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

fusl/src/fcntl/fcntl.c

+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <stdarg.h>
+#include <errno.h>
+#include "syscall.h"
+#include "libc.h"
+
+int fcntl(int fd, int cmd, ...)
+{
+        unsigned long arg;
+        va_list ap;
+        va_start(ap, cmd);
+        arg = va_arg(ap, unsigned long);
+        va_end(ap);
+        if (cmd == F_SETFL) arg |= O_LARGEFILE;
+        if (cmd == F_SETLKW) return syscall_cp(SYS_fcntl, fd, cmd, (void *)arg);
+        if (cmd == F_GETOWN) {
+                struct f_owner_ex ex;
+                int ret = __syscall(SYS_fcntl, fd, F_GETOWN_EX, &ex);
+                if (ret == -EINVAL) return __syscall(SYS_fcntl, fd, cmd, (void *)arg);
+                if (ret) return __syscall_ret(ret);
+                return ex.type == F_OWNER_PGRP ? -ex.pid : ex.pid;
+        }
+        if (cmd == F_DUPFD_CLOEXEC) {
+                int ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, arg);
+                if (ret != -EINVAL) {
+                        if (ret >= 0)
+                                __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+                        return __syscall_ret(ret);
+                }
+                ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, 0);
+                if (ret != -EINVAL) {
+                        if (ret >= 0) __syscall(SYS_close, ret);
+                        return __syscall_ret(-EINVAL);
+                }
+                ret = __syscall(SYS_fcntl, fd, F_DUPFD, arg);
+                if (ret >= 0) __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+                return __syscall_ret(ret);
+        }
+        switch (cmd) {
+        case F_SETLK:
+        case F_GETLK:
+        case F_GETOWN_EX:
+        case F_SETOWN_EX:
+                return syscall(SYS_fcntl, fd, cmd, (void *)arg);
+        default:
+                return syscall(SYS_fcntl, fd, cmd, arg);
+        }
+}