diff --git a/options/posix/include/unistd.h b/options/posix/include/unistd.h
index 26f407d8f5..4cc750763a 100644
--- a/options/posix/include/unistd.h
+++ b/options/posix/include/unistd.h
@@ -305,7 +305,11 @@ extern int optopt;
 #endif /* !__MLIBC_ABI_ONLY */
 
 // Non-POSIX functions supported by Linux.
-typedef unsigned useconds_t;
+#if UINTPTR_MAX == UINT64_MAX
+typedef __mlibc_uint64 useconds_t;
+#else
+typedef __mlibc_uint32 useconds_t;
+#endif
 
 #ifndef __MLIBC_ABI_ONLY