guile: Change return value of gdbscm_with_guile for const char *

The documentation of gdbscm_with_guile says that it returns a statically
allocated string (IOW, a const char *).  We can reflect that in its
return value type, and get rid of C++ build errors.

Initially fixes:

/home/simark/src/binutils-gdb/gdb/guile/scm-disasm.c: In function ‘void* gdbscm_disasm_read_memory_worker(void*)’:
/home/simark/src/binutils-gdb/gdb/guile/scm-disasm.c:93:12: error: invalid conversion from ‘const void*’ to ‘void*’ [-fpermissive]
     return "seek error";

gdb/ChangeLog:

	* guile/guile-internal.h (gdbscm_with_guile): Change return
	types to const char *.
	* guile/scm-safe-call.c (gdbscm_with_guile): Likewise.
	(struct c_data) <func>: Likewise.
	(struct c_data) <result>: Change type to const char *.
	(scscm_eval_scheme_string): Change return type to
	const char *.
	(scscm_source_scheme_script): Likewise.
	(gdbscm_safe_eval_string): Change type of result variable to
	const char * and remove cast.
	(gdbscm_safe_source_script): Likewise.
	* guile/scm-disasm.c (gdbscm_disasm_read_memory_worker):
	Change return type to const char *.
	(gdbscm_disasm_read_memory): Change type of status to
	const char *.

gdb/ChangeLog

@@ -1,3 +1,21 @@
+2015-10-27  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* guile/guile-internal.h (gdbscm_with_guile): Change return
+	types to const char *.
+	* guile/scm-safe-call.c (gdbscm_with_guile): Likewise.
+	(struct c_data) <func>: Likewise.
+	(struct c_data) <result>: Change type to const char *.
+	(scscm_eval_scheme_string): Change return type to
+	const char *.
+	(scscm_source_scheme_script): Likewise.
+	(gdbscm_safe_eval_string): Change type of result variable to
+	const char * and remove cast.
+	(gdbscm_safe_source_script): Likewise.
+	* guile/scm-disasm.c (gdbscm_disasm_read_memory_worker):
+	Change return type to const char *.
+	(gdbscm_disasm_read_memory): Change type of status to
+	const char *.
+
 2015-10-27  Pedro Alves  <palves@redhat.com>
 
 	* source.c (openp): New local 'last_errno'.  Use it to

gdb/guile/guile-internal.h

@@ -384,7 +384,7 @@ extern void gdbscm_memory_error (const char *subr, const char *msg, SCM args)
 
 /* scm-safe-call.c */
 
-extern void *gdbscm_with_guile (void *(*func) (void *), void *data);
+extern const char *gdbscm_with_guile (const char *(*func) (void *), void *data);
 
 extern SCM gdbscm_call_guile (SCM (*func) (void *), void *data,
 			      excp_matcher_func *ok_excps);

gdb/guile/scm-disasm.c

@@ -76,7 +76,7 @@ dascm_make_insn (CORE_ADDR pc, const char *assembly, int insn_len)
    Scheme port.  Called via gdbscm_call_guile.
    The result is a statically allocated error message or NULL if success.  */
 
-static void *
+static const char *
 gdbscm_disasm_read_memory_worker (void *datap)
 {
   struct gdbscm_disasm_read_data *data
@@ -109,7 +109,7 @@ gdbscm_disasm_read_memory (bfd_vma memaddr, bfd_byte *myaddr,
 			   struct disassemble_info *dinfo)
 {
   struct gdbscm_disasm_read_data data;
-  void *status;
+  const char *status;
 
   data.memaddr = memaddr;
   data.myaddr = myaddr;

gdb/guile/scm-safe-call.c

@@ -28,10 +28,10 @@
 
 struct c_data
 {
-  void *(*func) (void *);
+  const char *(*func) (void *);
   void *data;
   /* An error message or NULL for success.  */
-  void *result;
+  const char *result;
 };
 
 /* Struct to marshall args through gdbscm_with_catch.  */
@@ -167,8 +167,8 @@ gdbscm_with_catch (void *data)
    The result if NULL if no exception occurred, otherwise it is a statically
    allocated error message (caller must *not* free).  */
 
-void *
-gdbscm_with_guile (void *(*func) (void *), void *data)
+const char *
+gdbscm_with_guile (const char *(*func) (void *), void *data)
 {
   struct c_data c_data;
   struct with_catch_data catch_data;
@@ -369,7 +369,7 @@ struct eval_scheme_string_data
 /* Wrapper to eval a C string in the Guile interpreter.
    This is passed to gdbscm_with_guile.  */
 
-static void *
+static const char *
 scscm_eval_scheme_string (void *datap)
 {
   struct eval_scheme_string_data *data
@@ -398,12 +398,12 @@ char *
 gdbscm_safe_eval_string (const char *string, int display_result)
 {
   struct eval_scheme_string_data data = { string, display_result };
-  void *result;
+  const char *result;
 
   result = gdbscm_with_guile (scscm_eval_scheme_string, (void *) &data);
 
   if (result != NULL)
-    return xstrdup ((char *) result);
+    return xstrdup (result);
   return NULL;
 }
 
@@ -411,7 +411,7 @@ gdbscm_safe_eval_string (const char *string, int display_result)
 
 /* Helper function for gdbscm_safe_source_scheme_script.  */
 
-static void *
+static const char *
 scscm_source_scheme_script (void *data)
 {
   const char *filename = (const char *) data;
@@ -439,7 +439,7 @@ gdbscm_safe_source_script (const char *filename)
      by default.  This function is invoked by the "source" GDB command which
      already has its own path search support.  */
   char *abs_filename = NULL;
-  void *result;
+  const char *result;
 
   if (!IS_ABSOLUTE_PATH (filename))
     {
@@ -452,7 +452,7 @@ gdbscm_safe_source_script (const char *filename)
 
   xfree (abs_filename);
   if (result != NULL)
-    return xstrdup ((char *) result);
+    return xstrdup (result);
   return NULL;
 }