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GNU Info (g77-300.info)Startup CodeStartup Code ------------ 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: 1. Calls `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'. 2. Calls `f_setsig' (with no arguments). This sets up the signaling and exception environment. 3. Calls `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.) 4. Arranges for `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'. 5. Calls `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__'.) 6. If no `onexit' or `atexit' is provided by the system, calls `f_exit'. 7. Calls `exit' with a zero argument, to signal a successful program termination. 8. Returns a zero value to the caller, to signal a successful program termination, in case `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'. automatically generated by info2www version 1.2.2.9 |