FIFO issue

runawayNinja · #1 (**permalink**) 04-29-2004

hello i am making a client server program
that communicates via FIFOS and i cannot get it to not hang
i also am forking exec-ing the client from the server

my logic (i will spare you the code )
in the server

i make two fifos in my server my file permissions
are 0644

i then open both with O_RDWR

i then fork and exec the client

inside the client

i open two fifos both with O_RDWR

then begin a while loop to write and read from the server

i understand that a FIFO hangs until a read has established itself
but that is why i have the RDWR flag instead of read and write

my questions are

should i fork/exec first?

do i need to mkfifos in the client as well
even tho i am forking and execing

can someone offer some insight i would really appreciate
this has been a thorn in myside

Neo · #2 (**permalink**) 04-29-2004

Quote:

A FIFO special file (a named pipe) is similar to a pipe, except that it is accessed as part of the file system. It can be opened by multiple pro cesses for reading or writing. When processes are exchanging data via the FIFO, the kernel passes all data internally without writing it to the file system. Thus, the FIFO special file has no contents on the file system, the file system entry merely serves as a reference point so that processes can access the pipe using a name in the file system.

The kernel maintains exactly one pipe object for each FIFO special file that is opened by at least one process. The FIFO must be opened on both ends (reading and writing) before data can be passed. Normally, opening the FIFO blocks until the other end is opened also.

A process can open a FIFO in non-blocking mode. In this case, opening for read only will succeed even if noone has opened on the write side yet; opening for write only will fail with ENXIO (no such device or address) unless the other end has already been opened.

Under Linux, opening a FIFO for read and write will succeed both in blocking and non-blocking mode. POSIX leaves this behaviour undefined. This can be used to open a FIFO for writing while there are no readers available. A process that uses both ends of the connection in order to communicate with itself should be very careful to avoid deadlocks.

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