Question

Background

I am writing a shared library in C, dynamically linked with LD_PRELOAD, that is meant to intercept and override network calls from the application preloading it, such as socket(), connect(), recv(), send(), etc.

In the init of the library I launch a thread with pthread_create(). This thread polls some kernel memory, which is mapped to user space.

The library is meant to be generic enough to deal with – but not limited to – networking benchmark applications such as netperf, iperf, sockperf.

My issue

Everything works fine, life is sweet in most of the cases except one. Deamonized applications. For instance, if I launch netserver (the server side of the netperf benchmarking application) as a deamon, i.e. without the -D parameter, one of the first things the application does is call fork(). On fork, the parent is closed using exit(EXIT_SUCCESS) and the child listens for connections. The fact that the parent exits kills my polling thread.

So what I’d like to achieve is have the child spawn a new polling thread if the parent has gone. I can intercept and override the fork() call but fundamentally, how can I have the child know whether the parent has gone or not? I can’t take any assumption because the library has to be generic.

Thanks.

Answer 1

You can poll periodically getppid() function. As soon as it starts to return ‘1’ (the id of init process) – your parent is dead.

Update

excerpt from ‘man pthread_create’:

The new thread terminates in one of the following ways: …

• Any of the threads in the process calls exit(3), or the main thread
  performs a return from main(). This causes the termination of all
  threads in the process.

So, if your thread is created by the netserver process that calls exit – yes this thread will be terminated