When g77 compiles a main program unit, it gives it the public
procedure name MAIN__.
The libg2c library has the actual main() procedure
as is typical of C-based environments, and
it is this procedure that performs some initial start-up
activity and then calls MAIN__.
Generally, g77 and libg2c are designed so that you need not
include a main program unit written in Fortran in your program--it
can be written in C or some other language.
Especially for I/O handling, this is the case, although g77 version 0.5.16
includes a bug fix for libg2c that solved a problem with using the
OPEN statement as the first Fortran I/O activity in a program
without a Fortran main program unit.
However, if you don't intend to use g77 (or f2c) to compile
your main program unit--that is, if you intend to compile a main()
procedure using some other language--you should carefully
examine the code for main() in libg2c, found in the source
file gcc/libf2c/libF77/main.c, to see what kinds of things
might need to be done by your main() in order to provide the
Fortran environment your Fortran code is expecting.
For example, libg2c's main() sets up the information used by
the IARGC and GETARG intrinsics.
Bypassing libg2c's main()
without providing a substitute for this activity would mean
that invoking IARGC and GETARG would produce undefined
results.
When debugging, one implication of the fact that main(), which
is the place where the debugged program "starts" from the
debugger's point of view, is in libg2c is that you won't be
starting your Fortran program at a point you recognize as your
Fortran code.
The standard way to get around this problem is to set a break
point (a one-time, or temporary, break point will do) at
the entrance to MAIN__, and then run the program.
A convenient way to do so is to add the gdb command
tbreak MAIN__
to the file .gdbinit in the directory in which you're debugging
(using gdb).
After doing this, the debugger will see the current execution point of the program as at the beginning of the main program unit of your program.
Of course, if you really want to set a break point at some
other place in your program and just start the program
running, without first breaking at MAIN__,
that should work fine.