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 can be used both as the system-wide C library for operating system installations and distributions, and as a tool for building individual application binaries deployable on a wide range of systems compatible with the Linux system calls API.
This manual covers many details of musl which may be of interest to programmers, systems integrators, and end users. It is a work in progress.
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.
cmpxchginstruction is added
The following additional targets are available for build, but may not work correctly and may not yet have ABI stability:
x32 (x86_64 ILP32 ABI)
The build system for musl uses the well-known
musl's configure script is not based on GNU autoconf, but is intended
to closely match the configure command line interface documented in
the GNU Coding Standards. Running configure produces a
file which can further be edited by hand, if necessary.
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 requirements
are placed on the compiler and linker. In particular, the linker must
-Bsymbolic-functions option, and the compiler must not
generate gratuitous GOT relocations where GOT-relative or PC-relative
addressing could be used instead.
A recent version of GCC or LLVM/clang is recommended for compiling musl. However, the following compilers are known to have built musl successfully:
GCC 4.6 or later. Versions back to 3.4.6 or possibly earlier will work but have floating point bugs which affect i386.
LLVM/clang 3.2 or later. Earlier versions are known to have bugs that produce a malfunctioning libc.
PCC 1.1.0.DEVEL or later.
Firm/cparser, for static libc only.
./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
avoid breaking an existing non-musl environment on the host. If musl
will be used as the primary system libc, prefix should usually be
--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
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
--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
DESTDIR variable can be set when running
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)
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
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
manually also works. In the future,
--enable-debug may also enable
additional debugging features at the source level.
./configure --help for additional options.
After running configure, run
make to compile and
make install to
install. If desired,
make install can be invoked directly without
make, but it may be desirable to do these as separate
steps if elevated 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
install target in musl's
Makefile is fully declaratory,
and its proper operation depends on file timestamps being correct. If
files with newer/future timestamps already exist in the destination,
updated files may fail to be installed. This can be avoided by
deleting the offending files, fixing their timestamps, or installing
first to a fresh
DESTDIR then moving the files into place.
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 under the heading Additional Files
Used later in this part of the manual.
In the following,
refer to the paths chosen at build time (by default,
$(prefix)/lib, respectively) and
refers to the full name for the target CPU architecture/ABI,
including the "subarch" component.
$(syslibdir)/ld-musl-$(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.
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.
libc.a installed in
$(libdir) provides the entire
standard library implementation for static linking. The file
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
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
-lpthread, etc. be accepted by the
Several bare object files are also included in
Scrt1.o are the normal and position-independent versions,
respectively, of the entry point code linked into every program.
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
crtn.o files if desired.
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
and a supporting GCC specs file it uses is located at
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 a 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
$(syslibdir)/ld-musl-$(ARCH).so.1 - must resolve to the musl
dynamic linker/libc.so binary in order for dynamic-linked programs
to run. For static-linked programs it is unnecessary.
While some programs may operate correctly even without some or all of the above, musl's behavior in their absence is unspecified.
/dev/log - a UNIX domain socket to which the
sends log messages. If absent, 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/pts - device node and devpts filesystem mount
point, respectively. If absent,
/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. If absent (and TCB shadow files are also absent),
shadow password lookups will fail.
/shadow - text file containing a single shadow
password record for user, based on the Openwall TCB alternative
for shadow password storage. If absent for user,
will be searched.
/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
/etc/hosts - text file mapping hostnames to IP addresses. If
absent, only DNS will be used.
/etc/services - text file mapping network service names to port
numbers. If absent, only numeric service/port strings can be used.
/etc/shells - a list of shell pathnames to be returned by the
getusershell interface. If absent a built-in default list will be
directories searched for time zone files when the
variable is set to a relative pathname. If absent, only absolute
zoneinfo file pathnames or POSIX timezone specifications can be
../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
PATH - Used by execvp, execlp, and posix_spawnp as specified in
POSIX. If unset, a default search path of
"/usr/local/bin:/bin:/usr/bin" is used.
TZ - Specifies the local timezone to be used for functions which
deal with local time. The value of
TZ can be either a POSIX
timezone specification in the form
stdoffset[dst[offset][,start[/time],end[/time]]] or the name of a
zoneinfo-binary-format timezone file (the form used by glibc and
most other systems). The latter is interpreted as an absolute
pathname if it begins with a slash, a relative pathname if it begins
with a dot, and otherwise is searched in
/etc/zoneinfo. When searching these paths,
strings including any dots are rejected. If the program was invoked
setuid, setgid, or with other elevated capabilities, the absolute
and relative pathname options are not available.
DATEMSK - Used by the
getdate function as a pathname for the
file containing date formats to scan, per POSIX.
PWD - Used by the nonstandard
get_current_dir_name function; if
it matches the actual current directory, it is returned instead of
getcwd to obtain the canonical pathname.
LOGNAME - The
getlogin function simply returns the value of the
LD_PRELOAD - Colon-separated list of shared libraries that will be
preloaded by the dynamic linker before processing the application's
dependency list. Components can be absolute or relative pathnames or
filenames in the default library search path. This variable is
completely ignored in programs invoked setuid, setgid, or with other
LD_LIBRARY_PATH - Colon-separated list of pathnames that will be
searched for shared libraries requested without an explicit
pathname. This path is searched prior to the default path (which is
built-in default fallback if this file is missing). This variable is
completely ignored in programs invoked setuid, setgid, or with other
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 targeting the same CPU architecture and ABI musl was built
for. C89 compilers are also supported provided that they accept the
long long type and wide character literals as extensions. A few
public header files do, however, require compiler-specific extensions
in order to provide the mandated standard features:
1.0fi to be accepted as a constant expression
suitable for defining
tgmath.h requires the
__typeof__ extension. Future versions may
offer a portable C11 version of
_Generic as well.
stdarg.h requires the
__builtin_va_list type and related builtin
netinet/tcp.h requires anonymous unions, available in C11 and as
an extension in GCC and compatible compilers, in the default feature
profile. In strict ISO C and POSIX profiles, anonymous unions are
In addition, the definitions of
stddef.h) require the
extensions, respectively, to provide fully conforming definitions.
When used with compilers which do not predefine
headers will fallback to alternate definitions.
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 application. 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.
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
-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.
If no feature test macros are defined, musl's headers operate in
"default features" mode, exposing the equivalent of the
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.
Adds nothing; only suppresses the default features. This macro is defined automatically by GCC and other major compilers in strict standards-conformance modes.
As specified by POSIX 2008; adds POSIX base. If defined to a value
200809L, or if the deprecated version
is defined at all, interfaces which were removed from the standard
but which are still in widespread use are also exposed.
As specified by POSIX 2008; adds all interfaces in POSIX including
the XSI option. If defined to a value less than
which were removed from the standard but which are still in
widespread use are also exposed.
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.
Adds everything above, plus interfaces modeled after GNU libc extensions and interfaces for making use of Linux-specific features.
Documentation of specific extensions provided by the nonstandard feature test macros will be added in a future edition of this manual.
For all interfaces provided by musl that are specified by standards to which musl aims for conformance, the relevant standards documents are the official documentation.
The current edition of the C99 standard with TC1/2/3 applied is available from http://www.open-std.org as document WG14 N1256.
The POSIX 2008 standard with TC1 applied is available from The Open Group at http://pubs.opengroup.org/onlinepubs/9699919799/.
This portion of the manual is incomplete. Future editions will document musl's behavior where the standards specify that it is implementation-defined, non-standard extensions musl implements, and additional properties of musl's implementation.