Building an executable shared library with ifort

Question

There are several excellent discussions on SO already covering how to produce an executable shared library on Linux:

• See https://unix.stackexchange.com/questions/7066
• and building a .so that is also an executable as examples

In C/C++, this seems relatively straightforward; essentially there are two parts:

1. Add an .interp section to the ELF (as ld doesn't include one for shared libraries) by including something along these lines in the library source:
   const char interp_section[] __attribute__((section(".interp"))) = "/path/to/dynamic/linker";
2. Set an appropriate entry point when linking, using -Wl,-e,entry_point

Does anyone know how to achieve this with a library written in Fortran? Specifically, how to add a .interp section to a shared library compiled with ifort?

Answer 1

With help of a C compiler to create an additional object file to be linked to the dynamic library, such a Fortran90 executable and dynamic link library can be created:

/* stub.c: compile e.g. with gcc -c stub.c
const char dl_loader[] __attribute__((section(".interp"))) =
    "/lib64/ld-linux-x86-64.so.2";
    /* adjust string if path or architecture is different */

! testif.f90: compile e.g. with ifort -c -fPIC testif.f90

subroutine execentry
    write(*,*) 'Written from executable.'
    ! without call to exit seems to lead to segmentation fault
    call exit(0)
end subroutine

subroutine libroutine
    write(*,*) 'Written by libroutine.'
end subroutine

! linktest.f90: compile e.g. with ifort -c linktest.f90
! main Fortran program for testing

program linktest

call libroutine

end

For compilation and linking:

gcc -c stub.c
ifort -c -fPIC testif.f90
ifort -c linktest.f90
ifort -shared -o libtestif.so testif.o stub.o -Wl,-e,execentry_
ifort -o linktest linktest.o -L. -ltestif

Executing the dynamic link library directly ./libtestif.so will call execentry, and running the link test program

LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH ./linktest

will call libroutine.

The C code is only needed to create the .interp section. The underscore in the ld flag -Wl,-e,execentry_ is added according to the symbol name mangling for Intel ifort (or GNU gfortran) vs. GNU or Intel C compilers.