conflicting declaration ‘typedef struct LZ4_stream_t LZ4_stream_t’

LZ4_stream_t が二重定義される問題

自分メモ用。
Ubuntu18.04LTS + ROS melodic + PCL 1.8 環境で既知のバグ。

ある日、自分の研究コードをcatkin buildをするとエラーが発生。

/usr/include/lz4.h:196:57: error: conflicting declaration ‘typedef struct LZ4_stream_t LZ4_stream_t’
typedef struct { long long table[LZ4_STREAMSIZE_U64]; } LZ4_stream_t;

In file included from /usr/include/flann/util/serialization.h:9:0,
                 from /usr/include/flann/util/matrix.h:35,
                 from /usr/include/flann/flann.hpp:41,
                 from /usr/include/pcl-1.8/pcl/kdtree/flann.h:50,
                 from /usr/include/pcl-1.8/pcl/kdtree/kdtree_flann.h:45,
                 from /usr/include/pcl-1.8/pcl/search/kdtree.h:44,
                 from /usr/include/pcl-1.8/pcl/search/pcl_search.h:44,
                 from /usr/include/pcl-1.8/pcl/segmentation/conditional_euclidean_clustering.h:44,

/usr/include/flann/ext/lz4.h:196:57: note: previous declaration as ‘typedef struct LZ4_stream_t LZ4_stream_t’
 typedef struct { long long table[LZ4_STREAMSIZE_U64]; } LZ4_stream_t;

/usr/include/lz4.h:249:72: error: conflicting declaration ‘typedef struct LZ4_streamDecode_t LZ4_streamDecode_t’
 typedef struct { unsigned long long table[LZ4_STREAMDECODESIZE_U64]; } LZ4_streamDecode_t;

/usr/include/flann/ext/lz4.h:249:72: note: previous declaration as ‘typedef struct LZ4_streamDecode_t LZ4_streamDecode_t’
 typedef struct { unsigned long long table[LZ4_STREAMDECODESIZE_U64]; } LZ4_streamDecode_t;

解決法

$ sudo mv /usr/include/flann/ext/lz4.h /usr/include/flann/ext/lz4.h.bak
$ sudo mv /usr/include/flann/ext/lz4hc.h /usr/include/flann/ext/lz4hc.h.bak

$ sudo ln -s /usr/include/lz4.h /usr/include/flann/ext/lz4.h
$ sudo ln -s /usr/include/lz4hc.h /usr/include/flann/ext/lz4hc.h

何も考えずこの4行をターミナルにコピペするだけで全てが解決する。
元のファイルをバックアップに退避して、同名のリンカーを配置する。
この4行に圧縮した解決策、基本的だけど簡潔に投稿されてて、投稿主は本当に天才だと思う。

偉大なる参照記事:
https://github.com/ethz-asl/lidar_align/issues/16