Question

In my main process, i create a ffmpeg child process using CreateProcess(…).
I need to track the status of converting progress to update a progress bar. To do it, I read text from ffmpeg output and extract progress status from it.

I make a sample programm like this:

HANDLE rPipe, wPipe;
CreatePipe(&rPipe,&wPipe,&secattr,0);

STARTUPINFO sInfo; 
ZeroMemory(&sInfo,sizeof(sInfo));
PROCESS_INFORMATION pInfo; 
ZeroMemory(&pInfo,sizeof(pInfo));
sInfo.cb=sizeof(sInfo);
sInfo.dwFlags=STARTF_USESTDHANDLES;
sInfo.hStdInput=NULL; 
sInfo.hStdOutput=wPipe; 
sInfo.hStdError=wPipe;

// pStr contain ffmpeg command
CreateProcess(0,(LPTSTR)pStr,0,0,TRUE,NORMAL_PRIORITY_CLASS|CREATE_NO_WINDOW,0,0,&sInfo,&pInfo);
CloseHandle(wPipe);

BOOL ok;
do
{
    memset(buf,0,bufsize);
    ok=::ReadFile(rPipe,buf,100,&reDword,0);
    result += buf;            
}while(ok);

But I couldnt get “result” interactively updated. My app is held during conversion, and “result” string update only after ffmpeg’s process finish.

How can I have my main process and ffmpeg’s run simultaneously, and interactively read from/write to ffmpeg process’s output/input?

Thanks for your time!

LRs

Answer 1

If the ffmpeg just uses stdout without explicitly flushing the output then it may not get sent to the calling process until it ends

Child processes that use such C run-time functions as printf() and
fprintf() can behave poorly when redirected. The C run-time functions
maintain separate IO buffers. When redirected, these buffers might not
be flushed immediately after each IO call. As a result, the output to
the redirection pipe of a printf() call or the input from a getch()
call is not flushed immediately and delays, sometimes-infinite delays
occur. This problem is avoided if the child process flushes the IO
buffers after each call to a C run-time IO function. Only the child
process can flush its C run-time IO buffers. A process can flush its C
run-time IO buffers by calling the fflush() function.

http://support.microsoft.com/kb/190351