Nov 062017
 

So, I’m doing some development on a Cortex M3-based device with access to only one serial port, and that serial port is doing double-duty as serial console and polling a Modbus energy meter.  How do I get log messages out?

My code actually implements the stubs to direct stdout and stderr transparently to the serial port, however this has to go to /dev/null when the Modbus port is in use.  That said, _write_r still gets called, in my code, it is possible to set a breakpoint inside the _write_r function when traffic is identified for the console.

As it happens, gdb can be told to not only break there, but to perform a series of actions.  In my case serial.c:659 is the file and line number inside an if branch that handles the console code.  Setting up gdb to print this data out requires the following statements:

(gdb) break serial.c:659
(gdb) commands
Type commands for breakpoint(s) 3, one per line.
End with a line saying just "end".
>set ((char*)buf)[cnt] = 0
>print (char*)buf
>continue
>end
(gdb) c

The result:

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=78) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$51 = 0x200068c0 "/home/stuartl/vrt/projects/widesky/hub/hal/demo/main.c:226 Registration sent\r\n"

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=46) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$52 = 0x200068c0 "Received NTP time is Mon Nov  6 04:13:41 2017\n"

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=2) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$53 = 0x200068c0 "\r\n"

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=89) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$54 = 0x200068c0 "/home/stuartl/vrt/projects/widesky/hub/hal/demo/main.c:115 Registration timeout: 30 sec\r\n"

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=83) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$55 = 0x200068c0 "/home/stuartl/vrt/projects/widesky/hub/hal/demo/main.c:130 Select source address:\r\n"

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=53) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$56 = 0x200068c0 " ? fdde:ad00:beef:0:0:ff:fe00:a400 Pref=Y Valid=Y\r\n"

Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=15) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$57 = 0x200068c0 " ? Selected\r\n"

---Type  to continue, or q  to quit---
Breakpoint 3, _write_r (ptr=, fd=0, buf=0x200068c0, cnt=78) at /home/stuartl/vrt/projects/widesky/hub/hal/src/serial.c:659
659			if (serial_console_target.port) {
$58 = 0x200068c0 "/home/stuartl/vrt/projects/widesky/hub/hal/demo/main.c:226 Registration sent\r\n"

Not as nice as having a dedicated port, but better than nothing.