Background
I was investigating an issue with the conan recipe for Poco 1.13.3 while using MinGW. I modified the recipe for Poco 1.14.0 (locally) and stumbled upon a possible issue with 1.14.0.

Generally there seem to be a problem with discovering the correct value of HAVE_SENDFILE.

Even though I noticed this while building with MinGW, it does not actually affect the output using MinGW, because of:

if (MINGW)
	target_link_libraries(Net PUBLIC "mswsock.lib")
endif()

I will get back to that later.

Describe the bug
When building Poco 1.14.0 with MinGW 12.2.0 and CMake 3.16.3 I noticed the following oddity:

-- Looking for include file sys/sendfile.h
-- Looking for include file sys/sendfile.h - not found
-- Looking for sendfile
-- Looking for sendfile - not found
-- OS has native sendfile function
-- CMake 3.16.3 successfully configured Poco using MinGW Makefiles generator

Net/CMakeLists.txt does not find neither 'sys/sendfile.h' nor a 'sendfile' symbol, but still prints out a STATUS message that 'OS has native sendfile support'.

There seem to be a number of problems with the following block:

if (MSVC)
	set(HAVE_SENDFILE ON)
else()
	include(CheckIncludeFiles)
	include(CheckSymbolExists)
	check_include_files(sys/sendfile.h HAVE_SYS_SENDFILE_H)
	if(HAVE_SYS_SENDFILE_H)
		check_symbol_exists(sendfile sys/sendfile.h HAVE_SENDFILE)
		if (NOT DEFINED HAVE_SENDFILE)
			check_symbol_exists(sendfile64 sys/sendfile.h HAVE_SENDFILE)
		endif()
	else()
		# BSD version
		check_symbol_exists(sendfile "sys/types.h;sys/socket.h;sys/uio.h" HAVE_SENDFILE)
	endif()
endif()

if (DEFINED HAVE_SENDFILE)
	message(STATUS "OS has native sendfile function")
	add_compile_definitions(POCO_HAVE_SENDFILE)
endif()

My initial issue stems from if (DEFINED HAVE_SENDFILE). check_symbol_existswill define the variable HAVE_SENDFILE but set it to false.

This means that OS has native send function will always be written to the screen and POCO_HAVE_SENDFILE will always be defined.

The same problem exists for the line if (NOT DEFINED HAVE_SENDFILE). I do not believe the above block will ever check for sendfile64 as HAVE_SENDFILE is always defined but possibly false.

Even with the two above issues fixed, there seems to be a problem with check_symbol_exists(sendfile64 sys/sendfile.h HAVE_SENDFILE).

Because HAVE_SENDFILE is already defined, the check does not occur at all. Not sure if this is a bug in CMake 3.16.3 or expected behaviour. But, either HAVE_SENDFILE needs to be undefined again, using unset(HAVE_SENDFILE CACHE), or we need to use a different variable name, eg HAVE_SENDFILE64.

From my testing, with both Linux gcc 12 and MinGW 12, I believe the block should be:

if (MSVC OR MINGW)
	set(HAVE_SENDFILE ON)
else()
	include(CheckIncludeFiles)
	include(CheckSymbolExists)
	check_include_files(sys/sendfile.h HAVE_SYS_SENDFILE_H)
	if(HAVE_SYS_SENDFILE_H)
		check_symbol_exists(sendfile sys/sendfile.h HAVE_SENDFILE)
		if (NOT HAVE_SENDFILE)
			check_symbol_exists(sendfile64 sys/sendfile.h HAVE_SENDFILE64)
		endif()
	else()
		# BSD version
		check_symbol_exists(sendfile "sys/types.h;sys/socket.h;sys/uio.h" HAVE_SENDFILE)
	endif()
endif()

if (HAVE_SENDFILE OR HAVE_SENDFILE64)
	message(STATUS "OS has native sendfile function")
	add_compile_definitions(POCO_HAVE_SENDFILE)
endif()

The reason I am adding OR MINGW to the initial line is the addition of mswsock.lib later in the CMakeLists.txt as described in the beginning.

This causes SocketImpl::sendFile to link to TransmitFile which matches the ìfdef POCO_OS_FAMILY_WINDOWS` block. (This is the issue I am investigating in the conan recipe, as it does not propagate this dependency to the recipe consumer).

To Reproduce
Made different changes for test purposes:

• Changed sys/sendfile.h to sys/sendfilex.h and re-ran configure
• Changed sendfile to sendfilex and re-ran configure
• Changed sendfile64 to sendfile64x and re-ran configure

Expected behavior
POCO_HAVE_SENDFILE should not be defined as a compile definition if sendfile is unavailable.

Please add relevant environment information:

• Linux GCC 12 and Windows MinGW (12.2.0)
• Poco version 1.14.0
• CMake version 3.16.3