diff --git a/wrapper.c b/wrapper.c
index f92b14759833b1ae77070b4188d8ad8ea8d3799e..cfaf23d387b16ef2491abc0b41fa52d7c9cc009d 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -135,8 +135,22 @@ void *xcalloc(size_t nmemb, size_t size)
  * 64-bit is buggy, returning EINVAL if len >= INT_MAX; and even in
  * the absense of bugs, large chunks can result in bad latencies when
  * you decide to kill the process.
+ *
+ * We pick 8 MiB as our default, but if the platform defines SSIZE_MAX
+ * that is smaller than that, clip it to SSIZE_MAX, as a call to
+ * read(2) or write(2) larger than that is allowed to fail.  As the last
+ * resort, we allow a port to pass via CFLAGS e.g. "-DMAX_IO_SIZE=value"
+ * to override this, if the definition of SSIZE_MAX given by the platform
+ * is broken.
  */
-#define MAX_IO_SIZE (8*1024*1024)
+#ifndef MAX_IO_SIZE
+# define MAX_IO_SIZE_DEFAULT (8*1024*1024)
+# if defined(SSIZE_MAX) && (SSIZE_MAX < MAX_IO_SIZE_DEFAULT)
+#  define MAX_IO_SIZE SSIZE_MAX
+# else
+#  define MAX_IO_SIZE MAX_IO_SIZE_DEFAULT
+# endif
+#endif
 
 /*
  * xread() is the same a read(), but it automatically restarts read()