FIFO doesn't block on read

Question

Why the following program doesn't block on the second read call?

int pid = fork(); if(pid) { int fifo = open("testfifo", O_RDWR); char buf[20]; while(1) { read(fifo, buf, 10); puts(buf); } } else { int fifo = open("testfifo", O_WRONLY); write(fifo, "teststring", 10); close(fifo); } return 0;

The second read call continues returning 0 even though the fifo become empty and it should block on the read call.

Am I missing something?

The OS is Windows and the pipe has been created with a mknod testfifo p.

Do not ignore the return value of read(). Interpret 0 as "end of file", < 0 as an error. You should get eof here, you closed the pipe. — Hans Passant
– Hans Passant, Commented Aug 31, 2011 at 12:31
@Simone Apologies for my dodgy edit - obviously didn't intend to damage your question like that! — razlebe
– razlebe, Commented Aug 31, 2011 at 12:48
@Hans But what i want is the program to block on the read waiting other processes to write on the pipe — Simone
– Simone, Commented Aug 31, 2011 at 12:48
Is this native Windows code or is it running under Cygwin or any similar environment? If native, whose C runtime library are you using - Microsoft Visual Studio? — Harry Johnston
– Harry Johnston, Commented Sep 1, 2011 at 2:40
@Harry Johnston Is cygwin. However i solved the problem; i have simply to open and close the file each time in the server cycle. — Simone
– Simone, Commented Sep 1, 2011 at 3:43

Simone · Accepted Answer · 2011-09-01 03:44:28Z

I found, from another stackoverflow question, that i should open and close the "server" pipe, in this case the pipe of the parent process, each time; so here's the correct code:

int pid = fork(); if(pid) { char buf[20]; while(1) { int fifo = open("testfifo", O_RDWR); read(fifo, buf, 15); close(fifo); puts(buf), fflush(stdout); } } else { int fifo = open("testfifo", O_WRONLY); write(fifo, "teststring", 15); close(fifo); }

Johannes · Accepted Answer · 2011-09-02 11:54:13Z

0

You did not close the file

EDIT: deleted something embarrassing.

edited Sep 2, 2011 at 11:54

answered Aug 31, 2011 at 11:58

Johannes

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1 Comment

Simone Over a year ago

Tried, but the problem still remain; actually I think that the fifo is closed by the system at the end of the process. Moreover if you look at the code, there is a infinite loop, so the close(fifo) you added is never executed.

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