Re: [PATCH c++ 06/12] Fix constness problem in ioscm_make_gdb_stdio_port

[Pedro Alves <palves@redhat.com>]

On 10/26/2015 03:46 AM, Simon Marchi wrote:
> ioscm_make_gdb_stdio_port passes const char pointers (literal strings) to
> scm_mode_bits, which takes a non-const char pointer.  Ideally, we would
> change scm_mode_bits to take a const char pointer, but it's not part of
> an API we control.
> 
> Instead, it's easy enough to build the string to pass to scm_mode_bits in
> a (non-const) char array and pass that.
> 
> gdb/ChangeLog:
> 
> 	* guile/scm-ports.c (ioscm_make_gdb_stdio_port): Pass non-const
> 	char pointer to scm_mode_bits.
> ---
>  gdb/guile/scm-ports.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/gdb/guile/scm-ports.c b/gdb/guile/scm-ports.c
> index 925f3b2..49e4fd6 100644
> --- a/gdb/guile/scm-ports.c
> +++ b/gdb/guile/scm-ports.c
> @@ -357,29 +357,38 @@ ioscm_init_stdio_buffers (SCM port, long mode_bits)
>  static SCM
>  ioscm_make_gdb_stdio_port (int fd)
>  {
> -  int is_a_tty = isatty (fd);
>    const char *name;
>    long mode_bits;
>    SCM port;
> +  char buf[3];
> +
> +  memset (buf, 0, sizeof (buf));
>  
>    switch (fd)
>      {
>      case 0:
>        name = input_port_name;
> -      mode_bits = scm_mode_bits (is_a_tty ? "r0" : "r");
> +      buf[0] = 'r';
>        break;
>      case 1:
>        name = output_port_name;
> -      mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
> +      buf[0] = 'w';
>        break;
>      case 2:
>        name = error_port_name;
> -      mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
> +      buf[0] = 'w';
>        break;
>      default:
>        gdb_assert_not_reached ("bad stdio file descriptor");
>      }
>  
> +  if (isatty (fd))
> +    {
> +      buf[1] = '0';
> +    }
> +
> +  mode_bits = scm_mode_bits (buf);
> +
>    port = ioscm_open_port (stdio_port_desc, mode_bits);
>  
>    scm_set_port_filename_x (port, gdbscm_scm_from_c_string (name));
> 

Hmmm, I have the below on my branch, which seems clearer/straightforward
to me.  I don't think the cast in the scm_mode_bits call is problem.

It's a deficiency of the scm_mode_bits api that it doesn't take a const
pointer, and we actually already have this in ioscm_parse_mode_bits:

  /* Kinda awkward to convert the mode from SCM -> string only to have Guile
     convert it back to SCM, but that's the API we have to work with.  */
  mode_bits = scm_mode_bits ((char *) mode);

WDYT?

From f1038881f2b79d26968f2552b86612324fdf72c3 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Thu, 22 Oct 2015 11:43:57 +0100
Subject: [PATCH] guile: 'const char *' -> 'char *' casts
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

/home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c: In function ‘scm_unused_struct* ioscm_make_gdb_stdio_port(int)’:
/home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c:369:55: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
       mode_bits = scm_mode_bits (is_a_tty ? "r0" : "r");
                                                       ^
In file included from /usr/include/guile/2.0/libguile/fports.h:28:0,
                 from /usr/include/guile/2.0/libguile.h:55,
                 from /home/pedro/gdb/mygit/src/gdb/guile/guile-internal.h:29,
                 from /home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c:28:
/usr/include/guile/2.0/libguile/ports.h:281:14: error:   initializing argument 1 of ‘long int scm_mode_bits(char*)’ [-fpermissive]
 SCM_API long scm_mode_bits (char *modes);
              ^
/home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c:373:55: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
       mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
                                                       ^
In file included from /usr/include/guile/2.0/libguile/fports.h:28:0,
                 from /usr/include/guile/2.0/libguile.h:55,
                 from /home/pedro/gdb/mygit/src/gdb/guile/guile-internal.h:29,
                 from /home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c:28:
/usr/include/guile/2.0/libguile/ports.h:281:14: error:   initializing argument 1 of ‘long int scm_mode_bits(char*)’ [-fpermissive]
 SCM_API long scm_mode_bits (char *modes);
              ^
/home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c:377:55: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
       mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
                                                       ^
In file included from /usr/include/guile/2.0/libguile/fports.h:28:0,
                 from /usr/include/guile/2.0/libguile.h:55,
                 from /home/pedro/gdb/mygit/src/gdb/guile/guile-internal.h:29,
                 from /home/pedro/gdb/mygit/src/gdb/guile/scm-ports.c:28:
/usr/include/guile/2.0/libguile/ports.h:281:14: error:   initializing argument 1 of ‘long int scm_mode_bits(char*)’ [-fpermissive]
 SCM_API long scm_mode_bits (char *modes);
---
 gdb/guile/scm-ports.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/gdb/guile/scm-ports.c b/gdb/guile/scm-ports.c
index 90bdb39..d8ee70e 100644
--- a/gdb/guile/scm-ports.c
+++ b/gdb/guile/scm-ports.c
@@ -359,6 +359,7 @@ ioscm_make_gdb_stdio_port (int fd)
 {
   int is_a_tty = isatty (fd);
   const char *name;
+  const char *mode_str;
   long mode_bits;
   SCM port;
 
@@ -366,20 +367,21 @@ ioscm_make_gdb_stdio_port (int fd)
     {
     case 0:
       name = input_port_name;
-      mode_bits = scm_mode_bits (is_a_tty ? "r0" : "r");
+      mode_str = is_a_tty ? "r0" : "r";
       break;
     case 1:
       name = output_port_name;
-      mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
+      mode_str = is_a_tty ? "w0" : "w";
       break;
     case 2:
       name = error_port_name;
-      mode_bits = scm_mode_bits (is_a_tty ? "w0" : "w");
+      mode_str = is_a_tty ? "w0" : "w";
       break;
     default:
       gdb_assert_not_reached ("bad stdio file descriptor");
     }
 
+  mode_bits = scm_mode_bits ((char *) mode_str);
   port = ioscm_open_port (stdio_port_desc, mode_bits);
 
   scm_set_port_filename_x (port, gdbscm_scm_from_c_string (name));
-- 
1.9.3