[musl] __MUSL__ macro

[Alastair Houghton <ahoughton@apple.com>]

[-- Attachment #1: Type: text/plain, Size: 2272 bytes --]

Hi all,

Before I start, I’m aware of

  <https://www.openwall.com/lists/musl/2013/03/29/13>

but I *still* want to add __MUSL__ (see attached patch).

Let me explain what we’re doing, why we want it and why we think musl *should* have it.  We’re trying to add support for musl to Swift <https://swift.org <https://swiftlang.org/>> and its attendant core libraries, and there are a number of things about musl that presently differ from other platforms/C libraries we support.

Examples include the use of `union`s in `pthread_mutex_t` et al (which means that we can’t write a C++ `constexpr` function that returns one, even if all it does is return `PTHREAD_MUTEX_INITIALIZER`), the fact that it doesn’t have the `d_namlen` member of `struct dirent`, or the fact that `dladdr()` is a no-op when statically linked.

I’m aware that there are other solutions for some of these cases - but for instance the `dladdr()` issue is not something you can test at configuration time when cross-compiling, since there’s no way to know what the result would be without running a program on the target system (which you don’t necessarily have available at configuration time in that case).  Likewise, configuration time tests are slow and  aren’t even a thing in some of the projects we need to add support in, whereas we do generally have the ability to use the preprocessor.

You might say we should just bite the bullet and add configuration steps to all of the other projects, but that is honestly a non-starter; the owners of those projects are not likely to accept patches to add such a thing, even as a short-term fix, and the patches to do so would be non-trivial in a number of cases also.  In the longer term we don’t want higher level projects to need to do this kind of thing and the lower level ones could in many cases do some kind of configuration time testing, but that’s a longer term goal and it doesn’t help us to add musl support in the interim, which we would like to do.

I’ve attached a proposed patch that adds `__MUSL__` (set to the major version) and `__MUSL_MINOR__` (set to the minor version) as well as `__MUSL_PREREQ()` (which works the same way `__GLIBC_PREREQ()` does).

Kind regards,

Alastair Houghton


[-- Attachment #2: musl-version.patch --]
[-- Type: application/octet-stream, Size: 2772 bytes --]

diff --git a/Makefile b/Makefile
index e8cc4436..ea91b9be 100644
--- a/Makefile
+++ b/Makefile
@@ -34,7 +34,7 @@ CRT_OBJS = $(filter obj/crt/%,$(ALL_OBJS))
 
 AOBJS = $(LIBC_OBJS)
 LOBJS = $(LIBC_OBJS:.o=.lo)
-GENH = obj/include/bits/alltypes.h obj/include/bits/syscall.h
+GENH = obj/include/bits/alltypes.h obj/include/bits/syscall.h obj/include/features.h
 GENH_INT = obj/src/internal/version.h
 IMPH = $(addprefix $(srcdir)/, src/internal/stdio_impl.h src/internal/pthread_impl.h src/internal/locale_impl.h src/internal/libc.h)
 
@@ -95,6 +95,17 @@ $(ALL_LIBS) $(ALL_TOOLS) $(ALL_OBJS) $(ALL_OBJS:%.o=%.lo) $(GENH) $(GENH_INT): |
 $(OBJ_DIRS):
 	mkdir -p $@
 
+MUSL_VERSION:=$(shell cd $(srcdir) && sh tools/version.sh)
+empty:=
+space:=$(empty) $(empty)
+musl_ver_parts:=$(subst -git-,$(space),$(subst .,$(space),$(MUSL_VERSION)))
+MUSL_MAJOR:=$(word 1, $(musl_ver_parts))
+MUSL_MINOR:=$(word 2, $(musl_ver_parts))
+MUSL_PATCH:=$(word 3, $(musl_ver_parts))
+
+obj/include/features.h: $(srcdir)/include/features.h.in $(wildcard $(srcdir)/VERSION $(srcdir)/.git)
+	sed -e "s/@MUSL_MAJOR@/$(MUSL_MAJOR)/g;s/@MUSL_MINOR@/$(MUSL_MINOR)/g;s/@MUSL_PATCH@/$(MUSL_PATCH)/g" $< > $@
+
 obj/include/bits/alltypes.h: $(srcdir)/arch/$(ARCH)/bits/alltypes.h.in $(srcdir)/include/alltypes.h.in $(srcdir)/tools/mkalltypes.sed
 	sed -f $(srcdir)/tools/mkalltypes.sed $(srcdir)/arch/$(ARCH)/bits/alltypes.h.in $(srcdir)/include/alltypes.h.in > $@
 
@@ -103,7 +114,7 @@ obj/include/bits/syscall.h: $(srcdir)/arch/$(ARCH)/bits/syscall.h.in
 	sed -n -e s/__NR_/SYS_/p < $< >> $@
 
 obj/src/internal/version.h: $(wildcard $(srcdir)/VERSION $(srcdir)/.git)
-	printf '#define VERSION "%s"\n' "$$(cd $(srcdir); sh tools/version.sh)" > $@
+	printf '#define VERSION "%s"\n' "$(MUSL_VERSION)" > $@
 
 obj/src/internal/version.o obj/src/internal/version.lo: obj/src/internal/version.h
 
@@ -209,6 +220,9 @@ $(DESTDIR)$(includedir)/bits/%: obj/include/bits/%
 $(DESTDIR)$(includedir)/%: $(srcdir)/include/%
 	$(INSTALL) -D -m 644 $< $@
 
+$(DESTDIR)$(includedir)/%: obj/include/%
+	$(INSTALL) -D -m 644 $< $@
+
 $(DESTDIR)$(LDSO_PATHNAME): $(DESTDIR)$(libdir)/libc.so
 	$(INSTALL) -D -l $(libdir)/libc.so $@ || true
 
diff --git a/include/features.h b/include/features.h.in
similarity index 80%
rename from include/features.h
rename to include/features.h.in
index 85cfb72a..07662804 100644
--- a/include/features.h
+++ b/include/features.h.in
@@ -1,6 +1,13 @@
 #ifndef _FEATURES_H
 #define _FEATURES_H
 
+#define __MUSL__       @MUSL_MAJOR@
+#define __MUSL_MINOR__ @MUSL_MINOR@
+#define __MUSL_PATCH__ @MUSL_PATCH@
+
+#define __MUSL_PREREQ(maj, min) \
+  ((__MUSL__ << 16) + __MUSL_MINOR >= ((maj) << 16) + (min))
+
 #if defined(_ALL_SOURCE) && !defined(_GNU_SOURCE)
 #define _GNU_SOURCE 1
 #endif