Re: CFLAGS/LDFLAGS for PIE vs PIC?

[Bryan Ischo]

On 04/01/12 11:30, Jeffrey Walton wrote:
> Hi All,
>
> I'm interested in trying to figure out how to "cleanly" set options
> for CFLAGS (and CXXFLAGS) and LDFLAGS. I also want it to intelligently
> apply to programs and shared objects. The problem I am having is
> programs need:
>
>    CFLAGS += -fPIE
>    LDFLAGS += -pie
>
> while shared objects need:
>
>    CFLAGS += -fPIC
>    LDFLAGS += -shared
>
> So I can't run configure to set up a project properly:
>
> $ configure CFLAGS="-Wall -Wextra -Wconversion -fPIE -pie
>      -Wno-unused-parameter -Wformat=2 -Wformat-security
>      -fstack-protector-all -Wstrict-overflow
>      -Wl,-z,noexecstack -Wl,-z,relro -Wl,-z,now"
>
> I see others have also felt the same discomfort
> (http://lists.fedoraproject.org/pipermail/devel-announce/2011-August/000821.html):
>
>      "All of this is only an issue because most build systems don't
>      let you say different CFLAGS or LDFLAGS for shared libraries
>      and executables.  Sigh."
>
> Any ideas?

Well, I don't use GNU autoconf for my projects, so I can't address that. 
 What I do though is to specify different rules for object files that 
are to be linked into shared libraries than those to be built into 
executables.  I use a different file extension for shared object files - 
typically, .po instead of .o - and then specify files with this 
extension as being the prerequisite for shared objects.  My compile rule 
for building .po objects will include the -fPIC flag; the compile rule 
for building .o objects will not.  If I am building the same library as 
both shared and static, the object files will be built twice - once as 
.po and once as .o - each to be linked into the corresponding shared or 
static library.  Files that only ever link directly into executables are 
only ever built as .o files.  I find this works very well.  I don't know 
how, or if it's possible, to easily institute this policy in autoconf 
generated Makefiles.

Bryan