[CMake] Bug? add_custom_command() ignores head file outputs

[Adam B]

Hello all,

I'm relatively new to CMake but I've encountered some unexpected behavior.  Perhaps it's by design or perhaps it's a bug - you tell me.  The short story is this:

It appears that add_custom_command() is ignored if the only OUTPUT is a C header file (.h).  The generated makefiles do not include the custom command.  However, if I also list a C source file (.c) as a second output, the makefiles get the command.

Here's a simplified setup to reproduce the problem:

---------project_dir/-----------

     CMakeLists.txt
     hello.c
     hello.h
     update_hello_h.sh

--------CMakeLists.txt---------

cmake_minimum_required(VERSION 2.8)
project(cmake_test)

set_directory_properties(PROPERTIES CLEAN_NO_CUSTOM ON)  #so hello.h never gets cleaned away

add_custom_command(OUTPUT hello.h COMMAND ./update_hello_h.sh DEPENDS update_hello_h.sh)

add_executable(hello hello.c)

------------hello.c----------------

#include <stdio.h>
#include "hello.h"
int main(int argc, char ** argv)
{
     printf(HELLO_MSG "\n");
     return 0;
}

-----------hello.h----------------

#define HELLO_MSG "Hello Fri Aug 24 09:01:53 PDT 2012"

-------update_hello_h.sh-----

#!/bin/sh
echo "#define HELLO_MSG" \"Hello `date`\" > hello.h

----------------------------------

What I would expect is for CMake to add a makefile rule to execute ./update_hello_h.sh if it's newer than hello.h.  No such rule gets generated.  However, if you add hello.c like so:

     add_custom_command(OUTPUT hello.h hello.c COMMAND ./update_hello_h.sh DEPENDS update_hello_h.sh)

Then a rule gets generated.  Is this a bug or am I missing something?  I feel like I'm lying to CMake by telling it that hello.c is an output when it really isn't.

I'm using CMake 2.8.0 on Ubuntu 10.04.

Much thanks!
- Adam B.


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