[cmake-developers] [CMake 0014874]: Static library can miss some object files

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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http://public.kitware.com/Bug/view.php?id=14874 
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Reported By:                zub
Assigned To:                
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Project:                    CMake
Issue ID:                   14874
Category:                   CMake
Reproducibility:            always
Severity:                   minor
Priority:                   normal
Status:                     new
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Date Submitted:             2014-04-14 11:50 EDT
Last Modified:              2014-04-14 11:50 EDT
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Summary:                    Static library can miss some object files
Description: 
When creating a static library using binutils (ar) and there exist a duplicate
object name (e.g. a/Foo.cpp.o, b/Foo.cpp.o), the resulting static library can
end up having only one of the duplicate objects.

My reading of ar man page implies ar in general is OK with duplicate object
names. Also, creating a simple CMake project (just a/foo.cpp, b/foo.cpp,
add_library(test STATIC a/foo.cpp b/foo.cpp) results in a correct libtest.a.

This bug only happens if there are many objects. This affects how CMake invokes
ar. By default CMake invokes ar for the first time for a library like this:

ar cr libtest.a path/to/a/foo.cpp.o path/to/b/foo.cpp.o ...

and if this is the only invocation, it produces a correct libtest.a. But if
there are many objects, CMake invokes ar several times, using the following for
subsequent calls:

ar r libtest.a other objects...

Now if it happens that the first duplicate object is added in one invocation of
ar, and then CMake invokes ar again to add another, ar just replaces the
previous object.

When I set:

set(CMAKE_CXX_ARCHIVE_APPEND "<CMAKE_AR> <LINK_FLAGS> q <TARGET> <OBJECTS>")

to use "q" instead of "r", the issue is resolved for me, so the fix could be
just using "q" when adding objects (CMAKE_CXX_ARCHIVE_APPEND,
CMAKE_C_ARCHIVE_APPEND and perhaps also CMAKE_CXX_ARCHIVE_CREATE and
CMAKE_C_ARCHIVE_CREATE, though this probably won't make a difference).

But I'm not sure how do other ar's handle this. I tested this using "GNU ar (GNU
Binutils for Debian) 2.24".


Steps to Reproduce: 
The issue is deterministic, but it does not occur for every project with
duplicate object names - it depends on the project.

To trigger this a project with many objects is required so that CMake in fact
does invoke ar multiple times. Also, the duplicate objects must find their way
into multiple invocations of ar, e.g.:

ar cr libtest.a objects... a/foo.cpp.o objects...
ar r libtest.a objects... b/foo.cpp.o objects...

Then ar removes the previous foo.cpp.o.

Creating a standalone testcase seems not so trivial; I run into this on a real
project. If necessary, I can try to create one.
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Issue History 
Date Modified    Username       Field                    Change               
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2014-04-14 11:50 zub            New Issue                                    
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