musl ==== Part I - About musl ------------------- ### Introduction and Scope musl is an implementation of the userspace portion of the standard library functionality described in the ISO C and POSIX standards, plus common extensions. It is both a component for use in Linux-based operating systems and a tool for building application binaries deployable on a wide range of Linux-based systems and non-Linux systems which can provide a compatible syscall API layer. ### Conformance The interfaces in musl are modeled upon and intended to conform to the requirements of the ISO C99 standard (ISO/IEC 9899-1999), including Annex F, and POSIX 2008 / Single Unix Standard Version 4, with all current technical corrigenda applied. However, musl has not been certified by any standards body, and no guarantee of conformance is made by the copyright holders or any other party with an interest in musl. Moreover, since musl provides only the userspace portion of the standard system interfaces, conformance to the requirements of POSIX depends in part on the behavior of the underlying kernel. Linux 2.6.39 or later is believed to be sufficient; earlier versions in the 2.6 series will work, but with varying degrees of non-conformance, particularly in the area of signal handling behavior and close-on-exec race conditions. Likewise, conformance to the requirements of ISO C, and especially Annex F (IEEE floating point semantics), depends in part on both the compiler used to build musl and the compiler used when building applications against musl. At this time there is no known fully conforming compiler. ### Supported Targets * i386 * Requires support or kernel emulation of `cmpxchg` instruction, introduced on the 80486 * x86_64 * ARM * EABI, standard or hard-float VFP variant * Little-endian default; big-endian variants also supported * Compiler toolchains only support armv4t and later * MIPS * ABI is o32 * Big-endian default; little-endian variants also supported * MIPS2 or later, or kernel emulation of ll/sc (standard in Linux) is required * FPU or kernel float emulation (standard in Linux but disabled on some OpenWRT builds) is required * PowerPC * Only 32-bit is supported * Compiler toolchain must provide 64-bit long double, not IBM double-double or IEEE quad * For dynamic linking, compiler toolchain must be configured for "secure PLT" variant * Microblaze * Big-endian default; little-endian variants also supported * Soft-float ### Build and Installation The build system for musl uses the well-known `./configure` idiom. musl's configure script is not based on GNU autoconf, but is intended to closely match the configure API documented in the GNU Coding Standards. Alternatively, the provided template for `config.mak` may be edited by hand in place of running `./configure`. #### Prerequisites The only build-time prerequisites for musl are the standard POSIX shell and utilities, GNU Make (version 3.81 or later) and a freestanding C99 compiler toolchain targeting the desired instruction set architecture and ABI, with support for gcc-style inline assembly, weak aliases, and stand-alone assembly source files. The system used to build musl does not need to be Linux-based, nor do the Linux kernel headers need to be available. If support for dynamic linking is desired, some further requriements are placed on the compiler and linker. In particular, the linker must support the `-Bsymbolic-functions` option, and the compiler must not generate gratuitous GOT relocations where GOT-relative or PC-relative addressing could be used instead. #### Build options Running `./configure --help` from the top-level source directory will print usage information for configure. In most cases, the only options which should be needed are: * `--prefix`, used to control where musl will be installed. The prefix for musl defaults to `/usr/local/musl` rather than `/usr/local` to avoid breaking an existing non-musl environment on the host. If musl will be used as the primary system libc, prefix should usually be set to `/usr` or `/`. * `--syslibdir`, used to specify the location at which the dynamic linker should be installed and found at runtime. The default of `/lib` should only be overridden when installing in `/lib` is impossible, since the pathname of the dynamic linker is stored in all dynamic-linked executables, and executables using non-standard paths for the dynamic linker may be difficult to deploy on other systems. Both `--prefix` and `--syslibdir` should reflect the final runtime location where musl will be installed. If musl should be installed to a different location to prepare a package file or new target system image, the `DESTDIR` variable can be set when running `make install`. In this case, `DESTDIR` will be prepended to all installation paths, but will not be saved anywhere in the files installed. Other build options of interest are: * `CC=...`, to choose a non-default compiler. * `CFLAGS=...`, to pass custom options to the compiler. * `--disable-shared`, to disable building shared `libc.so` if it will not be needed. This cuts the build time in half. * `--disable-static`, to disable building `libc.a`. Other (empty) `.a` files are still built. This also cuts the build time in half. * `--enable-optimize=`*list*, where *list* is a comma-separated list of components (subdirectories of `src`, or glob patterns) which will be optimized at `-O3` rather than the default optimization level `-Os`. Manually specifying an optimization level in the provided `CFLAGS`, or using `--enable-debug` or `--disable-optimize`, will turn off default optimizations. * `--enable-warnings`, to turn on the recommended set of GCC warning options with which musl is intended to compile warning-free. * `--enable-debug`, to turn on debugging. Adding `-g` to `CFLAGS` manually also works. In the future, `--enable-debug` may also enable additional debugging features at the source level. See `./configure --help` for additional options. #### Compiling and Installing After running configure, run `make` to compile and `make install` to install. If desired, `make install` can be invoked directly without first running `make`, but it may be desirable to do these as separate steps if eleveated privileges are needed to install to the final destination. musl's makefile is fully declarative and non-recursive, and may be arbitrarily parallelized with the `-j` option. #### After Installation If installing for the first time and using dynamic linking, it may be necessary to create a path file for the dynamic linker. See `../etc/ld-musl-$(ARCH).path` in the next section of the manual. ### Installed Components In the following, `$(syslibdir)`, `$(includedir)`, and `$(libdir)` refer to the paths chosen at build time (by default, `/lib`, `$(prefix)/include`, and `$(prefix)/lib`, respectively) and `$(ARCH)` refers to the *full* name for the target CPU architecture/ABI, including the "subarch" component. #### Dynamic linking runtime `$(syslibdir)/ld-musl-$(ARCH).so.1` provides the dynamic linker, or "program interpreter", for dynamically linked ELF programs using musl. The absolute pathname to this file must be stored in all such programs. The build and installation system provided with musl sets it up as a symbolic link to `$(libdir)/libc.so`, but system integrators may choose to make it available in whichever ways they find suitable. #### Development environment Header files for use by the C compiler are installed in `$(includedir)`. The standard headers are fully self-contained, and do not make use of kernel-provided or compiler-provided headers or otherwise require such headers to be present. The file `libc.a` installed in `$(libdir)` provides the entire standard library implementation for static linking. The file `libc.so` provides the linker with access to the standard library's symbols for use at link-time in producing dynamic-linked binaries. It is not searched at runtime; the standard library is resolved as part of the program interpreter at `$(syslibdir)/ld-musl-$(ARCH).so.1`. Additional files `libm.a`, `librt.a`, `libpthread.a`, `libcrypt.a`, `libutil.a`, `libxnet.a`, `libresolv.a`, and `libdl.a` are provided in `$(libdir)` as empty library archives. They contain no code, but are present to satisfy the POSIX requirement that options of the form `-lm`, `-lpthread`, etc. be accepted by the `c99` compiler. Several bare object files are also included in `$(libdir)`: `crt1.o` and `Scrt1.o` are the normal and position-independent versions, respectively, of the entry point code linked into every program. `crti.o` and `crtn.o`, also linked into every program and into shared libraries, provide support for legacy means by which the compiler can arrange for global constructors and destructors to be executed. It is possible to setup a legacy-free compiler toolchain that does not need the `crti.o` and `crtn.o` files if desired. #### Compiler wrapper To be written. ### Filesystem Layout Dependencies musl aims to avoid imposing filesystem policy; however, the following minimal set of filesystems dependencies must be met in order for programs using musl to function correctly: * `/dev/null` - required by POSIX * `/dev/tty` - required by POSIX * `/tmp` - required by POSIX to exist as a directory, and used by various temporary file creation functions. * `/dev/shm` - must be a directory, and should have permissions 01777. If absent, POSIX shared memory and named semaphore interfaces will fail; programs not using these features will be unaffected. * `/dev/ptmx` - must exist and be accessible for read/write in order for pseudo-terminal opening to work. * `/dev/pts` - must be a mounted devpts filesystem in order for pseudo-terminal opening to work. * `/proc` - must be a mount point for Linux procfs or a symlink to such. Several functions such as realpath, fexecve, and a number of the "at" functions added in POSIX 2008 need access to /proc to function correctly. * `/etc/resolv.conf` - needed to provide addresses of nameservers to be used for DNS lookups, unless a working nameserver is available on the loopback address. * `../etc/ld-musl-$(ARCH).path`, taken relative to the location of the "program interpreter" specified in the program's headers - if present, this will be processed as a text file containing the shared library search path, with components delimited by newlines or colons. If absent, a default path of `"/lib:/usr/local/lib:/usr/lib"` will be used. Not used by static-linked programs. Part II - Usage --------------- To be written. This part of the manual will deal with documenting implementation-defined behavior and further behaviors that are not required to be documented but for which musl makes additional guarantees. Part III - Implementation ------------------------- To be written. This part of the manual will document the implementation of musl, including matters such as source tree layout, built system, algorithms used, musl-internal APIs, coding style, and information on porting.