我声明我从未使用过autoconf工具。
现在我正在将一个小项目从autoconf迁移到cmake，然后我遇到了AC_SYS_LARGEFILE。除了AC_SYS_LARGEFILE是4字节还是8字节这是一种检查，我对off_t不太了解。
出于好奇，我想了解AC_SYS_LARGEFILE的典型用例。
真正的问题，我还是需要AC_SYS_LARGEFILE做CMake吗？
如果是，是否在CMake中已经存在一种机制，还是我应该自己实现？
据我所知，目前对该CMake的本机不提供任何支持。但是，有些CMake项目已经创建了自己的解决方案。请参见WireShark和OpenJPEG存储库中的这些文件。这些代码中的每个代码似乎都是从Gromacs Project改编而成。
为了完整起见，这是WireShark CMake文件，显示了如何测试程序中是否需要大文件支持：
# This code was copied from http://www.gromacs.org/
# and its toplevel COPYING file starts with:
#
# GROMACS is free software, distributed under the GNU General Public License
# (GPL) Version 2.
# - Define macro to check large file support
#
# GMX_TEST_LARGE_FILES(VARIABLE)
#
# VARIABLE will be set to true if 64-bit file support is available.
# This macro will also set defines as necessary to enable large file
# support, for instance:
# _LARGE_FILES
# _LARGEFILE_SOURCE
# _FILE_OFFSET_BITS=64
#
# However, it is YOUR job to make sure these defines are set in a cmakedefine so they
# end up in a config.h file that is included in your source if necessary!
MACRO(GMX_TEST_LARGE_FILES VARIABLE)
IF(NOT DEFINED ${VARIABLE})
if(CMAKE_C_COMPILER_ID MATCHES "MSVC")
#
# This is Visual Studio; Visual Studio has supported
# _fseeki64 and _ftelli64 since Visual Studio 2005 / MSVCR80,
# and we require newer versions, so we know we have them.
#
MESSAGE(STATUS "_fseeki64 and _ftelli64 are present")
else(CMAKE_C_COMPILER_ID MATCHES "MSVC")
#
# This is UN*X, or some other Windows compiler.
#
# For UN*X, we do the Large File Support tests, to see
# whether it's present and, if so what we need to define
# to enable it.
#
# XXX - what's the right thing to do on Windows with,
# for example, MinGW?
#
# On most platforms it is probably overkill to first test
# the flags for 64-bit off_t, and then separately fseeko.
# However, in the future we might have 128-bit seek offsets
# to support 128-bit filesystems that allow 128-bit offsets
# (ZFS), so it might be dangerous to indiscriminately set
# e.g. _FILE_OFFSET_BITS=64.
#
MESSAGE(STATUS "Checking for 64-bit off_t")
# First check without any special flags
TRY_COMPILE(FILE64_OK "${CMAKE_BINARY_DIR}"
"${CMAKE_SOURCE_DIR}/cmake/TestFileOffsetBits.c")
if(FILE64_OK)
MESSAGE(STATUS "64-bit off_t is present with no special flags")
endif(FILE64_OK)
if(NOT FILE64_OK)
# Test with _FILE_OFFSET_BITS=64
TRY_COMPILE(FILE64_OK "${CMAKE_BINARY_DIR}"
"${CMAKE_SOURCE_DIR}/cmake/TestFileOffsetBits.c"
COMPILE_DEFINITIONS "-D_FILE_OFFSET_BITS=64" )
if(FILE64_OK)
MESSAGE(STATUS "64-bit off_t is present with _FILE_OFFSET_BITS=64")
set(_FILE_OFFSET_BITS 64 CACHE INTERNAL "64-bit off_t requires _FILE_OFFSET_BITS=64")
endif(FILE64_OK)
endif(NOT FILE64_OK)
if(NOT FILE64_OK)
# Test with _LARGE_FILES
TRY_COMPILE(FILE64_OK "${CMAKE_BINARY_DIR}"
"${CMAKE_SOURCE_DIR}/cmake/TestFileOffsetBits.c"
COMPILE_DEFINITIONS "-D_LARGE_FILES" )
if(FILE64_OK)
MESSAGE(STATUS "64-bit off_t is present with _LARGE_FILES")
set(_LARGE_FILES 1 CACHE INTERNAL "64-bit off_t requires _LARGE_FILES")
endif(FILE64_OK)
endif(NOT FILE64_OK)
if(NOT FILE64_OK)
# Test with _LARGEFILE_SOURCE
TRY_COMPILE(FILE64_OK "${CMAKE_BINARY_DIR}"
"${CMAKE_SOURCE_DIR}/cmake/TestFileOffsetBits.c"
COMPILE_DEFINITIONS "-D_LARGEFILE_SOURCE" )
if(FILE64_OK)
MESSAGE(STATUS "64-bit off_t is present with _LARGEFILE_SOURCE")
set(_LARGEFILE_SOURCE 1 CACHE INTERNAL "64-bit off_t requires _LARGEFILE_SOURCE")
endif(FILE64_OK)
endif(NOT FILE64_OK)
if(NOT FILE64_OK)
MESSAGE(STATUS "64-bit file offset support not available")
else(NOT FILE64_OK)
# Set the flags we might have determined to be required above
configure_file("${CMAKE_SOURCE_DIR}/cmake/TestLargeFiles.c.cmakein"
"${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/TestLargeFiles.c")
#
# If this is Windows, assume we have _fseeki64/_ftelli64
# available. If this is UN*X, check for fseeko/ftello.
#
if(NOT WIN32)
MESSAGE(STATUS "Checking for fseeko/ftello")
# Test if ftello/fseeko are available
TRY_COMPILE(FSEEKO_COMPILE_OK "${CMAKE_BINARY_DIR}"
"${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/TestLargeFiles.c")
if(FSEEKO_COMPILE_OK)
MESSAGE(STATUS "Checking for fseeko/ftello - present")
endif(FSEEKO_COMPILE_OK)
if(NOT FSEEKO_COMPILE_OK)
# glibc 2.2 neds _LARGEFILE_SOURCE for fseeko (but not 64-bit off_t...)
TRY_COMPILE(FSEEKO_COMPILE_OK "${CMAKE_BINARY_DIR}"
"${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/TestLargeFiles.c"
COMPILE_DEFINITIONS "-D_LARGEFILE_SOURCE" )
if(FSEEKO_COMPILE_OK)
MESSAGE(STATUS "Checking for fseeko/ftello - present with _LARGEFILE_SOURCE")
set(_LARGEFILE_SOURCE 1 CACHE INTERNAL "64-bit fseeko requires _LARGEFILE_SOURCE")
endif(FSEEKO_COMPILE_OK)
endif(NOT FSEEKO_COMPILE_OK)
if(FSEEKO_COMPILE_OK)
SET(${VARIABLE} 1 CACHE INTERNAL "Result of test for large file support" FORCE)
set(HAVE_FSEEKO 1 CACHE INTERNAL "64-bit fseeko is available" FORCE)
else(FSEEKO_COMPILE_OK)
MESSAGE(STATUS "Checking for fseeko/ftello - not found")
SET(${VARIABLE} 0 CACHE INTERNAL "Result of test for large file support" FORCE)
endif(FSEEKO_COMPILE_OK)
endif(NOT WIN32)
endif(NOT FILE64_OK)
endif(CMAKE_C_COMPILER_ID MATCHES "MSVC")
ENDIF(NOT DEFINED ${VARIABLE})
ENDMACRO(GMX_TEST_LARGE_FILES VARIABLE)