Cannot find netdb.h when compiling ZMQ in C to wasm

[HaoxinSEU]

Hi community,

Thanks for your work on WASI SDK!

I'm developing a C plugin using ZMQ, so that the plugin can send some messages. However, when I compiled it with ./bin/wasm32-wasip2-clang and use ./bin/lld as the linker, I got the following errors:

In file included from /opt/miniz_extism/dependencies/zmq/src/ws_address.cpp:3:
In file included from /opt/miniz_extism/dependencies/zmq/src/precompiled.hpp:30:
/opt/miniz_extism/dependencies/zmq/src/../include/zmq.h:769:33: error: unknown type name 'sigset_t'
  769 |                           const sigset_t *sigmask_);
      |                                 ^
In file included from /opt/miniz_extism/dependencies/zmq/src/ws_address.cpp:8:
In file included from /opt/miniz_extism/dependencies/zmq/src/ws_address.hpp:11:
/opt/miniz_extism/dependencies/zmq/src/ip_resolver.hpp:9:10: fatal error: 'netdb.h' file not found
    9 | #include <netdb.h>
      |          ^~~~~~~~~
2 errors generated.
make[2]: *** [CMakeFiles/objects.dir/build.make:76: CMakeFiles/objects.dir/src/ws_address.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:391: CMakeFiles/objects.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

I'm quite confused with this error, because the header file netdb.h is there: ./share/wasi-sysroot/include/wasm32-wasip2/netdb.h.

My command to run cmake is:

cmake -D CMAKE_C_COMPILER=$(pwd)/../wasi-sdk/bin/wasm32-wasip2-clang -DCMAKE_LINKER=$(pwd)/../wasi-sdk/bin/lld ..

and the CMakeLists.txt file includes

add_compile_options(-D_WASI_EMULATED_SIGNAL)
target_link_libraries(libzmq wasi-emulated-signal)
set(CMAKE_C_FLAGS "--target=wasm32-wasip2")

Does anyone have a hint on why I got this error? Thanks a lot!