[PATCH v5 2/2] gdb/ser-unix: add POSIX cfsetispeed/cfsetospeed support for custom baud rates

sunilkumar.dora
2026-03-24T16:47:26Z

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

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 <kevinb@redhat.com>
Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>
---
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 03f46df5400..e46b3a56a1f 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 72e69c9749f..d10c4fdce34 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -27741,6 +27741,45 @@ _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 277eb945e84..42d7594fa31 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);
--
2.49.0