Using special debugger breakpoints and commands, the Pico can read and write to the debugging console as well as read and write files on a development PC. The Semihosting library allows applications to use the semihosting support as a normal filesystem or serial port.
NOTE Semihosting only works when connected to an OpenOCD + GDB debug session. Running an application compiled for Semihosting without the debugger will cause a panic and hang the chip.
As of now, only ARM has support for Semihosting.
Running Semihosting on the Development HostStart OpenOCD normally from inside a directory that you can read and write files within (i.e. do not run from C:\\Program Files\\.. on Windows where general users aren’t allowed to write). The starting directory will be where the Pico will read and write files using the SemiFS class. Be sure to keep the terminal window you ran OpenOCD in open, because all SerialSemi input and output will go to that terminal and not gdb’s.
Start GDB normally and connect to the OpenOCD debugger and enable semihosting support
(gdb) target extended-remote localhost:3333 (gdb) monitor arm semihosting enable
At this point load and run your ELF application as normal. Again, all SerialSemi output will go to the OpenOCD window, not GDB.
See the hellosemi example in the Semihosting library.
Simply include <Semihosting.h> in your application and use SerialSemi as you would any other Serial port with the following limitations:
Baud rate, bit width, etc. are all ignored
Input is limited because read may hang indefinitely in the host and available is not part of the spec
SerialSemi can also be selected as the debug output port in the IDE, in which case ::printf will write to the debugger directly.
Use SemiFS the same way as any other file system. Note that only file creation and renaming are supported, with no provision for iterating over directories or listing files. In most cases simply opening a File and writing out a debug dump is all that’s needed:
SemiFS.begin();
File f = SemiFS.open("debug.dmp", "w");
f.write(buffer, size);
f.close();
SerialSemi.printf("Debug dump now available on host.\n");
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