get process name from process ID in a c/cpp program (I can't use /proc/<pid>/cmdline)

I know this question has been asked a few times, but unfortunately I haven't been able to find an answer that matches my restrictions.

I have a PID (not my process), and I want to find its name. Now, I can't access /proc/<pid>/cmdline (restrictions), and I can't find a way to get the output of ps in my program, besides sending its output to a file and then parsing it (which I need to avoid).

Is there another option?

I am coding in Linux/Android user space in C/C++

Solution

It sounds like ps does work (?) but you can't write to a temporary file. (Why? Maybe AppArmor is restricting access to just some processes?)

If that's true, then you can use a pipe to read the ps output directly into your program, without a temporary file.

int fds[2];
pipe(fds);

int pid = fork();
if (pid == 0) {
  // child
  close(fds[0]);  // close the read end of the pipe in the child
  dup2(1,fds[1]); // move the write end to be stdout
  close(fds[1]);

  execlp("ps", "ps", "-p", "blah", NULL);
} else {
  // parent
  close(fds[1]);  // close the write end of the pipe in the parent.

  // read data from fds[0] here

  close(fds[0]);
  int status;
  wait(&status); // clean up the zombie ps process
}

That example leaves out all the error checking (which you must add), and might not be allowed (depending what the nature of your access restrictions might be).