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

fusl/src/stdio/__fopen_rb_ca.c

+#include "stdio_impl.h"
+#include <fcntl.h>
+#include <string.h>
+
+FILE *__fopen_rb_ca(const char *filename, FILE *f, unsigned char *buf, size_t len)
+{
+        memset(f, 0, sizeof *f);
+
+        f->fd = sys_open(filename, O_RDONLY|O_CLOEXEC);
+        if (f->fd < 0) return 0;
+        __syscall(SYS_fcntl, f->fd, F_SETFD, FD_CLOEXEC);
+
+        f->flags = F_NOWR | F_PERM;
+        f->buf = buf + UNGET;
+        f->buf_size = len - UNGET;
+        f->read = __stdio_read;
+        f->seek = __stdio_seek;
+        f->close = __stdio_close;
+        f->lock = -1;
+
+        return f;
+}