12-15-2007
Doubt regarding Select()
Please provide the solution for the following scenario:
1) There are two process named as ProcessA and ProcessB
2) ProcessA has opend a named pipe in read mode.This has been made as blocking mode.
3) ProcessB opens this pipe and writes in Blocking mode.So wat happens is even if Process A goes down ,ProcessB still waits
that ProcessA will come up since it is in blocking mode.
Solution tried:
1) Made the open system call as non -blocking mode
But here the problem is, when the packet size is more to be sent it keeps on writing and at one time PIPEBUFFER gets filled.
2) So we made the write system as blocking using select
when pipe buffer is full the return value is EAGAIN
code:
retval =write();
if (retval==-1)
{
if(errno==EAGAIN)
{
select()
}
}
so now when pipe buffer is full it is made to blocking,
AGAIN the problem we face is:
For example:
assume : pipe buffer size =100
according to our product, it should write only 10 bytes,assume now the pipe buffer is full
so write() system call will return EAGAIN,it goes to blocking.now even if there is a SINGLE BYTE space ,write will proceed.
kindly provide me some solution -how to fix this issue
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FIFO(7) Linux Programmer's Manual FIFO(7)
NAME
fifo - first-in first-out special file, named pipe
DESCRIPTION
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 multi-
ple processes for reading or writing. When processes are exchanging data via the FIFO, the kernel passes all data internally without writ-
ing 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 refer-
ence 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 nonblocking mode. In this case, opening for read only will succeed even if no-one 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 nonblocking mode. POSIX leaves this behavior 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.
NOTES
When a process tries to write to a FIFO that is not opened for read on the other side, the process is sent a SIGPIPE signal.
FIFO special files can be created by mkfifo(3), and are indicated by ls -l with the file type 'p'.
SEE ALSO
mkfifo(1), open(2), pipe(2), sigaction(2), signal(2), socketpair(2), mkfifo(3), pipe(7)
COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-03 FIFO(7)