Subprocess icon bounces in dock - c++

Question

My application starts a subprocess program to read video using the QuickTime framework via fork() and pipes. The subprocess goes into a wait loop when it is not busy, i.e. it does usleep until there is input. The subprocess is not a GUI application and it is written in C++.
When opening AVI video coded using the MSVC codec, a second copy of the application icon shows in the dock and bounces. After about 30 seconds in the Activity Monitor I can see that the subprocess changes to "not responding" even though CPU appears to be ~0%. The subprocess is still running and responding; it's just that Activity Monitor says otherwise.
If I look at the state of the subprocess, via gdb attach or check its output; everything looks fine. I can tell the subprocess to close the file and open another one and continue using it at which point the bouncing dock icon disappears and the process is not marked as not responding.
It's as if OSX thinks my subprocess has crashed (?) but I cannot detect an exception.
How can I stop the subprocess showing an icon in the dock, bouncing and being marked as not responding ?
This is how I set up communication with the subprocess:
#include <unistd.h>
#define READ 0
#define WRITE 1
// Start process
pid_t popen2(const char *command, char * const argv[], int *infp, int *outfp)
{
int p_stdin[2], p_stdout[2];
pid_t pid;
// Set up pipes
if(pipe(p_stdin) != 0 || pipe(p_stdout) != 0)
return(-1);
pid = fork();
if(pid < 0)
return(pid);
else if(pid == 0)
{
// Set up communication via stdin/out
close(p_stdin[WRITE]);
dup2(p_stdin[READ], READ);
close(p_stdout[READ]);
dup2(p_stdout[WRITE], WRITE);
execvp(command, argv); // run subprocess
perror("execvp");
exit(1);
}
// Provide pointers to the file descriptors to the caller
if(infp == NULL)
close(p_stdin[WRITE]);
else
*infp = p_stdin[WRITE];
if(outfp == NULL)
close(p_stdout[READ]);
else
*outfp = p_stdout[READ];
return(pid);
}
See this SO question for more discussion of popen2().
Note: this code may or may not be the cause of my problem. As a first step, I would really like to prove what is the cause.

Answer 1

The “not responding” part is simple: Your subprocess is not running a runloop of any type, so from the POV of the system, it’s not handling events.
I’m a bit hazier on why your new process is getting a dock icon, but it basically boils down to fork() creating a process that inherits the attributes of the parent process (in this case, of it being a foreground application). OS X has a number of mechanisms to launch subprocesses in more sensible ways than fork(). If your app is in Cocoa, use NSTask, otherwise, take a look at posix_spawn(2).
This should be a drop in replacement for your routine:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <spawn.h>
#include <signal.h>
#include <crt_externs.h>
#define READ 0
#define WRITE 1
#define environ (*_NSGetEnviron())
pid_t popen2(const char *command, char * const argv[], int *infp, int *outfp)
{
int p_stdin[2], p_stdout[2];
pid_t pid;
if(pipe(p_stdin) != 0 || pipe(p_stdout) != 0)
return(-1);
posix_spawn_file_actions_t file_actions;
posix_spawn_file_actions_init(&file_actions);
posix_spawn_file_actions_adddup2(&file_actions, p_stdin[READ], 0);
posix_spawn_file_actions_adddup2(&file_actions, p_stdout[WRITE], 1);
posix_spawn_file_actions_adddup2(&file_actions, 2, 2);
posix_spawnattr_t spawnAttributes;
posix_spawnattr_init(&spawnAttributes);
sigset_t no_signals;
sigset_t all_signals;
sigemptyset (&no_signals);
sigfillset (&all_signals);
posix_spawnattr_setsigmask(&spawnAttributes, &no_signals);
posix_spawnattr_setsigdefault(&spawnAttributes, &all_signals);
short flags = POSIX_SPAWN_CLOEXEC_DEFAULT | POSIX_SPAWN_SETSIGMASK | POSIX_SPAWN_SETSIGDEF;
posix_spawnattr_setflags(&spawnAttributes, flags);
if (posix_spawn(&pid, command, &file_actions, &spawnAttributes, argv, environ)) {
perror("posix_spawn");
exit(1);
}
close(p_stdin[READ]);
if(infp == NULL)
close(p_stdin[WRITE]);
else
*infp = p_stdin[WRITE];
close(p_stdout[WRITE]);
if(outfp == NULL)
close(p_stdout[READ]);
else
*outfp = p_stdout[READ];
return(pid);
}

Answer 2

If you're using a compiler that supports c++11, I'd recommend checking out packaged_tasks.
Alternatively, you could also use condition_variables, but I'd try to get it working with packaged tasks first. Condition variables are more of the primitive than the higher-level packaged tasks. Either way, it's a lot easier (and standards compliant) to use these mechanisms than tradition IPC techniques.
That is, of course, if you don't have to have a separate process.
For further information, I'd highly recommend checking out The C++ Programming Language, 4th Edition. It has simple example of a producer/consumer mechanism with a simple vector that may suit you well. If you don't spring for the book, I'm sure you can find similar examples online for using condition_variables, futures or promises.
HTH

Answer 3

I ended up rewriting the subprocess as a MacOS XPC service. In the XPC service's property list I added LSBackgroundOnly to get Launch Services to ignore system event handling.