GDB: Add `info main' command

Allow consumers of GDB to extract the name of the main method.  This is
most useful for Fortran programs which have a variable main method.

Used by both MAP and DDT e.g. it is used to detect the presence of debug
information.

Co-Authored-By: Maciej W. Rozycki <macro@embecosm.com>

gdb/NEWS

@@ -83,6 +83,9 @@ show always-read-ctf
   When off, CTF is only read if DWARF is not present.  When on, CTF is
   read regardless of whether DWARF is present.  Off by default.
 
+info main
+  Get main symbol to identify entry point into program.
+
 * New convenience function "$_shell", to execute a shell command and
   return the result.  This lets you run shell commands in expressions.
   Some examples:

gdb/doc/gdb.texinfo

@@ -20162,6 +20162,12 @@ The optional flag @samp{-q}, which stands for @samp{quiet}, disables
 printing header information and messages explaining why no functions
 or variables have been printed.
 
+@kindex info main
+@item info main
+Print the name of the starting function of the program.  This serves
+primarily Fortran programs, which have a user-supplied name for the
+main subroutine.
+
 @kindex info classes
 @cindex Objective-C, classes and selectors
 @item info classes

gdb/symtab.c

@@ -5282,6 +5282,14 @@ info_modules_command (const char *args, int from_tty)
 		      from_tty);
 }
 
+/* Implement the 'info main' command.  */
+
+static void
+info_main_command (const char *args, int from_tty)
+{
+  gdb_printf ("%s\n", main_name ());
+}
+
 static void
 rbreak_command (const char *regexp, int from_tty)
 {
@@ -6873,6 +6881,9 @@ Options:\n\
 		_("All module names, or those matching REGEXP."));
   set_cmd_completer_handle_brkchars (c, info_types_command_completer);
 
+  add_info ("main", info_main_command,
+	    _("Get main symbol to identify entry point into program."));
+
   add_basic_prefix_cmd ("module", class_info, _("\
 Print information about modules."),
 			&info_module_cmdlist, 0, &infolist);

gdb/testsuite/gdb.fortran/info-main.exp

@@ -0,0 +1,42 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+require allow_fortran_tests
+
+load_lib fortran.exp
+
+standard_testfile .f90
+
+set old_gdbflags $GDBFLAGS
+set GDBFLAGS [string map {"-readnow" ""} "$GDBFLAGS"]
+
+if {[prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} {debug f90}]} {
+    return -1
+}
+
+gdb_test "info main" "simple" "info main prior to start"
+
+if ![fortran_runto_main] {
+    untested "could not run to main"
+    return -1
+}
+
+gdb_test "info main" "simple" "info main post start"
+
+set GDBFLAGS "$GDBFLAGS -readnow"
+clean_restart $testfile
+gdb_test "info main" "simple" "info main with readnow"
+
+set GDBFLAGS $old_gdbflags

gdb/testsuite/gdb.fortran/info-main.f90

@@ -0,0 +1,19 @@
+! Copyright 2023 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+program simple
+    implicit none
+    print *, "Nothing."
+end program simple