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

fusl/src/stdio/__fdopen.c

+#include "stdio_impl.h"
+#include <stdlib.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+
+FILE *__fdopen(int fd, const char *mode)
+{
+        FILE *f;
+        struct winsize wsz;
+
+        /* Check for valid initial mode character */
+        if (!strchr("rwa", *mode)) {
+                errno = EINVAL;
+                return 0;
+        }
+
+        /* Allocate FILE+buffer or fail */
+        if (!(f=malloc(sizeof *f + UNGET + BUFSIZ))) return 0;
+
+        /* Zero-fill only the struct, not the buffer */
+        memset(f, 0, sizeof *f);
+
+        /* Impose mode restrictions */
+        if (!strchr(mode, '+')) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
+
+        /* Apply close-on-exec flag */
+        if (strchr(mode, 'e')) __syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
+
+        /* Set append mode on fd if opened for append */
+        if (*mode == 'a') {
+                int flags = __syscall(SYS_fcntl, fd, F_GETFL);
+                if (!(flags & O_APPEND))
+                        __syscall(SYS_fcntl, fd, F_SETFL, flags | O_APPEND);
+                f->flags |= F_APP;
+        }
+
+        f->fd = fd;
+        f->buf = (unsigned char *)f + sizeof *f + UNGET;
+        f->buf_size = BUFSIZ;
+
+        /* Activate line buffered mode for terminals */
+        f->lbf = EOF;
+        if (!(f->flags & F_NOWR) && !__syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz))
+                f->lbf = '\n';
+
+        /* Initialize op ptrs. No problem if some are unneeded. */
+        f->read = __stdio_read;
+        f->write = __stdio_write;
+        f->seek = __stdio_seek;
+        f->close = __stdio_close;
+
+        if (!libc.threaded) f->lock = -1;
+
+        /* Add new FILE to open file list */
+        return __ofl_add(f);
+}
+
+weak_alias(__fdopen, fdopen);