Linux 2.6 FIFO vs Linux 2.4 FIFO

gjpc · 03-19-2008, 11:13 AM

Hi,

I am back porting a Linux 2.6 project to Linux 2.4. The main process is a daemon that accepts control inputs over a named pipe FIFO. It works just fine on the 2.6 kernel. The 2.4 kernel appears to have a very aggressive FIFO buffering scheme, 2.4 does not allow the reading end to see the data in the FIFO until either there is a lot of data or the write end closes. This is very inconvenient because some of the command sequences are interactive.

Must I close a 2.4 pipe every time I wish to transmit a short message to the reader? I tried getc's and they do not return anything until the writer closes the pipe or it fills up. I am using iostream objects on each end of the pipe. Readers open with:

in.open( fifoInName.c_str(), ios::in );

Writers open with:

out.open( fifoOutName.c_str(), ios::trunc );

I have proof that the pipe code is functional in Linux 2.4: If I cat the output FIFO to the screen in one shell and echo the help command into the input FIFO in another shell I get full text of the help command.

Any suggestions are appreciated.

ta0kira · 03-19-2008, 01:14 PM

Have you tried non-blocking reads?
ta0kira

gjpc · 03-19-2008, 01:29 PM

Peek does not reveal anything in the FIFO. The problem is that there is data in the FIFO and the reciever does not see it until the FIFO file is closed.

ta0kira · 03-19-2008, 03:34 PM

Peeking isn't a system-level I/O operation. It's associated with the FILE* buffer placed on top of the file descriptor. The only way to tell if the FIFO is closed is to have a read operation return 0 bytes read, which the libc interface handles internally (you don't deal with it directly when using FILE*.)
ta0kira

PS Place this right after fdopen on the reading end and see what happens:

Code:

setvbuf(/*FILE**/ your_file, NULL, _IONBF, 0);
fcntl( /*int*/ your_fd, F_SETFL, fcntl(/*int*/ your_fd, F_GETFL) | O_NONBLOCK ); //(needs <fcntl.h>)

You'll need to allow for empty reads that aren't necessarily indicative of EOF.

gjpc · 03-19-2008, 08:26 PM

Blocking is not a factor.

After much experimentation, I discovered that it must be the g++ 3.2 runtime vs g++ 3.4 runtime.

g++ 3.4 must have the smarts to open a FIFO in O_DIRECT mode.

I had to drop using the ostream and istream objects and do

out = open( "fifoOut", O_RDONLY | O_DIRECT)

and

in = open( "fifoIn", O_TRUNC | O_WRONLY | O_DIRECT)

Of course all my token processing has to be redone. I do not have the option of installing the new gnu c++ runtime on the old systems. Thank goodness for ostrstream!

I could not find a way to have the 3.2 runtime open with O_DIRECT mode.