[LCP]read error

[Andrew Weaver]

Does process B terminate after the write?

> -----Original Message-----
> From:	Hemant Mohan [SMTP:hemantm at pune.tcs.co.in]
> Sent:	Tuesday, April 23, 2002 11:46 AM
> To:	linuxcprogramming at lists.linux.org.au
> Subject:	[LCP]read error
> 
> 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
> 
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