Why -lm option is not required in MinGW GCC? - gcc

Compiler option -lm is required when compiling and linking program containing functions in math.h on Linux GCC, otherwise a link error will appear. However, -lm option is not required in MinGW GCC on Windows.

The -lm option in't required with MinGW(-w64) because the math functions are already part of MinGW(-w64).

Related

What default libraries does -nolibc remove?

I am working on a project which eventually requires me to link my code against my customized version of libc, libstdc++ and libgcc. Now I am learning what libraries are linked by default.
I tried to compile my project code with -nolibc and manually provide the needed libraries in the argument list.
g++ -fuse-ld=gold -O2 -o main -static -nolibc main.cpp -lgcc -lm -lc
I got the following error, showing that I missed some libraries.
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc_eh.a(unwind-dw2-fde-dip.o):function _Unwind_Find_FDE: error: undefined reference to 'dl_iterate_phdr'
Which library is missing here?

How do I link libraries statically with gcc?

I am new to C++ compiling/linking.
I am trying to link all libraries statically with gcc, I tried using LDFLAGS=-static but did not work. Error message showed:
/bin/sh ./libtool --tag=CXX --mode=link /home/dizhang/lib/hdf5/bin/h5c++ -g -O2 -L/home/dizhang/lib/blitz/lib -L/home/dizhang/lib/libconfig/lib -o angora src/libangora.la -lblitz -lconfig++
libtool: link: /home/dizhang/lib/hdf5/bin/h5c++ -g -O2 -o angora -L/home/dizhang/lib/blitz/lib -L/home/dizhang/lib/libconfig/lib src/.libs/libangora.a -L/bgsys/drivers/V1R2M2/ppc64/comm/lib64 -L/bgsys/drivers/V1R2M2/ppc64/comm/lib -L/bgsys/drivers/V1R2M2/ppc64/spi/lib -lpthread -lm /home/dizhang/lib/blitz/lib/libblitz.a /bgsys/drivers/V1R2M2/ppc64/comm/lib/libmpichcxx-gcc.so /bgsys/drivers/toolchain/V1R2M2_base/gnu-linux/powerpc64-bgq-linux/lib/libstdc++.so /bgsys/drivers/V1R2M2/ppc64/comm/lib/libmpich-gcc.so /bgsys/drivers/V1R2M2/ppc64/comm/lib/libopa-gcc.so /bgsys/drivers/V1R2M2/ppc64/comm/lib/libmpl-gcc.so /home/dizhang/lib/libconfig/lib/libconfig++.a /bgsys/drivers/toolchain/V1R2M2_base-efix014/gnu-linux/powerpc64-bgq-linux/lib/libstdc++.so -Wl,-rpath -Wl,/bgsys/drivers/toolchain/V1R2M2_base-efix014/gnu-linux/powerpc64-bgq-linux/lib -Wl,-rpath -Wl,/bgsys/drivers/V1R2M2/ppc64/comm/lib -Wl,-rpath -Wl,/bgsys/drivers/toolchain/V1R2M2_base/gnu-linux/powerpc64-bgq-linux/lib -Wl,-rpath -Wl,/bgsys/drivers/toolchain/V1R2M2_base-efix014/gnu-linux/powerpc64-bgq-linux/lib -Wl,-rpath -Wl,/bgsys/drivers/V1R2M2/ppc64/comm/lib -Wl,-rpath -Wl,/bgsys/drivers/toolchain/V1R2M2_base/gnu-linux/powerpc64-bgq-linux/lib
/bgsys/drivers/toolchain/V1R2M2_base-efix014/gnu-linux/lib/gcc/powerpc64-bgq-linux/4.4.7/../../../../powerpc64-bgq-linux/bin/ld: attempted static link of dynamic object `/bgsys/drivers/V1R2M2/ppc64/comm/lib/libmpichcxx-gcc.so'
collect2: ld returned 1 exit status
I did some search and found that, telling Makefile -Wl -Bstatic may solve this problem, but how exactly I should change this in my Makefile?
I tried searching -Wl in Makefile but it was not in the text.
Thanks,
Di
Looks like you are trying to build HDF5 with MPI support on BGQ.
As long as you end up passing a ".so" version of a lib to gcc on BGQ -- you will see this error. You may need to check how you are passing MPI info to HDF5's configure script.
In my case (building another project that uses CMake) passing BGQ's MPI compiler wrappers to CMake always created a problem where it would try to link MPI using the shared libs instead of the static ones. To resolve this, I had to make sure to explicitly specify the ".a" variants of the MPI libs.

Order of linkinkg libc and libpthread on FreeBSD

I have this simple c-code
It works fine when i link it as
gcc -g app.c.o -o app -lrt -lpthread -lc
But if we change order of linking libc and libthread
gcc -g app.c.o -o app -lrt -lc -lpthread
it does not work.
We know on FreeBSD stubs of pthread-functions in libc are made as weak reference.
For example
objdump -T /lib/libc.so.7 | grep pthread_cond_signal
00000000000e2bf0 w DF .text 0000000000000011 FBSD_1.0 pthread_cond_signal
It means order of linking is not matter. Why so?
Compile and link using the option -pthread. Note the missing "ell".
Update:
-pthread instructs all tools involved in creating a binary (pre-processor, compiler, linker) to take care that the application/library to be build runs as intended. (This obviously is only necessary if the source makes use of any member(s) of the pthread_*-family of functions.)
Whereas -lpthread links a library called libpthread, nothing more and nothing less.
The difference in detail is implementation specific.
Note: if -pthread has been specified -lpthread is not necessary as well as not recommended to be specfied.
The reason for the behavior is that the linker only does a single pass through the libraries in the provided order to resolve symbols.
So, it needs to know about the pthread_* functions from -lpthread before it can resolve references to those functions in later libraries.
I believe the use of weak references is so that you can have functions in -lc that need -lpthread, but if you don't reference those functions in -lc than it isn't an error that its references to pthread_* couldn't be resolved.

CMake: correctly linking system library using gcc

I have a static libary mylib that depends on the math library.
If I first link mylib with math and then to my executable it works:
add_executable(myapp main.c)
target_link_libraries(mylib m)
target_link_libraries(myapp mylib)
But if I do the linking directly with the executable it fails when using gcc (with clang it works!)
add_executable(myapp main.c)
target_link_libraries(myapp m mylib)
Why does this make any difference?
I thought that it is anyway not possible to link libraries together?
When using cmake's target_link_libraries it does not mean you will link anything. It rather will create a dependency between a target and a library of type/action link.
I guess that the actually build line of the first example will result in something like that:
gcc -o myapp myapp.o -lmylib -lm
and the second one
gcc -o myapp myapp.o -lm -lmylib
. If mylib has references to m the second example (might) not link.
Try to run make VERBOSE=1 and study the command-line of the link-process to really understand what's happening. The linker of clang is maybe intelligent and waits for all calls to be linked before actually dropping a library during the link-process.
When using target_link_libraries it matters in which order you specify linked libraries.
This does not work when using gcc (at least in v4.6.3):
target_link_libraries(myapp m mylib)
while this works:
target_link_libraries(myapp mylib m)
So all libraries mylib depends on have to come after mylib.
If you track down the actual linker invocation with make VERBOSE=1 you will find this for the broken example:
gcc main.c.o -o luatest -rdynamic -lm mylib.a
and this for the working one:
gcc main.c.o -o luatest -rdynamic mylib.a -lm
Invoking clang with the exact same parameters works in both cases!
So #PatrickB seems to be right:
The linker of clang is maybe intelligent and waits for all calls to be
linked before actually dropping a library during the link-process.

Compiling a dynamically linked library

I'm currently trying to compile a dynamically linked library (for a plugin system) using Windows and MinGW.
I compile each objects using this command line :
mingw-g++ -fPIC test.cpp
And the library using this line:
mingw-g++ -rdynamic -shared -Wl,-soname,test.so.1 -o test.so test.o
It doesn't work at all (using GCC with Linux, a similar line works though) : fPIC and rdynamic are ignored for some reason.
And while trying to make the library, it fails because the compiler try to link it with objects that are supposed to be resolved as I dynamically link it with the main binary.
So how do you compile this using MinGW?
Thanks :) !
-fPIC and -rdynamic are ignored because they are unused for Windows.
Also, .so is not the correct output extension for libraries on Windows.
To make a shared library for/on windows with GCC:
mingw-g++ -c file.cpp -o file.o
mingw-g++ -shared -Wl,--out-implib,libfile.a -o file.dll file.o
No more, no less.
And, documentation is always lovely to have: http://www.mingw.org/wiki/sampleDLL

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