ARM GCC 11.3 Rel1 newlib-nano linker warning _read is not implemented and will always fail

Bump.

I'm having the exact same issue.

Sounds great!  I've read that all you have to do to implement output to your favorite console-like device is "implement _write()"
Of course, that ends up sucking in all of newlib's stdio, which is based on filsystem stuff, so it's pretty bloated.  And it references a bunch of other functions, even if you never use code paths that would call those functions to be executed (_read() for a character LCD, etc.)

I though you used to have to implement a bunch of "stub" functions for each symbol you didn't care about.  Now, it seems that 'something' creates failing stub functions for you, and issues a warning that it's used a (failure-returning) stub.  That sounds ... mostly like I'd want it to be.

(some stubs may need to return success, though.)

I am facing the same problem, is there any statement about why this warnings are present despite of setting 

--specs=nano.specs --specs=nosys.specs ?

I can provide empty/dummy implementations of functions that warnings are about. But I am not sure if I am able to prevent runtime errors if these functions would be used somewhere. If this is official solution for that warnings there must be a compile time solution to forbid using this dummy implementations. 

I was in the same situation (got these warnings after upgrading projects toolchain to ARM GCC v11.3.rel1), precisely:

/opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld: /opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/libc.a(libc_a-closer.o): in function `_close_r':

/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/closer.c:47: warning: _close is not implemented and will always fail

/opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld: /opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/libc.a(libc_a-lseekr.o): in function `_lseek_r':

/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/lseekr.c:49: warning: _lseek is not implemented and will always fail

/opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld: /opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/libc.a(libc_a-readr.o): in function `_read_r':

/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/readr.c:49: warning: _read is not implemented and will always fail

/opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld: /opt/arm-gnu-toolchain-11.3.rel1-x86_64-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/libc.a(libc_a-writer.o): in function `_write_r':

/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/writer.c:49: warning: _write is not implemented and will always fail

I confirmed that the functions that the linker compained about were not even emitted in the generated ELF file (NO symbols containing "read", "lseek", "open").

But for me, changing the linker specs from --specs=nosys.specs (which used to work flawlesssly with older GCC versions) to --specs=nano.specs fixed all these warnings, requiring no further changes (I did NOT provide function definitions for _write, _read, _lseek or similar)

Additional compiler flags I used: -Wall -Wextra -ffunction-sections -fdata-sections

Linker flags I used:  -Wl,--gc-sections

TL;DR:

Just switching linker specs from --specs=nosys.specs to --specs=nano.specs might fix this for you!

this warning message defined by macro 'stub_warning' in newlib source 'libgloss/libnosys/warning.h'