On Windows, Rust exports the name of DllMain (std) / _DllMainCRTStartup (no_std), which is not standard behaviour and unwanted in most cases (that I am aware of). As far as I understand this happens because they must be marked with either #[no_mangle] or #[export_name = ".."] so the link.exe can find the symbol. The extern keyword must also be used to mark them as stdcall for 32 bit targets.

Example project:

• Cargo.toml

  [lib]
  crate-type = ["cdylib"]

• lib.rs (std)

  #[no_mangle]
  extern "system" fn DllMain(_: *const u8, _: u32, _: *const u8) -> u32 { 1 }

• lib.rs (no_std)

  #![no_std]
  
  #[panic_handler]
  fn panic(_: &core::panic::PanicInfo) -> ! { loop {} }
  
  #[no_mangle]
  extern "system" fn _DllMainCRTStartup(_: *const u8, _: u32, _: *const u8) -> u32 { 1 }

dumpbin output

• Example Windows dll file:

  Dump of file C:\Windows\System32\httpprxp.dll
  
  File Type: DLL
  
    Section contains the following exports for httpprxp.dll
  
      00000000 characteristics
          0.00 version
             1 ordinal base
             8 number of functions
             8 number of names
  
      ordinal hint RVA      name
  
            1    0 00001870 ProxyHelperGetProxyEventInformation
            2    1 00001D00 ProxyHelperProviderConnectToServer
            3    2 00001DC0 ProxyHelperProviderDisconnectFromServer
            4    3 000010E0 ProxyHelperProviderFreeMemory
            5    4 00001220 ProxyHelperProviderRegisterForEventNotification
            6    5 00001AC0 ProxyHelperProviderSetProxyConfiguration
            7    6 00001BA0 ProxyHelperProviderSetProxyCredentials
            8    7 00001620 ProxyHelperProviderUnregisterEventNotification
  
    Summary
  
          1000 .data
          1000 .didat
          1000 .pdata
          2000 .rdata
          1000 .reloc
          1000 .rsrc
          2000 .text
  

• Rust example from above (std):

  Dump of file std.dll
  
  File Type: DLL
  
    Section contains the following exports for std.dll
  
      00000000 characteristics
          0.00 version
             1 ordinal base
             2 number of functions
             2 number of names
  
      ordinal hint RVA      name
  
            1    0 00001000 DllMain = DllMain
            2    1 00001010 rust_eh_personality = rust_eh_personality
  
    Summary
  
          1000 .data
          1000 .pdata
          1000 .rdata
          1000 .reloc
          1000 .text
  

• Rust example from above (no_std):

  Dump of file no_std.dll
  
  File Type: DLL
  
    Section contains the following exports for no_std.dll
  
      00000000 characteristics
          0.00 version
             1 ordinal base
             1 number of functions
             1 number of names
  
      ordinal hint RVA      name
  
            1    0 00001000 _DllMainCRTStartup = _DllMainCRTStartup
  
    Summary
  
          1000 .rdata
          1000 .text
  

Expected output

DllMain and _DllMainCRTStartup should not be exported by name, just as in the first dumpbin output example of a Windows system dll.

Tested compiler version

rustc 1.41.0-nightly (c8ea4ace9 2019-12-14)
binary: rustc
commit-hash: c8ea4ace9213ae045123fdfeb59d1ac887656d31
commit-date: 2019-12-14
host: x86_64-pc-windows-msvc
release: 1.41.0-nightly
LLVM version: 9.0