Select not blocking on FIFO pipe

I am trying to continuously read from a FIFO pipe and just print the
data to a file. To do this, I am using the select() function to block
until the program
sees some data on the pipe.  But for some odd reason, the select is
constantly returning - even when there is no new data in the FIFO
pipe.  Here's my code if it helps.



fd_set rfds;
int retval;
char *buf;
size_t n = 0;
struct timeval tv;
FILE *output = fopen("out", "a");
FILE *log = fopen("fifo", "r");

FD_ZERO(&rfds);
FD_SET(fileno(log), &rfds);

while(1) {
tv.tv_sec = 300;
tv.tv_usec = 0;
retval = select(fileno(log) + 1, &rfds, NULL, NULL, &tv);

if (retval) {
while(getline(&buf, &n, log) != -1) {
fprintf(out, "%s", buf);
}
}
}

Any suggestions appreciated.

--Sachin

In article <beb6d765.0410221354.645c6910@posting.google.com>,
ssdas4eva@yahoo.com (Sachin Doshi) wrote:

> I am trying to continuously read from a FIFO pipe and just print the
> data to a file. To do this, I am using the select() function to block
> until the program
> sees some data on the pipe.  But for some odd reason, the select is
> constantly returning - even when there is no new data in the FIFO
> pipe.  Here's my code if it helps.
>
>
>
> fd_set rfds;
> int retval;
> char *buf;
> size_t n = 0;
> struct timeval tv;
> FILE *output = fopen("out", "a");
> FILE *log = fopen("fifo", "r");
>
> FD_ZERO(&rfds);
> FD_SET(fileno(log), &rfds);
>
> while(1) {
>   tv.tv_sec = 300;
>   tv.tv_usec = 0;
>   retval = select(fileno(log) + 1, &rfds, NULL, NULL, &tv);
>
>   if (retval) {
>     while(getline(&buf, &n, log) != -1) {
>       fprintf(out, "%s", buf);
>     }
>   }
> }
>
> Any suggestions appreciated.

Several problems:

1) You're trying to use select() and also stdio on the same stream.
This is very tricky because of stdio's buffering.

2) You have to do FD_SET() each time through the loop, since select()
modifies rfds.

3) You're not checking the return value of select() for errors.  -1 will
be treated as true, so you'll go into the getline() loop.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***