os_mon: Elucidate port program error messages

Try to avoid confusion of the message 'Erlang has closed' by
clarifying that the message is sent from os_mon port programs.

1 files changed, 12 insertions, 2 deletions

diff --git a/lib/os_mon/c_src/cpu_sup.c b/lib/os_mon/c_src/cpu_sup.c
index 9c5f9a6aa5..a0432b3093 100644
--- a/lib/os_mon/c_src/cpu_sup.c
+++ b/lib/os_mon/c_src/cpu_sup.c
@@ -458,8 +458,18 @@ static void error(char* err_msg) {
    * if we get error here we have trouble,
    * silence unnecessary warnings
    */
-  if(write(FD_ERR, err_msg, strlen(err_msg)));
-  if(write(FD_ERR, "\n", 1));
+  char buffer[256] = "[os_mon] cpu supervisor port (cpu_sup): ";
+  int i = strlen(buffer), j = 0;
+  int n = strlen(err_msg);
+
+  while(i < 253 && j < n) {
+      buffer[i++] = err_msg[j++];
+  }
+  buffer[i++] = '\r';
+  buffer[i++] = '\n';
+
+  /* try to use one write only */
+  if(write(FD_ERR, buffer, i));
   exit(-1);
 }