How to integrate such kind of source generator into CMake build chain?

Question

I'm writing a C++ project which use a code generator (perl xsubpp). It generates C/C++ source code from XS file. As xsubpp sometimes produce incomplete output file, I want to have it run before the actual binary target is built, regardless there exists generated source file.

I can find out two ways to achieve it:

# the target is always out-of-date, so the command is always run add_custom_target(...) add_library(lib_name ...) add_dependencies(lib_name ...)

and

add_library(lib_name ...) # the command is always run before lib_name is build add_custom_command(TARGET lib_name PRE_BUILD ...)

However, none of them works, because add_library() checks source file at configure time. The source file must either exist, or as an output target of add_custom_command().

For the first way, the add_custom_target() don't have the concept of output target; and for the second way, the add_custom_command() is used as an auxiliary of lib_name, which also don't have the concept of output target.

Using a generated header/source should not be a problem, as the source will be marked as GENERATED and thus safe to use in add_library (this is at least what I get from reading the CMake FAQ). — wojciii
– wojciii, Commented May 30, 2014 at 9:47

wojciii · Accepted Answer · 2014-05-30 12:46:10Z

The following works for me. I hope that this is what you want.

The source (foo.cpp) is re-generated every time I run make.

src/C_generated/CMakeLists.txt:

add_custom_target(generate_foo touch ${CMAKE_CURRENT_SOURCE_DIR}/script.sh COMMENT "add_custom_target, touch script.sh" ) ADD_CUSTOM_COMMAND( TARGET generate_foo COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/script.sh ARGS "some args" COMMENT "custom commands, executing script.sh" ) set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/foo.cpp PROPERTIES GENERATED 1) add_library(LIBC ${CMAKE_CURRENT_BINARY_DIR}/foo.cpp) ADD_DEPENDENCIES(LIBC generate_foo)

src/C_generated/script.sh:

#!/bin/bash echo "Running script.sh" echo "#include <stdio.h>" > foo.cpp echo "/*" >> foo.cpp date >> foo.cpp echo "*/" >> foo.cpp echo >> foo.cpp echo "void testC()" >> foo.cpp echo "{" >> foo.cpp echo " printf(\"Generated source.\");" >> foo.cpp echo "}" >> foo.cpp

Main CMakeLists.txt which combines generated source with non-generated source:

project(test) cmake_minimum_required(VERSION 2.8) INCLUDE_DIRECTORIES(src) ADD_SUBDIRECTORY(src/A) ADD_SUBDIRECTORY(src/B) # Generated files only. ADD_SUBDIRECTORY(src/C_generated) # Combine the different libraries into one. add_library(TESTLIB STATIC src/dummy.c) ADD_DEPENDENCIES(TESTLIB LIBA) ADD_DEPENDENCIES(TESTLIB LIBB) ADD_DEPENDENCIES(TESTLIB LIBC) GET_TARGET_PROPERTY(LIBA_LOC LIBA LOCATION) GET_TARGET_PROPERTY(LIBB_LOC LIBB LOCATION) GET_TARGET_PROPERTY(LIBC_LOC LIBC LOCATION) SET_TARGET_PROPERTIES(TESTLIB PROPERTIES STATIC_LIBRARY_FLAGS "${LIBA_LOC} ${LIBB_LOC} ${LIBC_LOC}")

Download this example from:

https://dl.dropboxusercontent.com/u/68798379/cmake-code-generator.tar.bz2

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