Add a "fork" of musl as //fusl.

fusl/src/passwd/putgrent.c

Index: fusl/src/passwd/putgrent.c
diff --git a/fusl/src/passwd/putgrent.c b/fusl/src/passwd/putgrent.c
new file mode 100644
index 0000000000000000000000000000000000000000..a0b320fc32692bc244d40f0c0191f8d94eefb388
--- /dev/null
+++ b/fusl/src/passwd/putgrent.c
@@ -0,0 +1,17 @@
+#define _GNU_SOURCE
+#include <grp.h>
+#include <stdio.h>
+
+int putgrent(const struct group *gr, FILE *f)
+{
+	int r;
+	size_t i;
+	flockfile(f);
+	if ((r = fprintf(f, "%s:%s:%d:", gr->gr_name, gr->gr_passwd, gr->gr_gid))<0) goto done;
+	if (gr->gr_mem) for (i=0; gr->gr_mem[i]; i++)
+		if ((r = fprintf(f, "%s%s", i?",":"", gr->gr_mem[i]))<0) goto done;
+	r = fputc('\n', f);
+done:
+	funlockfile(f);
+	return r<0 ? -1 : 0;
+}