fpermissive
起因
在使用命令编译cpp文件的时候:
gcc -I. -I../../../ -I../../../../ -I../../../../../ -I../../../../../../ -I../../../../cres/Include/ -I../../../../ldp/include/ -I../../../include/ -I../../include/ -I../common/new_logger/ -I./api_app -I./linenoise -I./sds -I./yaml-cpp-yaml-cpp-0.6.3/include/ -Wp,-MT,build/main.o -Wp,-MMD,build/main.o.d -c -fvisibility=hidden -std=c++11 -Wno-unknown-pragmas -Wno-deprecated -Wno-write-strings -g -DOS_COMMON_EXT -DNDEBUG -D_FILE_OFFSET_BITS=64 -c -o build/main.o main.cpp
遇到了如下编译报错:
sds/sds.h: 在函数‘size_t sdsavail(sds)’中:
sds/sds.h:83:87: 错误：从类型‘void*’到类型‘sdshdr8*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
sds/sds.h:114:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(8,s);
^
sds/sds.h:83:87: 错误：从类型‘void*’到类型‘sdshdr16*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
sds/sds.h:118:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(16,s);
^
sds/sds.h:83:87: 错误：从类型‘void*’到类型‘sdshdr32*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
sds/sds.h:122:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(32,s);
^
sds/sds.h:83:87: 错误：从类型‘void*’到类型‘sdshdr64*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
sds/sds.h:126:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(64,s);
^
In file included from main.cpp:1:0:
api_shell.h: 在函数‘void freeHintsCallback(void*)’中:
api_shell.h:50:13: 错误：从类型‘void*’到类型‘sds {aka char*}’的转换无效 [-fpermissive]
sdsfree(ptr);
^
In file included from util.h:16:0,
from api_shell.h:13,
from main.cpp:1:
sds/sds.h:225:6: 错误： 初始化‘void sdsfree(sds)’的实参 1 [-fpermissive]
void sdsfree(sds s);
1、sds.h是antirez/**sds**中的source code，它是使用C编写的，在C中，是允许void pointer到其它类型的implicit conversion的，但是这在C++中是非法的，所以报了上述错误。
2、上述报错中的-fpermissive引起了我的注意，"permissive"的含义是允许的、自由的。
3、参见 thegreenplace void* and casts, in C and C++
4、显然，我的上述问题的来源是mix C and C++
5、后来，我在编译指令中添加了-fpermissive，然后上述报错全部变成了warning:
./api_app/../sds/sds.h: 在函数‘size_t sdsavail(sds)’中:
./api_app/../sds/sds.h:83:87: 警告：从类型‘void*’到类型‘sdshdr8*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
./api_app/../sds/sds.h:114:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(8,s);
^
./api_app/../sds/sds.h:83:87: 警告：从类型‘void*’到类型‘sdshdr16*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
./api_app/../sds/sds.h:118:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(16,s);
^
./api_app/../sds/sds.h:83:87: 警告：从类型‘void*’到类型‘sdshdr32*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
./api_app/../sds/sds.h:122:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(32,s);
^
./api_app/../sds/sds.h:83:87: 警告：从类型‘void*’到类型‘sdshdr64*’的转换无效 [-fpermissive]
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
^
./api_app/../sds/sds.h:126:13: 附注：in expansion of macro ‘SDS_HDR_VAR’
SDS_HDR_VAR(64,s);
stackoverflow What does the fpermissive flag do?
Right from the docs:
-fpermissiveDowngrade some diagnostics about nonconformant code from errors to warnings. Thus, using-fpermissivewill allow some nonconforming code to compile.
Bottom line: don't use it unless you know what you are doing!
NOTE:
1、简而言之，将error变成waring
gcc 3.5 Options Controlling C++ Dialect # -fpermissive
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some nonconforming code to compile.