[binutils-gdb] gdb/ser-unix: add POSIX cfsetispeed/cfsetospeed support for custom baud rates

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=7d53467645b0430e94a3cfd13fc3c16ef19efda5

commit 7d53467645b0430e94a3cfd13fc3c16ef19efda5
Author: Sunil Dora <[email protected]>
Date:   Tue Mar 24 09:45:27 2026 -0700

    gdb/ser-unix: add POSIX cfsetispeed/cfsetospeed support for custom baud rates
    
    glibc 2.42 and later, and GNU Hurd, accept arbitrary baud rates
    directly via cfsetispeed and cfsetospeed.  This behaviour is expected
    to be standardized in a future POSIX revision.
    
    Introduce a configure-time test (HAVE_CFSETSPEED_ARBITRARY) that
    detects this capability and add a new platform-agnostic helper
    set_custom_baudrate_posix.  The main dispatcher now prefers the POSIX
    path when it is available, falling back to the existing Linux (BOTHER)
    or Darwin (IOSSIOSPEED) implementations otherwise.
    
    The HAVE_CUSTOM_BAUDRATE_SUPPORT guard is extended to also cover the
    new POSIX case.
    
    Suggested-by: Kevin Buettner <[email protected]>
    Suggested-by: Maciej W. Rozycki <[email protected]>
    Signed-off-by: Sunil Dora <[email protected]>
    Reviewed-By: Eli Zaretskii <[email protected]>
    Approved-by: Kevin Buettner <[email protected]>

Diff:
---
 gdb/NEWS         |  6 ++++++
 gdb/config.in    |  3 +++
 gdb/configure    | 39 +++++++++++++++++++++++++++++++++++++++
 gdb/configure.ac | 19 +++++++++++++++++++
 gdb/ser-unix.c   | 49 ++++++++++++++++++++++++++++++++++++++++---------
 5 files changed, 107 insertions(+), 9 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index 4cf91053c95..89c2ac4384f 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -36,6 +36,12 @@
   subsequent runs of the inferior will use the same arguments as the
   first run.
 
+* GDB now supports arbitrary (non-standard) baud rates for serial
+  connections to remote targets on systems where the host libc accepts
+  them via cfsetispeed/cfsetospeed, such as glibc 2.42 and later and
+  GNU Hurd.  On other systems, existing platform-specific mechanisms
+  are used.
+
 * 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 6ad5240397b..e357c22e411 100644
--- a/gdb/config.in
+++ b/gdb/config.in
@@ -110,6 +110,9 @@
    the CoreFoundation framework. */
 #undef HAVE_CFPREFERENCESCOPYAPPVALUE
 
+/* Define if cfsetispeed/cfsetospeed accept arbitrary baud rates. */
+#undef HAVE_CFSETSPEED_ARBITRARY
+
 /* Define to 1 if you have the `clearenv' function. */
 #undef HAVE_CLEARENV
 
diff --git a/gdb/configure b/gdb/configure
index 64470a856ad..2fc0b583702 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -27743,6 +27743,45 @@ _ACEOF
 fi
 
 
+# Check whether cfsetispeed/cfsetospeed accept arbitrary baud rates.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether cfsetispeed/cfsetospeed accept arbitrary baud rates" >&5
+$as_echo_n "checking whether cfsetispeed/cfsetospeed accept arbitrary baud rates... " >&6; }
+if ${gdb_cv_cfsetspeed_arbitrary+:} 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 "$LINENO"; then :
+  gdb_cv_cfsetspeed_arbitrary=yes
+else
+  gdb_cv_cfsetspeed_arbitrary=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_cfsetspeed_arbitrary" >&5
+$as_echo "$gdb_cv_cfsetspeed_arbitrary" >&6; }
+
+if test "$gdb_cv_cfsetspeed_arbitrary" = yes; then
+
+$as_echo "#define HAVE_CFSETSPEED_ARBITRARY 1" >>confdefs.h
+
+fi
+
 
 
 # Check whether --with-jit-reader-dir was given.
diff --git a/gdb/configure.ac b/gdb/configure.ac
index 76a22c9cf3d..56ab86d9356 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -728,6 +728,25 @@ AC_SEARCH_LIBS(dlopen, dl)
 # Check for members required by the legacy Linux custom baud rate path.
 AC_CHECK_MEMBERS([struct termios.c_ospeed], [], [], [[#include <termios.h>]])
 
+# Check whether cfsetispeed/cfsetospeed accept arbitrary baud rates.
+AC_CACHE_CHECK([whether cfsetispeed/cfsetospeed accept arbitrary baud rates],
+  [gdb_cv_cfsetspeed_arbitrary], [
+  AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([[#include <termios.h>]],
+      [[
+       #if B9600 != 9600
+       #error B-constants are not numeric symbols
+       #endif
+      ]])],
+    [gdb_cv_cfsetspeed_arbitrary=yes],
+    [gdb_cv_cfsetspeed_arbitrary=no])
+])
+
+if test "$gdb_cv_cfsetspeed_arbitrary" = yes; then
+  AC_DEFINE([HAVE_CFSETSPEED_ARBITRARY], [1],
+	    [Define if cfsetispeed/cfsetospeed accept arbitrary baud rates.])
+fi
+
 GDB_AC_WITH_DIR([JIT_READER_DIR], [jit-reader-dir],
 		[directory to load the JIT readers from],
 		[${libdir}/gdb])
diff --git a/gdb/ser-unix.c b/gdb/ser-unix.c
index 72c46d29e8f..1bb21a08572 100644
--- a/gdb/ser-unix.c
+++ b/gdb/ser-unix.c
@@ -55,9 +55,10 @@
 
 #include "gdbsupport/scoped_ignore_sigttou.h"
 
-#if (defined(HAVE_SYS_IOCTL_H) \
-     && ((defined(BOTHER) && defined(HAVE_STRUCT_TERMIOS_C_OSPEED)) \
-	  || defined(IOSSIOSPEED)))
+#if (defined(HAVE_CFSETSPEED_ARBITRARY) \
+     || (defined(HAVE_SYS_IOCTL_H) \
+	 && ((defined(BOTHER) && defined(HAVE_STRUCT_TERMIOS_C_OSPEED)) \
+	     || defined(IOSSIOSPEED))))
 #  define HAVE_CUSTOM_BAUDRATE_SUPPORT 1
 #endif
 
@@ -510,7 +511,32 @@ set_baudcode_baudrate (struct serial *scb, int baud_code)
     perror_with_name (_("could not set tty state"));
 }
 
-#if HAVE_CUSTOM_BAUDRATE_SUPPORT && defined(BOTHER)
+#if HAVE_CUSTOM_BAUDRATE_SUPPORT && defined(HAVE_CFSETSPEED_ARBITRARY)
+
+/* Set a custom baud rate using the POSIX cfsetispeed/cfsetospeed
+   interface.  Supported in glibc 2.42 and later and expected to be
+   standardized in a future POSIX revision.  It is platform-agnostic
+   and also covers systems like GNU Hurd that do not provide BOTHER.  */
+
+static void
+set_custom_baudrate_posix (int fd, int rate)
+{
+  struct termios tio;
+
+  if (tcgetattr (fd, &tio) < 0)
+    perror_with_name (_("Cannot get current baud rate"));
+
+  if (cfsetispeed (&tio, rate) < 0)
+    perror_with_name (_("Cannot set custom input baud rate"));
+
+  if (cfsetospeed (&tio, rate) < 0)
+    perror_with_name (_("Cannot set custom output baud rate"));
+
+  if (tcsetattr (fd, TCSANOW, &tio) < 0)
+    perror_with_name (_("Cannot set custom baud rate"));
+}
+
+#elif HAVE_CUSTOM_BAUDRATE_SUPPORT && defined(BOTHER)
 
 /* Set a custom baud rate using the termios BOTHER.  */
 
@@ -557,19 +583,24 @@ set_custom_baudrate_darwin (int fd, int rate)
 }
 
 #endif /* HAVE_CUSTOM_BAUDRATE_SUPPORT
-	  && (defined(BOTHER) || defined(IOSSIOSPEED)) */
+	  && (defined(HAVE_CFSETSPEED_ARBITRARY)
+	     || defined(BOTHER)
+	     || defined(IOSSIOSPEED)) */
 
 #if HAVE_CUSTOM_BAUDRATE_SUPPORT
 
 /* Set a baud rate that differs from the OS B_codes.
-   This is possible if one of the following macros is available:
-   - BOTHER (Linux).
-   - IOSSIOSPEED (Darwin).  */
+   Prefer the POSIX cfsetispeed/cfsetospeed interface when the host
+   libc accepts arbitrary baud rates.  Otherwise fall back to
+   Linux-specific (BOTHER) or Darwin-specific (IOSSIOSPEED)
+   interfaces.  */
 
 static void
 set_custom_baudrate (int fd, int rate)
 {
-#if defined(BOTHER)
+#if defined(HAVE_CFSETSPEED_ARBITRARY)
+  set_custom_baudrate_posix (fd, rate);
+#elif defined(BOTHER)
   set_custom_baudrate_linux (fd, rate);
 #elif defined(IOSSIOSPEED)
   set_custom_baudrate_darwin (fd, rate);