gdb: make string-like set show commands use std::string variable

String-like settings (var_string, var_filename, var_optional_filename,
var_string_noescape) currently take a pointer to a `char *` storage
variable (typically global) that holds the setting's value.  I'd like to
"mordernize" this by changing them to use an std::string for storage.

An obvious reason is that string operations on std::string are often
easier to write than with C strings.  And they avoid having to do any
manual memory management.

Another interesting reason is that, with `char *`, nullptr and an empty
string often both have the same meaning of "no value".  String settings
are initially nullptr (unless initialized otherwise).  But when doing
"set foo" (where `foo` is a string setting), the setting now points to
an empty string.  For example, solib_search_path is nullptr at startup,
but points to an empty string after doing "set solib-search-path".  This
leads to some code that needs to check for both to check for "no value".
Or some code that converts back and forth between NULL and "" when
getting or setting the value.  I find this very error-prone, because it
is very easy to forget one or the other.  With std::string, we at least
know that the variable is not "NULL".  There is only one way of
representing an empty string setting, that is with an empty string.

I was wondering whether the distinction between NULL and "" would be
important for some setting, but it doesn't seem so.  If that ever
happens, it would be more C++-y and self-descriptive to use
optional<string> anyway.

Actually, there's one spot where this distinction mattered, it's in
init_history, for the test gdb.base/gdbinit-history.exp.  init_history
sets the history filename to the default ".gdb_history" if it sees that
the setting was never set - if history_filename is nullptr.  If
history_filename is an empty string, it means the setting was explicitly
cleared, so it leaves it as-is.  With the change to std::string, this
distinction doesn't exist anymore.  This can be fixed by moving the code
that chooses a good default value for history_filename to
_initialize_top.  This is ran before -ex commands are processed, so an
-ex command can then clear that value if needed (what
gdb.base/gdbinit-history.exp tests).

Another small improvement, in my opinion is that we can now easily
give string parameters initial values, by simply initializing the global
variables, instead of xstrdup-ing it in the _initialize function.

In Python and Guile, when registering a string-like parameter, we
allocate (with new) an std::string that is owned by the param_smob (in
Guile) and the parmpy_object (in Python) objects.

This patch started by changing all relevant add_setshow_* commands to
take an `std::string *` instead of a `char **` and fixing everything
that failed to build.  That includes of course all string setting
variable and their uses.

string_option_def now uses an std::string also, because there's a
connection between options and settings (see
add_setshow_cmds_for_options).

The add_path function in source.c is really complex and twisted, I'd
rather not try to change it to work on an std::string right now.
Instead, I added an overload that copies the std:string to a `char *`
and back.  This means more copying, but this is not used in a hot path
at all, so I think it is acceptable.

Change-Id: I92c50a1bdd8307141cdbacb388248e4e4fc08c93
Co-authored-by: Lancelot SIX <lsix@lancelotsix.com>

1 files changed, 23 insertions, 23 deletions

diff --git a/gdb/compile/compile.c b/gdb/compile/compile.c
index e815348..25cfd28 100644
--- a/gdb/compile/compile.c
+++ b/gdb/compile/compile.c
@@ -495,7 +495,22 @@ get_expr_block_and_pc (CORE_ADDR *pc)
 }
 
 /* String for 'set compile-args' and 'show compile-args'.  */
-static char *compile_args;
+static std::string compile_args =
+  /* Override flags possibly coming from DW_AT_producer.  */
+  "-O0 -gdwarf-4"
+  /* We use -fPIE Otherwise GDB would need to reserve space large enough for
+     any object file in the inferior in advance to get the final address when
+     to link the object file to and additionally the default system linker
+     script would need to be modified so that one can specify there the
+     absolute target address.
+     -fPIC is not used at is would require from GDB to generate .got.  */
+  " -fPIE"
+  /* We want warnings, except for some commonly happening for GDB commands.  */
+  " -Wall "
+  " -Wno-unused-but-set-variable"
+  " -Wno-unused-variable"
+  /* Override CU's possible -fstack-protector-strong.  */
+  " -fno-stack-protector";
 
 /* Parsed form of COMPILE_ARGS.  */
 static gdb_argv compile_args_argv;
@@ -505,7 +520,7 @@ static gdb_argv compile_args_argv;
 static void
 set_compile_args (const char *args, int from_tty, struct cmd_list_element *c)
 {
-  compile_args_argv = gdb_argv (compile_args);
+  compile_args_argv = gdb_argv (compile_args.c_str ());
 }
 
 /* Implement 'show compile-args'.  */
@@ -520,7 +535,7 @@ show_compile_args (struct ui_file *file, int from_tty,
 }
 
 /* String for 'set compile-gcc' and 'show compile-gcc'.  */
-static char *compile_gcc;
+static std::string compile_gcc;
 
 /* Implement 'show compile-gcc'.  */
 
@@ -696,13 +711,13 @@ compile_to_object (struct command_line *cmd, const char *cmd_string,
 
   compiler->set_verbose (compile_debug);
 
-  if (compile_gcc[0] != 0)
+  if (!compile_gcc.empty ())
     {
       if (compiler->version () < GCC_FE_VERSION_1)
 	error (_("Command 'set compile-gcc' requires GCC version 6 or higher "
 		 "(libcc1 interface version 1 or higher)"));
 
-      compiler->set_driver_filename (compile_gcc);
+      compiler->set_driver_filename (compile_gcc.c_str ());
     }
   else
     {
@@ -1029,23 +1044,9 @@ String quoting is parsed like in shell, for example:\n\
   -mno-align-double \"-I/dir with a space/include\""),
 			  set_compile_args, show_compile_args, &setlist, &showlist);
 
-  /* Override flags possibly coming from DW_AT_producer.  */
-  compile_args = xstrdup ("-O0 -gdwarf-4"
-  /* We use -fPIE Otherwise GDB would need to reserve space large enough for
-     any object file in the inferior in advance to get the final address when
-     to link the object file to and additionally the default system linker
-     script would need to be modified so that one can specify there the
-     absolute target address.
-     -fPIC is not used at is would require from GDB to generate .got.  */
-			 " -fPIE"
-  /* We want warnings, except for some commonly happening for GDB commands.  */
-			 " -Wall "
-			 " -Wno-unused-but-set-variable"
-			 " -Wno-unused-variable"
-  /* Override CU's possible -fstack-protector-strong.  */
-			 " -fno-stack-protector"
-  );
-  set_compile_args (compile_args, 0, NULL);
+
+  /* Initialize compile_args_argv.  */
+  set_compile_args (compile_args.c_str (), 0, NULL);
 
   add_setshow_optional_filename_cmd ("compile-gcc", class_support,
 				     &compile_gcc,
@@ -1058,5 +1059,4 @@ It should be absolute filename of the gcc executable.\n\
 If empty the default target triplet will be searched in $PATH."),
 				     NULL, show_compile_gcc, &setlist,
 				     &showlist);
-  compile_gcc = xstrdup ("");
 }