libc/common: Add memalign

Memalign is another name for the posix aligned_alloc function, although it
has weaker restrictions on the relationship between the alignment and size.

memalign() is used internally by the libstdc++ when built for 'newlib'
targets (which includes picolibc) instead of aligned_alloc() due to a bug
in gcc, so we need to provide an implementation of this when using that
library, even though it's not part of the Zephyr C library API.

When a fix for the libstdc++ is merged upstream and can be consider a
reasonable dependency for Zephyr, this work-around can be removed.

Closes: #57899

Signed-off-by: Keith Packard <[email protected]>

lib/libc/common/source/stdlib/malloc.c

@@ -153,6 +153,27 @@ void *aligned_alloc(size_t alignment, size_t size)
 	return ret;
 }
 
+#ifdef CONFIG_GLIBCXX_LIBCPP
+
+/*
+ * GCC's libstdc++ may use this function instead of aligned_alloc due to a
+ * bug in the configuration for "newlib" environments (which includes picolibc).
+ * When toolchains including that bug fix can become a dependency for Zephyr,
+ * this work-around can be removed.
+ *
+ * Note that aligned_alloc isn't defined to work as a replacement for
+ * memalign as it requires that the size be a multiple of the alignment,
+ * while memalign does not. However, the aligned_alloc implementation here
+ * is just a wrapper around sys_heap_aligned_alloc which doesn't have that
+ * requirement and so can be used by memalign.
+ */
+
+void *memalign(size_t alignment, size_t size)
+{
+	return aligned_alloc(alignment, size);
+}
+#endif
+
 static int malloc_prepare(void)
 {
 	void *heap_base = NULL;