Open Issues

This wiki page is for tracking known, mostly longstanding issues that are non-trivial to fix. Before adding an issue here, it should have been discussed at least once on the mailing list or IRC channel. Simple bugs found can usually be fixed right away.

Locale limitations

Locale support is very limited, and barely works. Translation of LC_TIME is not possible because the key strings for ABMON_5 and MON_5 (“May”) are identical. Custom collation orders (LC_COLLATE) are not implemented at all, despite there always having been an intent to support them. LC_NUMERIC and LC_MONETARY also admit no variation by locale. Solving these problems requires a major overhaul, but the main missing prerequisite is involvement from users who want the functionality.

Building officially recognized locales for musl is also a recognized open problem. The bulk of the data should be derived mechanically from the Unicode CLDR where possible, but the CLDR seems to lack certain time format variants corresponding to the ones C/POSIX needs for nl_langinfo/strftime. This probably requires a great deal of manual work to remedy, ideally getting the missing formats added upstream with Unicode.

Sanitizer compatibility

The upstream GCC and LLVM sanitizer library-side implementations rely heavily on access to libc internals and are not compatible with musl. UBSan is usable in trap-only mode but the other sanitizers are mostly unusable. Support for these tools is desired, but exposing internals as public interfaces is not, and sanitizer support will probably eventually be implemented as providing the necessary library functions in musl itself rather than use of the compiler-provided libraries.

Complex math

Currently most complex functions have dummy implementations. Correct implementation is non-trivial (small ULP errors for both real and imaginary parts without spurious floating-point exceptions and getting branch cuts right with correctly signed zeros), some work has been done in FreeBSD libm (based on the paper of Hull et al. about complex asin and acos) that might be usable in musl.

fnmatch logic

The fnmatch_internal function can be simplified (the check of the last '*' pattern component can be removed) and it has corner-cases where EILSEQ might not be handled correctly.

sbrk emulation

sbrk cannot be used safely, but some applications rely on it anyway. A possible solution is to emulate it with mmap.

Legacy non-libc headers

Some structs in procfs.h and user.h are only used by gdb, but for historical reasons glibc provided them. Needs investigation who uses these and possibly provide a legacy header set separate from the libc headers.


Legacy functions operating on ucontext_t (getcontext, setcontext, makecontext, swapcontext) are not implemented. They are no longer part of POSIX, but cooperative multi-tasking applications use them. ucontext_t also appears as an argument to sigaction handlers which cannot be used portably.

Previously posted issues which were resolved

Missing libresolv functions

The following issue was fixed in musl 1.1.2:

The resolver (DNS) framework in musl is a legacy-free implementation from scratch, but it was designed without any awareness of the legacy libresolv interfaces on which traditional gethostbyname/getaddrinfo implementations were built, and thus we have run into several instances of applications needing libresolv-style functions that did not exist in musl. Most of them had been added, haphazardly, in ways that either result in code duplication or lack the desired simplicity and efficiency we could have if the resolver itself were integrated with these functions.

Lack of features in resolver

The following issue was fixed across musl 1.1.2 and subsequent work on 1.1.3:

The traditional resolver supports searching multiple domains for a hostname, returning a mix of IPv4 and IPv6 addresses from /etc/hosts, etc. While low-priority, it would be nice to be able to duplicate some of these features. Addressing this issue is mainly a matter of refactoring getaddrinfo and the DNS code on which it depends; it should probably be done at the same time as the libresolv functions.

After some discussion, search domains were deemed a largely-harmful feature, so support for them was not initially added. Later, version 1.1.13 introduced search domains, but with stronger constraints than traditional implementations designed so that transient, potentially attacked-controlled failures cannot alter the results of a query. This establishes compatibility with systems using search domains in many but not all ways; the use cases that do not work were always unsafe and should be changed.

C++ ABI compatibility

As of musl 0.9.12, the C++ ABI is stabilized and compatible with glibc’s C++ ABI. This is sufficient for loading a glibc-built libstdc++ under musl, and for running C++ programs that were linked to glibc as long as they otherwise meet the requirements for working with musl. However, it’s still insufficient for musl-gcc to be able to build C++ programs using the existing toolchain on a glibc-based host.

Getting musl-gcc working with C++ is still an open issue, but it’s not an issue in musl so much as an issue of figuring out an approach to getting C++ headers that are usable with musl, without first needing to build a new libstdc++, and installing them, setting up the right paths, etc.

Removing lazy initialization of the thread pointer

The following was addressed in musl 1.1.0, with follow-up to improve compatibility with pre-2.6 kernels between 1.1.2 and 1.1.3:

This will add a one-syscall overhead at startup even in programs which don’t need it, but will allow large optimizations in many places, including optimizations which reduce code size and bloat and eliminate complexity. However, making such a change is contingent on making sure it does not preclude support for ancient kernels where the thread pointer setup syscalls are missing.

brk failures

The following issue was fixed in musl 1.1.0:

Currently malloc uses brk for small allocations. If brk fails then malloc fails even if there is still mmapable space. A common cause of brk failure is a kernel bug in ASLR for PIE binaries which allows very small heaps, but brk can fail whenever an mmaped region is in the way. The malloc design needs some changes to allow fallback to mmap.

Glibc does do the fallback, but that results significantly slower allocation than with brk adding a huge penalty for using PIE (until the kernel bug is fixed).

C11 Conformance

Full C11 coverage was added in musl 1.1.5; the main addition was the C11 threads API. The optional (and controversial) Annex K is not included. The current plan is to hold off on taking any action towards supporting Annex K unless/until other implementations adopt it and applications are using it.

Private futexes

Private futex support was also added in musl 1.1.5. Fallback for old kernels is not cached to avoid imposing high costs (e.g. global accesses via PIC and/or TLS accesses) on modern kernels.


Up through musl 1.1.6, the POSIX AIO implementation had significant conformance and quality of implementation issues. The most serious was that AIO was not synchronized with the close() function, meaning pending AIO operations could act upon (and possibly corrupt) another file if their original fd was closed and the file descriptor was reassigned. An analogous bug is also present in glibc’s implementation.

Details are available on this mailing list thread:

C locale conformance

Starting with version 1.1.11, musl’s C locale conforms to the future POSIX requirement (resolution to Austin Group issue #663) that the C locale’s character encoding be single-byte. Previously, the default locale was called “C.UTF-8”, was fully multibyte, and the results of some of the wctype.h functions in this locale were not conforming to the C language’s requirements on the C locale.

Stateful encodings in iconv

Up through 1.1.18, musl’s iconv implementation did not support stateful encodings; iconv_t descriptors were pure values encoding the source and destination charsets. Beginning with 1.1.19, stateful encodings will also be supported, most notably ISO-2022-JP.

Source code used to generate charset/Unicode data

This code was previously unpublished, making it difficult for anyone but the maintainer to update or customize the tables. It is now available on GitHub at:

NIS/LDAP/other user databases

Up through 1.1.6, only the passwd/group files were supported for user and group database functionality. musl 1.1.7 added support for alternative backends via the NSCD protocol. There is an implementation of the server side of this protocol that can use glibc-style NSS modules for backends at:

Substitute for _FORTIFY_SOURCE

This has been implemented in a libc-agnostic way as a second set of headers on top of the libc headers, using “GNU C” features such as #include_next and __builtin_object_size to provide a fully-inline (no use of special libc functions) version of FORTIFY. See:

Building musl itself with stack-protector

musl 1.1.9 introduced the ability to build libc itself with stack protector options. Previously, early-init-stage considerations precluded this.

Further malloc hardening

The adoption of mallocng in musl 1.2.1 mostly resolved the existing malloc-hardening wishlist. Still, further gains may be possible in the future including of MTE and other hardware-assisted protection.