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

fusl/src/stdio/freopen.c

+#include "stdio_impl.h"
+#include <fcntl.h>
+
+/* The basic idea of this implementation is to open a new FILE,
+ * hack the necessary parts of the new FILE into the old one, then
+ * close the new FILE. */
+
+/* Locking IS necessary because another thread may provably hold the
+ * lock, via flockfile or otherwise, when freopen is called, and in that
+ * case, freopen cannot act until the lock is released. */
+
+int __dup3(int, int, int);
+
+FILE *freopen(const char *restrict filename, const char *restrict mode, FILE *restrict f)
+{
+        int fl = __fmodeflags(mode);
+        FILE *f2;
+
+        FLOCK(f);
+
+        fflush(f);
+
+        if (!filename) {
+                if (fl&O_CLOEXEC)
+                        __syscall(SYS_fcntl, f->fd, F_SETFD, FD_CLOEXEC);
+                fl &= ~(O_CREAT|O_EXCL|O_CLOEXEC);
+                if (syscall(SYS_fcntl, f->fd, F_SETFL, fl) < 0)
+                        goto fail;
+        } else {
+                f2 = fopen(filename, mode);
+                if (!f2) goto fail;
+                if (f2->fd == f->fd) f2->fd = -1; /* avoid closing in fclose */
+                else if (__dup3(f2->fd, f->fd, fl&O_CLOEXEC)<0) goto fail2;
+
+                f->flags = (f->flags & F_PERM) | f2->flags;
+                f->read = f2->read;
+                f->write = f2->write;
+                f->seek = f2->seek;
+                f->close = f2->close;
+
+                fclose(f2);
+        }
+
+        FUNLOCK(f);
+        return f;
+
+fail2:
+        fclose(f2);
+fail:
+        fclose(f);
+        return NULL;
+}
+
+LFS64(freopen);