Question

How can the following code

while(1) {
  client_name_len = sizeof(struct sockaddr_in);
  newsockfd = accept(sockfd, (struct sockaddr *)&client_name, &client_name_len);
  if (newsockfd < 0) {
    perror("ERROR on accept");
    printf("Finished\n");
    close(sockfd);
    exit (EXIT_FAILURE);
  }
  printf("a:");
  pthread_t thread1;
  int *addr = (int*)malloc(sizeof(int));
  *addr = newsockfd;
  pthread_create( &thread1, NULL, &ProcessClient, (void*)addr);
}

produce the following output:

a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:ERROR on accept: Too many open files
a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:a:Finished

The problem is that perror("ERROR on accept"), printf("Finished\n") and printf("a:") are executed in the same thread, but the output is mixed.

ProcessClient does not output anything and does not create any threads.
sockfd is a standard listening tcp socket.

Answer 1

The printf("a:"); and printf("Finished\n"); are buffered I/O going to stdout.

perror("ERROR on accept"); is unbuffered (or immediately flushed) I/O going to stderr.

So the many a:‘s that show up after the ERROR on accept: Too many open files were actually printf()‘ed before, but buffering makes them show up later on the console.

See https://stackoverflow.com/a/3746795/12711 for details and methods to solve the problem.