Data queued in a named pipe.
 

Is there any way to find out how much data is queued in/written to a POSIX named pipe?

Thanks,
Abhishek

Other than reading the pipe, not that I know of.

When using sockets (for example, a socket pair created using socketpair(2)), one can use recv(descriptor,buffer,buffersize,MSG_NOWAIT|MSG_PEEK) to read but not consume already buffered data from the socket. It will not work with named pipes, however; only with sockets.

Thank you for the helpful reply. For the problem I had at hand I only needed to know when the pipe is empty, so the reader can stop reading. After your conformation and going through the manpage again I guess a call to read(int fildes, void *buf, size_t nbyte) is enough for that.

But just out of curiosity and for possible utility, is there a way to peek into the underlying kernel data structures to find out the fifo size, if anyone knows that?

You may wish to open the pipe nonblocking (or set it nonblocking using fcntl(descriptor,F_SETFL,O_NONBLOCK)), so the call will return immediately if there is no data to read from the pipe.

Only if using kernel 2.6.35 or later. Then, you have fcntl(descriptor,F_GETPIPE_SZ) to find out the capacity of the pipe (if the descriptor refers to a pipe), and fcntl(descriptor,F_GETPIPE_SZ,size) to set the pipe capacity. If setting a pipe capacity that is not a multiple of page size (sysconf(_SC_PAGESIZE)), the kernel will round it upwards to the next multiple of page size.

An userspace application may adjust the pipe capacity freely between one page (sysconf(_SC_PAGESIZE) bytes) and /proc/sys/fs/pipe-size-max bytes.