[AVR] error: reinterpret_cast from 'const __FlashStringHelper *' (aka 'const __attribute__((address_space(1))) char *') to 'const char *' is not allowed

[KOLANICH]

//toolchain/hardware/tools/avr/avr/include/avr/pgmspace.h
#ifndef PGM_P
#define PGM_P const char *
// or #define PGM_P __attribute__((address_space(1))) const char * - the same issue
#endif

//toolchain/hardware/arduino/avr/cores/arduino/WString.h
using __FlashStringHelper = __attribute__((address_space(1))) char;

//toolchain/hardware/arduino/avr/cores/arduino/Print.cpp
PGM_P p = reinterpret_cast<PGM_P>(ifsh);

error: reinterpret_cast from 'const __FlashStringHelper *' (aka 'const __attribute__((address_space(1))) char *') to 'const char *' is not allowed

Workaround:

PGM_P p = (PGM_P)(ifsh);

Related:
arduino/ArduinoCore-avr#524
arduino/ArduinoCore-API#184

[benshi001]

I have made contrast between the latest clang and avr-gcc-5.4.0, for the follow C code, both of them accepted.

__flash const char ifsh[4] = {100, 200};
char foo(void) {
        // const char *p = reinterpret_cast<const char *>(ifsh);
        const char *p = (const char *)(ifsh);
        return p[0];
}

But for the following C++ code, neither of clang and avr-gcc-5.4.0 accepted.

__flash const char ifsh[4] = {100, 200};
char foo(void) {
#ifdef __cplusplus
        //const char *p = (const char *)(ifsh);
        const char *p = reinterpret_cast<const char *>(ifsh);
        return p[0];
}

[benshi001]

It seems convert from program memory pointer to data memory pointer in pure C are allowed by both clang and avr-gcc, but in c++ code are allowed by neither.

[llvmbot]

@llvm/issue-subscribers-c-1

[AaronBallman]

This is behaving by design; it was changed in https://reviews.llvm.org/D58346 -- in general, casting away the address space is not a safe operation, so you have to use a C-style cast to force it to "work". Closing the issue as not a bug, but please reopen if you have further information.