C Interoperability: command-line arguments

[ivanpribec]

@RonShepard

That is, I would not have thought that the actual command line arguments would have been copied or otherwise manipulated by the fortran runtime library, and that it is accessing the same data that would be accessed from a C or C++ program (or any other program running on that OS).

I assumed that too, but looking at the symbol tables on Linux and Mac, I noticed that all three compilers (gfortran, ifort, and nvfortran) have this thing with MAIN_ and main going on. A colleague remarked that this must be some aboriginal Unix/ELF convention. The gfortran documentation for “Non-Fortran Main Program’s” states this is for historical reasons:

When you compile a PROGRAM with GNU Fortran, a function with the name main (in the symbol table of the object file) is generated, which initializes the libgfortran library and then calls the actual program which uses the name MAIN__, for historic reasons. If you link GNU Fortran compiled procedures to, e.g., a C or C++ program or to a Fortran program compiled by a different compiler, the libgfortran library is not initialized and thus a few intrinsic procedures do not work properly, e.g. those for obtaining the command-line arguments.

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@RonShepard

Of course, once a command line argument has been returned through the fortran intrinsic, that is a separate copy with whatever fortran metadata there is to define it as a fortran character string, and that local copy can be modified without affecting the original OS structure.

In libgfortran/runtime/main.c you can inspect that the runtime simply makes a copy of the input arguments from the C main:

static int argc_save;
static char **argv_save;


/* Set the saved values of the command line arguments.  */

void
set_args (int argc, char **argv)
{
  argc_save = argc;
  argv_save = argv;
}
iexport(set_args);