在编译OpenCV以及其附加模块时，有时会需要一些第三方的库，如果本地没有，会自动下载，下载地址一般为GitHub，结果当然就是卡死在那里，根本无法下载，下面教大家如何解决这种问题。
问题重现比如我在使用cmake配置的过程中出现以下情况：
Call Stack (most recent call first): /home/ryan/build/opencv_contrib-master/modules/dnn/cmake/download_protobuf.cmake:23 (ocv_download) /home/ryan/build/opencv_contrib-master/modules/dnn/cmake/download_protobuf.cmake:51 (ocv_protobuf_download) /home/ryan/build/opencv_contrib-master/modules/dnn/cmake/OpenCVFindLibProtobuf.cmake:30 (include) /home/ryan/build/opencv_contrib-master/modules/dnn/CMakeLists.txt:38 (include) -- Downloading v3.1.0/protobuf-cpp-3.1.0.tar.gz... 解决思路从上面的日志中可以得到以下信息：
需要下载protobuf-cpp-3.1.0.tar.gz这个文件
最先报错的是download_protobuf.cmake这个文件
一般情况下，最先报错的那个文件能获得一些与错误相关的信息，于是，我们打开这个文件，如下
set(PROTOBUF_CPP_NAME "libprotobuf") set(PROTOBUF_CPP_DOWNLOAD_HASH "bd5e3eed635a8d32e2b99658633815ef") set(PROTOBUF_CPP_PATH "${CMAKE_CURRENT_BINARY_DIR}/3rdparty/protobuf/sources") # /protobuf-3.1.0 subdirectory set(OPENCV_PROTOBUF_CPP_DOWNLOAD_URL ${OPENCV_PROTOBUF_URL};$ENV{OPENCV_PROTOBUF_URL};https://github.com/google/protobuf/releases/download/) function(ocv_protobuf_download file ID) if(DEFINED ${ID}_DOWNLOADED_HASH AND ${ID}_DOWNLOADED_HASH STREQUAL ${ID}_DOWNLOAD_HASH AND EXISTS ${${ID}_PATH}) # Files have been downloaded and checked by the previous CMake run return() else() if(EXISTS ${${ID}_PATH}) message(STATUS "${${ID}_NAME}: Removing previous unpacked files: ${${ID}_PATH}") file(REMOVE_RECURSE ${${ID}_PATH}) endif() endif() unset(${ID}_DOWNLOADED_HASH CACHE) file(MAKE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/.download) file(WRITE "${CMAKE_CURRENT_SOURCE_DIR}/.download/.gitignore" "*\n") ocv_download(PACKAGE ${file} HASH ${${ID}_DOWNLOAD_HASH} URL ${OPENCV_${ID}_DOWNLOAD_URL} DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/.download) set(${ID}_ARCHIVE "${DOWNLOAD_PACKAGE_LOCATION}") ocv_assert(EXISTS "${${ID}_ARCHIVE}") ocv_assert(NOT EXISTS "${${ID}_PATH}") file(MAKE_DIRECTORY ${${ID}_PATH}) ocv_assert(EXISTS "${${ID}_PATH}") file(WRITE "${${ID}_PATH}/.gitignore" "*\n") message(STATUS "${${ID}_NAME}: Unpacking ${file} to ${${ID}_PATH}...") execute_process(COMMAND ${CMAKE_COMMAND} -E tar xz "${${ID}_ARCHIVE}" WORKING_DIRECTORY "${${ID}_PATH}" RESULT_VARIABLE __result) if(NOT __result EQUAL 0) message(FATAL_ERROR "${${ID}_NAME}: Failed to unpack ${ID} archive from ${${ID}_ARCHIVE} to ${${ID}_PATH} with error ${__result}") endif() ocv_assert(EXISTS "${${ID}_PATH}") set(${ID}_DOWNLOADED_HASH "${${ID}_DOWNLOAD_HASH}" CACHE INTERNAL "${ID} hash") #message(STATUS "${${ID}_NAME}: Successfully downloaded") endfunction() ocv_protobuf_download(v3.1.0/protobuf-cpp-3.1.0.tar.gz PROTOBUF_CPP)可以看出，他需要libprotobuf这个库，并且需要下载到 ${CMAKE_CURRENT_BINARY_DIR}/3rdparty/protobuf/sources 目录下，而下载地址就是 https://github.com/google/protobuf/releases/download/ ，于是我们只要手动下载，然后把文件放到这个目录下即可。
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