- /* Copy user buffer. */
- if (!(port->write_buf = malloc(count)))
- RETURN_ERROR(SP_ERR_MEM, "buffer copy malloc failed");
- memcpy(port->write_buf, buf, count);
-
- /* Start asynchronous write. */
- if (WriteFile(port->hdl, buf, count, NULL, &port->write_ovl) == 0) {
- if (GetLastError() == ERROR_IO_PENDING) {
- DEBUG("Asynchronous write started");
- port->writing = 1;
- RETURN_VALUE("%d", count);
+ /* Keep writing data until the OS has to actually start an async IO for it.
+ * At that point we know the buffer is full. */
+ while (written < count)
+ {
+ /* Copy first byte of user buffer. */
+ port->pending_byte = *ptr++;
+
+ /* Start asynchronous write. */
+ if (WriteFile(port->hdl, &port->pending_byte, 1, NULL, &port->write_ovl) == 0) {
+ if (GetLastError() == ERROR_IO_PENDING) {
+ DEBUG("Asynchronous write started");
+ port->writing = 1;
+ RETURN_VALUE("%d", ++written);
+ } else {
+ /* Actual failure of some kind. */
+ RETURN_FAIL("WriteFile() failed");
+ }