examples/natmod/btree: Fix build on RV32 with Picolibc. #17642
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dpgeorge
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Jul 9, 2025
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dpgeorge
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Jul 10, 2025
This commit fixes building the "btree" example natmod on RV32 when Picolibc is being used and uses thread-local storage for storing the errno variable. The fix is surprisingly simple: Picolibc allows overriding the function that will provide a pointer to the "errno" variable, and the btree natmod integration code already has all of this machinery set up as part of its library integration. Redirecting Picolibc to the already existing pointer provider function via a compile-time definition is enough to let the module compile and pass QEMU tests. This workaround will work on any Picolibc versions (Arm, RV32, Xtensa, etc.) even if TLS support was not enabled to begin with, and will effectively do nothing if the toolchain used will rely on Newlib to provide standard C library functions. Given that the btree module now builds and passes the relevant natmod tests, said module is now part of the QEMU port's natmod testing procedure, and CI now will build the btree module for RV32 as part to its checks. Signed-off-by: Alessandro Gatti <a.gatti@frob.it>
This commit adds some documentation on what are the limitations of using Picolibc as a standard C library for native modules. This also contains a reference to the "errno" issue when building natmods on RV32 that the PR this commit is part of, as it is not obvious how to approach this issue when encountered for the first time. Signed-off-by: Alessandro Gatti <a.gatti@frob.it>
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Summary
This PR fixes building the "btree" example natmod on RV32 when Picolibc is being used and uses thread-local storage for storing the errno variable.
The fix is surprisingly simple: Picolibc allows overriding the function that will provide a pointer to the "errno" variable, and the btree natmod integration code already has all of this machinery set up as part of its library integration. Redirecting Picolibc to the already existing pointer provider function via a compile-time definition is enough to let the module compile and pass QEMU tests.
This workaround will work on any Picolibc versions (Arm, RV32, Xtensa, etc.) even if TLS support was not enabled to begin with, and will effectively do nothing if the toolchain used will rely on Newlib to provide standard C library functions.
Given that the btree module now builds and passes the relevant natmod tests, said module is now part of the QEMU port's natmod testing procedure, and CI now will build the btree module for RV32 as part to its checks.
Testing
The
btreenatmod was built successfully when targeting RV32, and then QEMU'smake test_natmodbtree tests pass. Relevant CI tests also pass in my repo's branch (and hopefully will pass on this branch too!).