7.4.7 Windows Socket Timeouts ¶

Microsoft Windows desktops older than 8.0 and Microsoft Windows Servers older than 2019 set a socket timeout 500 milliseconds longer than the value set by setsockopt with SO_RCVTIMEO and SO_SNDTIMEO options. The GNAT runtime makes a correction for the difference in the corresponding Windows versions. For Windows Server starting with version 2019, you must provide a manifest file for the GNAT runtime to be able to recognize that the Windows version does not need the timeout correction. The manifest file should be located in the same directory as the executable file and its file name must match the executable name suffixed by .manifest. For example, if the executable name is sock_wto.exe, the manifest file name must be sock_wto.exe.manifest. The manifest file must contain at least the following data:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
<application>
   <!-- Windows Vista -->
   <supportedOS Id="{e2011457-1546-43c5-a5fe-008deee3d3f0}"/>
   <!-- Windows 7 -->
   <supportedOS Id="{35138b9a-5d96-4fbd-8e2d-a2440225f93a}"/>
   <!-- Windows 8 -->
   <supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}"/>
   <!-- Windows 8.1 -->
   <supportedOS Id="{1f676c76-80e1-4239-95bb-83d0f6d0da78}"/>
   <!-- Windows 10 -->
   <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}"/>
</application>
</compatibility>
</assembly>

Without the manifest file, the socket timeout will be overcorrected on these Windows Server versions and the actual time wil be 500 milliseconds shorter than what was set with GNAT.Sockets.Set_Socket_Option. Note that on Microsoft Windows versions where correction is necessary, there is no way to set a socket timeout shorter than 500 ms. If a socket timeout shorter than 500 ms is needed on these Windows versions, you should add a call to Check_Selector before any socket read or write operations.