[Interest] QProcess: child read from stdin - no data received?

Till Oliver Knoll till.oliver.knoll at gmail.com
Sun May 13 19:51:22 CEST 2012

Am 13.05.2012 um 18:36 schrieb Till Oliver Knoll <till.oliver.knoll at gmail.com>:

> Hello,
> this seems to be an ever recurring topic: How does one get notified in a *child* process (started via QProcess) once there is data to be read on stdin (in a platform-independent matter - but for now mostly OS X)?

I came to realise that stdin/out are really *text*-based and trying to re-open them in binary mode is also not guaranteed to work everywhere:


As I really want to send raw JPEG binary data from parent to child this IPC solution is a dead-end anyway.

Next: QLocalServer/Socket. I assume in the parent process I start a QLocalServer, start a child process via QProcess and pass along the "server connect information" as arguments, and then in the child process connect to the parent process' server. That should hopefully give me a bi-directional (local socket or pipe) connection with "bytesAvailable" signals on both ends.

It remains to be seen whether the parent process with *no* network entitlement ("OS X sandboxing") can actually create a local server...

If that would fail, what remains is QSharedMemory. But to my understanding that would require to "poll some memory addresses" wheter the other process wants to "send a message". Not to mention that I need to send data of variable size (JPEG), so I would probably end up in a "memory-based block read/write protocol": "I have written 1024 KBytes of JPEG data into the shared memory block, more to follow (poll for ACK) - Ok, got it (poll for more data) - Here is the next block of 1024 KBytes - Ok, bring it on... - Here are the last 42 KBytes - Got it..." doh!

I hope the local socket approach works...


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