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I have the following situation: two applications, "interface" (Qt) and "center" (C), work together as part of a embedded Linux solution communicating mainly by sockets. "Interface" starts "center" and must be able to send a "close" command to it. "Center" has a main event loop where it should constantly read if the "close" command was send by the "interface".

First I would call "Center" with system("./center&") and I would use read() in the following way:

char readBuffer[10];
memset(readBuffer,'\0',sizeof(char) * 10);

read(STDIN_FILENO, readBuffer, 10);

if (strcmp(readBuffer, "close") == 0)
{
     DEBUG_MAIN("CENTER: Closing center normally");
     break;
}

(this inside a while(true) loop in main()).

The problem is that this way "Interface" unable to communicate directly to "Center" - I can only use the socket system. That would work fine, but it seems it would have some drawbacks: the socket is read from another thread, so I'll have to create mutex etc. to constantly see a bool flag inside main(), the call to the ill-advised system, etc..

So I decided that instead I should call "Center" with QProcess, what would give me some logging advantages and I would have the traditional QProcess way of communicating with that app, namely writing to the stdin with write().

So I did implemented QProcess successfully and everything is working fine with the exception that now the code copied above with read() became a blocking one: when calling "Center" with system(), the code would pass that part noticing there was nothing in STDIN to be read. Now that I call Center with QProcess, read() is behaving in a blocking way.

So my first (and specific) question is: why read() behaves in non-blocking fashion when "Center" is started with system() but blocking when is started with QProcess?

And the second (generic) question is: how can I read from STDIN in a non-blocking way? I already did this research before (example of results) and AFAIR the code above was the solution I got for doing this; this is why I was using it.

Answer to the second question: to read the STDIN in a non-blocking way, just write

#include <fcntl.h>

int flagsTemp = fcntl(STDIN_FILENO, F_GETFL, 0);
fcntl(STDIN_FILENO, F_SETFL, flagsTemp | O_NONBLOCK);

Reference

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  • Don't use `system`. First of all it will *block* until the program you're executing exits, secondly you can't read output from it, or write output to it. – Some programmer dude Mar 12 '15 at 14:01
  • As for your problem, is the code you show in the "UI" or the "Center" program? What are you reading with that? How do the two programs fit together? How are you using `QProcess`? And on POSIX systems you can make *any* file-descriptor non-blocking the same way, doesn't matter if it's a file, socket, pipe or something else. – Some programmer dude Mar 12 '15 at 14:02
  • @JoachimPileborg that's precisely what I'm trying to do by using QProcess! (but it's not true that the program will block till the other closes if you use & after the call to the program: `system("./center&")`) – Momergil Mar 12 '15 at 14:05
  • @JoachimPileborg so you're essentially saying that I could somehow control `read()` so it will be non-blocking when I use QProcess? Well how do I do this? Not to forget that, as I sad, read() only become blocking after I started using QProcess instead of a call with system(). – Momergil Mar 12 '15 at 14:06
  • Are you on a POSIX system (like Linux or OSX)? Then have you ever made a socket non-blocking? That works for *any* descriptor, not just sockets. But wouldn't it be better to use the Qt classes and functions to handle the I/O if you're using `QProcess`? Why do you resort to using low-level system calls? – Some programmer dude Mar 12 '15 at 14:10
  • @JoachimPileborg Yes I'm on Embedded Linux. As I sad, I AM using sockets as part of the communication - and they are non-blocking. What I want is a read to STDIN that is non blocking so I can use QProcess' write() method to send a "close" message to "center" instead of using that socket system. – Momergil Mar 12 '15 at 14:13

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