Unlike with some runtime systems, it shouldn't be necessary (unless there are bugs) to use a Fortran main program unit to ensure the runtime—specifically the I/O system—is initialized.
However, to use the g77 intrinsics GETARG and IARGC,
either the main routine from the libg2c library must be used,
or the f_setarg routine
(new as of egcs version 1.1 and g77 version 0.5.23)
must be called with the appropriate argc and argv arguments
prior to the program calling GETARG or IARGC.
To provide more flexibility for mixed-language programming
involving g77 while allowing for shared libraries,
as of egcs version 1.1 and g77 version 0.5.23,
g77's main routine in libg2c
does the following, in order:
f_setarg
with the incoming argc and argv arguments,
in the same order as for main itself.
This sets up the command-line environment
for GETARG and IARGC.
f_setsig (with no arguments).
This sets up the signaling and exception environment.
f_init (with no arguments).
This initializes the I/O environment,
though that should not be necessary,
as all I/O functions in libf2c
are believed to call f_init automatically,
if necessary.
(A future version of g77 might skip this explicit step, to speed up normal exit of a program.)
f_exit to be called (with no arguments)
when the program exits.
This ensures that the I/O environment is properly shut down before the program exits normally. Otherwise, output buffers might not be fully flushed, scratch files might not be deleted, and so on.
The simple way main does this is
to call f_exit itself after calling
MAIN__ (in the next step).
However, this does not catch the cases where the program
might call exit directly,
instead of using the EXIT intrinsic
(implemented as exit_ in libf2c).
So, main attempts to use
the operating environment's onexit or atexit
facility, if available,
to cause f_exit to be called automatically
upon any invocation of exit.
MAIN__ (with no arguments).
This starts executing the Fortran main program unit for
the application.
(Both g77 and f2c currently compile a main
program unit so that its global name is MAIN__.)
onexit or atexit is provided by the system,
calls f_exit.
exit with a zero argument,
to signal a successful program termination.
exit doesn't exit on the system.
All of the above names are C extern names,
i.e. not mangled.
When using the main procedure provided by g77
without a Fortran main program unit,
you need to provide MAIN__
as the entry point for your C code.
(Make sure you link the object file that defines that
entry point with the rest of your program.)
To provide your own main procedure
in place of g77's,
make sure you specify the object file defining that procedure
before -lg2c on the g77 command line.
Since the -lg2c option is implicitly provided,
this is usually straightforward.
(Use the --verbose option to see how and where
g77 implicitly adds -lg2c in a command line
that will link the program.
Feel free to specify -lg2c explicitly,
as appropriate.)
However, when providing your own main,
make sure you perform the appropriate tasks in the
appropriate order.
For example, if your main does not call f_setarg,
make sure the rest of your application does not call
GETARG or IARGC.
And, if your main fails to ensure that f_exit
is called upon program exit,
some files might end up incompletely written,
some scratch files might be left lying around,
and some existing files being written might be left
with old data not properly truncated at the end.
Note that, generally, the g77 operating environment
does not depend on a procedure named MAIN__ actually
being called prior to any other g77-compiled code.
That is, MAIN__ does not, itself,
set up any important operating-environment characteristics
upon which other code might depend.
This might change in future versions of g77,
with appropriate notification in the release notes.
For more information, consult the source code for the above routines. These are in gcc/libf2c/libF77/, named main.c, setarg.c, setsig.c, getarg_.c, and iargc_.c.
Also, the file gcc/gcc/f/com.c contains the code g77
uses to open-code (inline) references to IARGC.