remapping stdin and stdout on the fly

[DarkElvenAngel]

How do you remap stdin and stdout once the program is running.

I need to switch stdin and stdout to another device while it's connected. I've tried the following code

    fclose(stdin);
    fopen("/dev/ttytelnet","r");
    fclose(stdout);
    fopen("/dev/ttytelnet","w");

I don't know if this is fixable in pfs I've had to force printf to become fprint(stdout,...) I used a define this was the only way to get printf to correctly redirect to the selected stdout.

[Memotech-Bill]

The problem is that the Pico SDK includes its own version of printf. This is in import library, pico_printf. I think that this is included as part of the pico_stdio import library. You need to make sure that you don't include these in your CMakeLists.txt file.

You probably also need to ensure that the configuration option PICO_PRINTF_ALWAYS_INCLUDED is set to zero. See the SDK documentation.

[DarkElvenAngel]

Okay I've added this to the main CMakeLists.txt pico_set_printf_implementation(${BINARY} compiler) This makes things mostly work. I haven't done full testing but this seems like something to note in the README.

I'll comment more once I've done more testing

[DarkElvenAngel]

An update changing the printf to complier works only for printf the following commands don't work properly puts & putchar they don't send to stdout if it's been changed. This is something in the SDK or newlib to be aware of.

[Memotech-Bill]

You must still have pico_stdio amongst your link libraries in CMake, either directly or indirectly.

How this is works is convoluted:

In file pico-sdk/src/rp2_common/pico_stdio/stdio.c:

int WRAPPER_FUNC(putchar)(int c) {
    char cc = (char)c;
    stdio_put_string(&cc, 1, false, false);
    return c;
}

int WRAPPER_FUNC(puts)(const char *s) {
    int len = (int)strlen(s);
    stdio_put_string(s, len, true, false);
    stdio_flush();
    return len;
}

In pico-sdk/src/rp2_common/pico_platform/include/pico/platform.h:

#define WRAPPER_FUNC_NAME(x) __wrap_##x

Which means that WRAPPER_FUNC(putchar) becomes __wrap_putchar, Likewise for puts.

Then in pico-sdk/src/rp2_common/pico_stdio/CMakeLists.txt:

    pico_wrap_function(pico_stdio printf)
    pico_wrap_function(pico_stdio vprintf)
    pico_wrap_function(pico_stdio puts)
    pico_wrap_function(pico_stdio putchar)
    pico_wrap_function(pico_stdio getchar)

And in pico-sdk/src/CMakeLists.txt:

# add a link option to wrap the given function name; i.e. -Wl:wrap=FUNCNAME for gcc
function(pico_wrap_function TARGET FUNCNAME)
    target_link_options(${TARGET} INTERFACE "LINKER:--wrap=${FUNCNAME}")
endfunction()

So as a result of those two CMakeLists.txt files, the following two switches are included in the link options:

--wrap=puts
--wrap=putchar

The result of those switches is that the linker translates calls to puts into calls to __wrap_puts and putchar into __wrap_putchar. See near the bottom of https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_3.html for details.

Without import library pico_stdio puts and putchar should go to the NewLib versions.

[DarkElvenAngel]

I found wherepico_stdio is linked in CmakeLists.txt

Is it required for pfs to work correctly?

[Memotech-Bill]

I think probably not. It was probably put there for initial debugging and never removed.

It should probably be in the CMakeLists.txt for pfs_test instead.

I am not set up to test myself at the moment.