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 Included with musl is a wrapper script `musl-gcc` which can be used with an existing GCC compiler toolchain to build programs using musl. If installed, the script itself is located at `$(bindir)/musl-gcc`, and a supporting GCC specs file it uses is located at `$(libdir)/musl-gcc.specs`. Part II - Runtime Usage ----------------------- ### 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` - device node, required by POSIX * `/dev/tty` - device node, required by POSIX * `/tmp` - required by POSIX to exist as a directory, and used by various temporary file creation functions. * `/bin/sh` - an executable file providing as POSIX-conforming shell * `/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. While some programs may operate correctly even without some or all of the above, musl's behavior in their absence is unspecified. ### Additional Pathnames Used * `/dev/log` - a UNIX domain socket to which the `syslog()` interface sends log messages. If absent or inaccessible, log messages will be discarded. * `/dev/shm` - a directory; 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` and `/dev/pts` - device node and devpts filesystem mount point, respectively. If absent or inaccessible, `posix_openpt()` and `openpty()` will fail. * `/etc/passwd` and `/etc/group` - text files containing the user and group databases, mappings between names and numeric ids, and group membership lists, in the standard traditional format. If absent, user and/or group lookups will fail. * `/etc/shadow` - text file containing shadow password hashes for some or all users. * `/etc/resolv.conf` - text file providing addresses of nameservers to be used for DNS lookups. If absent, DNS requests will be sent to the loopback address and will fail unless the host has its own nameserver. * `/etc/hosts` - text file mapping hostnames to IP addresses. * `/etc/services` - text file mapping network service names to port numbers. * `/usr/share/zoneinfo`, `/share/zoneinfo`, and `/etc/zoneinfo` - directories searched for time zone files when the `TZ` environment variable is set to a relative pathname. * `../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 III - Library Usage for Development ---------------------------------------- ### Compiler Support All public interfaces in musl, at both the header file and library level, are intended to be mostly compatible with any C99, C11, or C++ compiler targetting the same CPU architecture and ABI musl was built for. C89 compilers are also supported provided that they accept the `long long` type as an extension. A few public header files do, however, require compiler-specific extensions in order to provide the mandated standard features: * `complex.h` requires `1.0fi` to be accepted as a constant expression suitable for defining `_Complex_I`. * `tgmath.h` requires the `__typeof__` extension. * FIXME: is this list complete? ### System Header Files (Add some intro text on musl's headers.) #### Introduction to Namespace Issues Any C program using a library, whether the standard library or a third-party library, needs to observe a contract with the library regarding usage of identifiers - in particular, which identifiers are used as part of the library's public interface or header file implementation, and which identifiers are used by the application. Having a clear contract is especially important when the library being used is not a single fixed implementation, but may have multiple versions or multiple independent implementations. The canonical example of such a library is the standard library. ISO C reserves all identifiers which are not explicitly defined or reserved by the standard for use by the appliction. POSIX, however, exposes a number of additional identifiers, and popular extensions outside of the standards define even more. In order to support applications which are written with different expectations on which identifiers may be used for the application's purposes, and which ones are defined by the system, a mechanism must be provided for choosing *which contract* will be used. #### Introduction to Feature Test Macros To solve this problem, POSIX introduced the concept of *feature test macros*. These are macros which an application may define *prior to the inclusion of any system header* (either at the source level, or via `-D` options passed as arguments to the compiler) in order to request a particular namespace contract. POSIX 2008 specifies two such feature test macros: * `_POSIX_C_SOURCE`, defined to `200809L` to request all interfaces defined in the POSIX base standard. * `_XOPEN_SOURCE`, defined to `700` to request all interfaces defined under the XSI option in addition to POSIX base. No requirements are placed on the namespace when neither of these macros is defined by the application. If one or both of these macros is defined by the application, two constraints are placed on the system headers: * They must define all macros and declare all functions and objects which the standard specifies for that header to provide. * They must not make use of any identifier not specified or reserved for that header. There is, however, an exception to the second rule: since the standard does not define behavior when the application has defined macros whose names are reserved for system use, implementations may specify their own feature test macros to expose additional identifiers alongside the standard ones. This is what musl, and most other implementations of the standard library, do. #### Feature Test Macros Supported by musl If no feature test macros are defined, musl's headers operate in "default features" mode, exposing the equivalent of the `_BSD_SOURCE` option below. This corresponds fairly well to what most applications unaware of feature test macros expect, and also provides a number of more modern features. Otherwise, if at least one of the below-listed feature test macros is defined, they are treated additively, starting from pure ISO C as a base. Unless otherwise specified, musl ignores the value of the macro and only checks whether it is defined. * `__STRICT_ANSI__` Adds nothing; only suppresses the default features. This macro is defined automatically by GCC and other major compilers in strict standards-conformance modes. * `_POSIX_C_SOURCE` (or `_POSIX_SOURCE`) As specified by POSIX 2008; adds POSIX base. If defined to a value less than `200809L`, or if the deprecated version `_POSIX_SOURCE` is defined at all, interfaces which were removed from the standard but which are still in widespread use are also exposed. * `_XOPEN_SOURCE` As specified by POSIX 2008; adds all interfaces in POSIX including the XSI option. If defined to a value less than `700`, interfaces which were removed from the standard but which are still in widespread use are also exposed. * `_BSD_SOURCE` Adds everything above, plus a number of traditional and modern interfaces modeled after BSD systems, or supported on current BSD systems based on older standards such as SVID. * `_GNU_SOURCE` (or `_ALL_SOURCE`) Adds everything above, plus interfaces modelef after GNU libc extensions and interfaces for making use of Linux-specific features. The specific extensions provided by the nonstandard feature test macros are documented in subsequent sections of this manual. ### Implementation-Defined Behavior To be written. ### BSD-Compatible Extensions To be written. ### GNU-Compatible Extensions To be written. ### Linux Extensions To be written. ### Other Nonstandard Behaviors and Extensions To be written. ### Quality of Implementation Guarantees To be written. Part IV - 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.