E4156

E4156#

extern “A” is unsupported in B backend.

This error happens when an FFI function written for a specific backend is discovered during the compilation of a different backend.

Erroneous example#

///|
pub extern "C" fn getchar() -> Int = "getchar"

Suggestion#

You can use conditional compilation to control when the function is available.

  • If you want a file level conditional compilation, configure the targets field.

    moon.pkg.json#
    {
  "targets": {
    "top.mbt": ["native"]
  }
}
    top.mbt#
    ///|
pub extern "C" fn getchar() ->  = "getchar"

  • If you want a function level conditional compilation, add the cfg attribute.

    other.mbt#
    ///|
#cfg(target="native")
pub extern "C" fn getchar2() -> Int = "getchar"
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