Read error

Read error

[Hemant Mohan]

I am writing a program whereby I am opening a FIFO for reading. However the 
read is getting blocked though another process has already written to the 
FIFO and no other process is accesiing this FIFO.
the program is something like:

Process A
main()
{
    int fd;
    fd=open(FIFO, RD_ONLY );
    read(fd, ....)
}

Process B
main()
{
    mkfifo(FIFO, 0664);
    fd =open(FIFO, WR_ONLY);
    write(fd, ...);
    printf("data written to fifo\n");
}

Process B is run before process A, and the printf statement is desiplayed on 
the screen.
But I find that inspite of this process B is blocked on the read as if there 
is no data in the FIFO.

However if in process B, the open is done in non blocking mode 
fd = open(FIFO, RD_ONLY | O_NONBOCK)
then it doesn't block on read, but read returns a negative value and the 
errorno is set, with the message "Resource temporarily unavailable".

My question is that why does the read block initially, when there is some 
data for it to read. Under what conditions does the read normally block.


Thanks,
Hemant Mohan

Re: Read error

[Glynn Clements]

Hemant Mohan wrote:

> My question is that why does the read block initially, when there is some 
> data for it to read. Under what conditions does the read normally block.

read() blocks if no data is available.

However, open(O_RDONLY) blocks until something opens the other end of
the pipe for writing, and open(O_WRONLY) blocks until something opens
the other end of the pipe for reading. So, whichever process is run
first will block on open() until the other process calls open().

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: Read error

[weener]

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