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Apache NuttX is a mature, real-time embedded operating system (RTOS)
https://nuttx.apache.org/
dcd93b0f67
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A module that names a shared library in DT_NEEDED now gets it loaded and its imports bound against it, rather than being refused. libelf_insert() does the loading, which is what dlopen() calls anyway: the library lands in the module registry like anything else, its exports come back through libelf_getsymbol() -- the same call dlsym() uses -- and a library named by two modules is loaded once. A bare name is looked for along LD_LIBRARY_PATH, where dlopen() looks for it. Undefined symbols resolve against the globally registered symbols first, then the modules this one depends on, then the table exec() supplied. Nothing here calls into dlfcn, because this loader is also the kernel's module loader, which has none. Each library becomes one of the module's dependencies[], and the dependency holds it in place of the reference libelf_insert() took. So a library loaded only for DT_NEEDED is kept by the modules that depend on it, and libelf_undepend() unloads it with the last of them; one that dlopen() or insmod also opened stays until that reference goes too. CONFIG_LIBC_ELF_MAXDEPEND bounds how many libraries a module may name, which is what it already meant. Six things had to be fixed to make it work, none of which a build shows. reldata was a file-scope global. Loading a library from inside libelf_relocatedyn() makes that function reentrant, so the nested load overwrote the outer one's relocation offsets and the module resumed binding with the library's DT_REL. It is now per call. A cross-object call needs the callee's data base, not the caller's. A symbol resolved from an FDPIC library comes back as a descriptor, and R_ARM_FUNCDESC_VALUE was treating it as a code address and pairing it with the importing module's GOT. It now copies both words, so the library runs with its own. An object with no imports has no PLT and so no DT_PLTGOT, but it still has a GOT and still has to be entered with it. Without the fallback its descriptors carried a data base of zero and the library read its globals through a null pointer. R_ARM_FUNCDESC, a pointer to a descriptor, wrapped a library's descriptor in a second one. It now stores the library's descriptor as it is. The flag that says a resolved value is a descriptor was set only for an import and never cleared, so the next relocation against a symbol of the module itself took that symbol for a descriptor too. It is cleared there. libelf_symname() was static, and reading a DT_NEEDED name needs it. A module with DT_NEEDED is refused where CONFIG_LIBC_ELF_MAXDEPEND is zero, since that is where the dependency logic is compiled out. A DT_NEEDED library is one shared instance, its data included, because the loader returns the object already in the registry. A module started with exec() is different: that path loads the module afresh each time, so two running instances have separate data while sharing one copy of the text. Built for mps3-an547:picostest with CONFIG_FDPIC both ways. Run on mps2-an500:xipfs under QEMU: fdpicxip solib loads libcounter.so by name out of DT_NEEDED, two instances share one pinned copy of its text, and the library is unloaded, and its pin given back, when the second one exits. A library also opened with dlopen() stays loaded after its DT_NEEDED user exits, and dlclose() unloads it. With CONFIG_ARCH_ADDRENV the program runs in its own address space, which a library libelf_insert() loads cannot reach, so DT_NEEDED is refused there as before. Assisted-by: Claude Code:claude-opus-5-5 Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com> |
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| dummy | ||
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| mm | ||
| net | ||
| openamp | ||
| pass1 | ||
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| CONTRIBUTING.md | ||
| INVIOLABLES.md | ||
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| README.md | ||
| ReleaseNotes | ||
Apache NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. Scalable from 8-bit to 64-bit microcontroller environments, the primary governing standards in NuttX are POSIX and ANSI standards. Additional standard APIs from Unix and other common RTOSs (such as VxWorks) are adopted for functionality not available under these standards, or for functionality that is not appropriate for deeply-embedded environments (such as fork()).
For brevity, many parts of the documentation will refer to Apache NuttX as simply NuttX.
Getting Started
First time on NuttX? Read the Getting Started guide! If you don't have a board available, NuttX has its own simulator that you can run on terminal.
Documentation
You can find the current NuttX documentation on the Documentation Page.
Alternatively, you can build the documentation yourself by following the Documentation Build Instructions.
The old NuttX documentation is still available in the Apache wiki.
Supported Boards
NuttX supports a wide variety of platforms. See the full list on the Supported Platforms page.
Contributing
If you wish to contribute to the NuttX project, read the Contributing guidelines for information on Git usage, coding standard, workflow and the NuttX principles.
License
The code in this repository is under either the Apache 2 license, or a license compatible with the Apache 2 license. See the License Page for more information.