[PATCH v2] PR gdb/33747: gdb/ser-unix: Avoid musl build failure when setting custom baud rates

[sunilkumar.dora@windriver.com]

From: Sunil Dora <sunilkumar.dora@windriver.com>

The Linux custom baud rate implementation accessed the struct termios 
members c_ispeed and c_ospeed directly. These fields are provided by 
glibc but are not exposed by musl, which causes the build to fail on 
musl-based systems.

Adjust set_custom_baudrate_linux to use a capability-based approach.
The Autoconf check HAVE_NUMERIC_BAUD_RATES determines whether
B-constants match numeric baud rates. If they do, use the standard
POSIX cfsetispeed and cfsetospeed interfaces. Otherwise, fall back
to the Linux-specific termios2 interface (TCGETS2) to support
arbitrary baud rates.

This preserves existing behavior on glibc systems while restoring
build compatibility with musl.

Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>
---
 gdb/NEWS         |  4 ++++
 gdb/config.in    |  3 +++
 gdb/configure    | 39 +++++++++++++++++++++++++++++++++++++++
 gdb/configure.ac | 19 +++++++++++++++++++
 gdb/ser-unix.c   | 42 ++++++++++++++++++++++++++----------------
 5 files changed, 91 insertions(+), 16 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index 47d8189d344..003a1700492 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -36,6 +36,10 @@
   subsequent runs of the inferior will use the same arguments as the
   first run.
 
+* GDB now detects if the host system uses numeric values for termios
+  B-constants.  If not, GDB now uses the Linux termios2 interface to
+  support custom baud rates.
+
 * Support for stabs debug information has been removed.
 
 * Support for mdebug debug information has been removed.
diff --git a/gdb/config.in b/gdb/config.in
index b11fcf18372..a7b8ca16d8c 100644
--- a/gdb/config.in
+++ b/gdb/config.in
@@ -346,6 +346,9 @@
 /* Define to 1 if you have the <nlist.h> header file. */
 #undef HAVE_NLIST_H
 
+/* Define if termios B-constants match their numeric baud rate */
+#undef HAVE_NUMERIC_BAUD_RATES
+
 /* Define to 1 if you have the `pipe' function. */
 #undef HAVE_PIPE
 
diff --git a/gdb/configure b/gdb/configure
index 12c54521682..aee49cc7d74 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -27728,6 +27728,45 @@ if test "$ac_res" != no; then :
 fi
 
 
+# Check whether termios B-constants match their numeric baud rate.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether termios B-constants are numeric" >&5
+$as_echo_n "checking whether termios B-constants are numeric... " >&6; }
+if ${gdb_cv_termios_numeric_baud+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <termios.h>
+int
+main ()
+{
+
+       #if B9600 != 9600
+       #error B-constants are not numeric symbols
+       #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_termios_numeric_baud=yes
+else
+  gdb_cv_termios_numeric_baud=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_termios_numeric_baud" >&5
+$as_echo "$gdb_cv_termios_numeric_baud" >&6; }
+
+if test "$gdb_cv_termios_numeric_baud" = yes; then
+
+$as_echo "#define HAVE_NUMERIC_BAUD_RATES 1" >>confdefs.h
+
+fi
+
 
 
 # Check whether --with-jit-reader-dir was given.
diff --git a/gdb/configure.ac b/gdb/configure.ac
index cf8078e1d89..ef8f085c824 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -725,6 +725,25 @@ AC_CONFIG_FILES([jit-reader.h:jit-reader.in])
 
 AC_SEARCH_LIBS(dlopen, dl)
 
+# Check whether termios B-constants match their numeric baud rate.
+AC_CACHE_CHECK([whether termios B-constants are numeric],
+  [gdb_cv_termios_numeric_baud], [
+  AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([[#include <termios.h>]],
+      [[
+       #if B9600 != 9600
+       #error B-constants are not numeric symbols
+       #endif
+      ]])],
+    [gdb_cv_termios_numeric_baud=yes],
+    [gdb_cv_termios_numeric_baud=no])
+])
+
+if test "$gdb_cv_termios_numeric_baud" = yes; then
+  AC_DEFINE([HAVE_NUMERIC_BAUD_RATES], [1],
+	    [Define if termios B-constants match their numeric baud rate])
+fi
+
 GDB_AC_WITH_DIR([JIT_READER_DIR], [jit-reader-dir],
 		[directory to load the JIT readers from],
 		[${libdir}/gdb])
diff --git a/gdb/ser-unix.c b/gdb/ser-unix.c
index c295a9c5ba1..410639ca460 100644
--- a/gdb/ser-unix.c
+++ b/gdb/ser-unix.c
@@ -515,31 +515,41 @@ set_baudcode_baudrate (struct serial *scb, int baud_code)
 static void
 set_custom_baudrate_linux (int fd, int rate)
 {
-#ifdef TCGETS2
-  struct termios2 tio;
-  const unsigned long req_get = TCGETS2;
-  const unsigned long req_set = TCSETS2;
-#else
+#if defined (HAVE_NUMERIC_BAUD_RATES)
   struct termios tio;
-  const unsigned long req_get = TCGETS;
-  const unsigned long req_set = TCSETS;
-#endif
 
-  if (ioctl (fd, req_get, &tio) < 0)
+  if (tcgetattr (fd, &tio) < 0)
+    perror_with_name (_("Can not get current baud rate"));
+
+  cfsetispeed (&tio, rate);
+  cfsetospeed (&tio, rate);
+
+  if (tcsetattr (fd, TCSANOW, &tio) < 0)
+    perror_with_name (_("Can not set custom baud rate"));
+
+#elif defined (TCGETS2)
+  /* Use the kernel-level termios2 structure.  */
+  struct termios2 tio2;
+
+  if (ioctl (fd, TCGETS2, &tio2) < 0)
     perror_with_name (_("Can not get current baud rate"));
 
   /* Clear the current output baud rate and fill a new value.  */
-  tio.c_cflag &= ~CBAUD;
-  tio.c_cflag |= BOTHER;
-  tio.c_ospeed = rate;
+  tio2.c_cflag &= ~CBAUD;
+  tio2.c_cflag |= BOTHER;
+  tio2.c_ospeed = rate;
 
   /* Clear the current input baud rate and fill a new value.  */
-  tio.c_cflag &= ~(CBAUD << IBSHIFT);
-  tio.c_cflag |= BOTHER << IBSHIFT;
-  tio.c_ispeed = rate;
+  tio2.c_cflag &= ~(CBAUD << IBSHIFT);
+  tio2.c_cflag |= BOTHER << IBSHIFT;
+  tio2.c_ispeed = rate;
 
-  if (ioctl (fd, req_set, &tio) < 0)
+  if (ioctl (fd, TCSETS2, &tio2) < 0)
     perror_with_name (_("Can not set custom baud rate"));
+
+#else
+  perror_with_name (_("Custom baud rate not supported on this platform"));
+#endif
 }
 
 #elif HAVE_CUSTOM_BAUDRATE_SUPPORT && defined(IOSSIOSPEED)
-- 
2.49.0