build：link error when using photon as cmake external project

[kernelai]

使用cmake external project 把photon到项目中时，编译报错。不使用fetch_content的原因是网络环境不好，build是下载会很慢。。。

ExternalProject_Add(photon
    SOURCE_DIR ${CMAKE_SOURCE_DIR}/third/photonlibos
    LOG_CONFIGURE
    1
    LOG_BUILD
    1
    LOG_INSTALL
    1
    CMAKE_ARGS
    -DCMAKE_INSTALL_PREFIX=${STAGED_INSTALL_PREFIX}/photon
    -DCMAKE_CXX_STANDARD=17
    INSTALL_COMMAND cmake -E echo "Skipping install step.")

set(PHOTON_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/third/photonlibos/include
    CACHE PATH "Path to internally built PhotonLibOS include directory"
    FORCE)
set(PHOTON_LIBRARY ${PROJECT_BINARY_DIR}/deps/Build/photon/output/libphoton.a
    CACHE PATH "Path to internally build PhotonLibOS library"
    FORCE)

ADD_LIBRARY(abc STATIC ${ABC_SOURCES})
TARGET_LINK_LIBRARIES(ts ${PHOTON_LIBRARY} ${FMT_LIBRARY})

link error：

[ 96%] Linking CXX executable cmake-external
/usr/bin/ld: /workspaces/cmake-external/build/deps/Build/photon/output/libphoton_sole.a(aio-wrapper.cpp.o): in function `photon::resume_libaio_requesters()':
aio-wrapper.cpp:(.text+0x3b8): undefined reference to `io_getevents'
/usr/bin/ld: /workspaces/cmake-external/build/deps/Build/photon/output/libphoton_sole.a(aio-wrapper.cpp.o): in function `libaio_wrapper_init':
aio-wrapper.cpp:(.text+0x954): undefined reference to `io_setup'
/usr/bin/ld: /workspaces/cmake-external/build/deps/Build/photon/output/libphoton_sole.a(aio-wrapper.cpp.o): in function `libaio_wrapper_fini':
aio-wrapper.cpp:(.text+0xabc): undefined reference to `io_destroy'
/usr/bin/ld: /workspaces/cmake-external/build/deps/Build/photon/output/libphoton_sole.a(aio-wrapper.cpp.o): in function `long photon::have_n_try<photon::libaiocb::cancel()::{lambda()#1}>(photon::libaiocb::cancel()::{lambda()#1} const&, char const*, long) [clone .isra.0]':
aio-wrapper.cpp:(.text._ZN6photon10have_n_tryIZNS_8libaiocb6cancelEvEUlvE_EElRKT_PKcl.isra.0[_ZN6photon8libaiocb15submit_and_waitEm]+0x54): undefined reference to `io_cancel'
/usr/bin/ld: /workspaces/cmake-external/build/deps/Build/photon/output/libphoton_sole.a(aio-wrapper.cpp.o): in function `photon::libaiocb::submit_and_wait(unsigned long)':

[beef9999]

photon目前没提供cmake的install，只是给用户生成了target，然后target附带了一些依赖，希望用户直接把这些target加到自己的cmake里面去。

如果不用target的话，那可以走编译的方式，即你这种方法。但需要注意的是，libphoton_sole.a是指包含photon源码的静态库，不包含第三方库，如libaio.a，liburing.a 等。libphoton.a是所有静态库的打包，是在make的最后一步进行的，如果不用target引用的话，至少需要你主动调用make编译。看起来你是因为没有make，所以链接到了错误的静态库上去了，导致没有aio的符号

[beef9999]

另外，不是 -D CMAKE_CXX_STANDARD ，而是 -D PHOTON_CXX_STANDARD ，参考官网文档 https://photonlibos.github.io/docs/introduction/how-to-build#build-options

[kernelai]

感谢回复。我试试