While trying to upgrade my ARM toolchain from GCC 9 to GCC 12 I started to get a lot of warnings from the linker, without any changes to source code or project configuration. The warnings look like this:
closer.c:(.text._close_r+0xc): warning: _close is not implemented and will always fail lseekr.c:(.text._lseek_r+0x10): warning: _lseek is not implemented and will always fail readr.c:(.text._read_r+0x10): warning: _read is not implemented and will always fail
I searched through the internet trying to find the cause of the problem, and found out that this was asked a lot of times, including on this forum. For example, here:
https://stackoverflow.com/questions/73742774/gcc-arm-none-eabi-11-3-is-not-implemented-and-will-always-fail
https://community.st.com/t5/stm32cubeide-mcus/build-error/td-p/587816
https://community.st.com/t5/stm32cubeide-mcus/linker-warnings-on-every-new-project/td-p/791910
https://community.silabs.com/s/question/0D5Vm0000047to9KAA/linker-errors-on-update-to-gnu-arm-v1221?language=en_US
https://community.arm.com/support-forums/f/compilers-and-libraries-forum/53519/arm-gcc-11-3-rel1-newlib-nano-linker-warning-_read-is-not-implemented-and-will-always-fail
It was also discussed in the newlib mailing list:
https://inbox.sourceware.org/newlib/CAAHv3KF9Gt0sFtm7Qsx4XfO0iaGHP2n9Z2qS8LdL2ytJcqR=eg@mail.gmail.com/
While I found no reliable and non-hacky solution, in the links above it is stated that the problem appeared in v11.3 of Arm GNU Toolchain. Indeed, after looking myself at the files, I found that starting with v11.3, file libnosys.a contains sections ".gnu.warning" (e.g. ".gnu.warning._write"), which cause the linker to produce the warnings. In 11.2 these sections are not present. In newlib codebase, these warnings have been there for at least 27 years, so something in the process of building the library changed between 11.2 and 11.3. The warnings are still there in Arm GNU Toolchain v12 and v14.
Can somebody please investigate why this change was made and whether it can be rolled back? From this discussion I see that there were some major changes in build configuration around v11.2 and some bugfixes were done to bring back some old functionality. So this could as well be by accident.
From my search on the internet, these warnings produce nothing but confusion among the developers. The confusion gets worse because, unlike gcc warnings, there is no straightforward way of silencing these linker warnings. Developers who link against "libnosys" should expect that certain syscalls would not work, that's why it's called "nosys" in the first place. So I think everybody is used to not seeing warnings when linking libnosys.