Date: Tue, 27 Feb 2001 16:26:23 -0500 (EST) From: Peter Dufault <dufault@hda.hda.com> To: Marc W <mwlist@lanfear.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: Where can I find out rules on blocking in threads? Message-ID: <200102272126.f1RLQNH24622@hda.hda.com> In-Reply-To: <200102272051.MAA42206@akira.lanfear.com> from Marc W at "Feb 27, 2001 12:51:11 pm"
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> it gets worse -- when i change my loop to be:
>
> while (1) {
>
> fifo = open(fifoPath, O_RDONLY | O_NONBLOCK);
> cread = read(fifo, buf, sizeof(buf));
> if (cread > 0) do_something();
> close(fifo);
> usleep(100000);
> }
>
> anybody who tries to open the write end of the fifo ends up hanging
> ..
It's a little messier than I hoped. As long as no one has opened the
FIFO for write read returns a zero with errno clear. If you don't have
time slicing turned on for the threads you'll hang forever.
You'll have to do something like this:
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <stdio.h>
struct threadstuff {
int rfid, wfid;
const char *name;
};
static void *
reader(void *arg) {
int flags;
ssize_t r;
char buff[32];
int cr;
struct threadstuff *pts = (struct threadstuff *)arg;
fprintf(stderr, "Opening %s for read, non-blocking...\n", pts->name);
pts->rfid = open(pts->name, O_RDONLY | O_NONBLOCK);
fprintf(stderr, "reader got fid %d, setting to blocking...\n", pts->rfid);
(void)fcntl(pts->rfid, F_GETFL, &flags);
flags &= ~O_NONBLOCK;
(void)fcntl(pts->rfid, F_SETFL, &flags);
fprintf(stderr, "Trying to read something from the FIFO...\n");
cr = 0;
/*
* You better have round robin turned on here or be sure
* the writer is of higher priority or you'll just buzz
* away the CPU.
*/
do {
errno = 0;
r = read(pts->rfid, buff, sizeof(buff) - 1);
if (errno == 0 && r == 0) {
/* No one has the FIFO open for write.
* You should delay here (or preferably synchronize)
* to avoid busy waiting.
*/
cr++;
fprintf(stderr, "%s(%3d) No writer yet.\r",
((cr % 5) == 1) ? "\n" : "", cr);
}
} while (r == 0);
if (r < 0) {
perror("Read of FIFO");
} else {
buff[r] = 0;
fprintf(stderr, "Read \"%s\".\n", buff);
}
return 0;
}
static void *
writer(void *arg) {
char something[] = "Something";
struct threadstuff *pts = (struct threadstuff *)arg;
fprintf(stderr, "Opening %s for write\n", pts->name);
pts->wfid = open(pts->name, O_WRONLY);
fprintf(stderr, "writer got fid %d.\n", pts->wfid);
write(pts->wfid, something, sizeof(something));
return 0;
}
int
main(int ac, char *av[]) {
pthread_t r, w;
void *value;
struct threadstuff ts;
ts.name = "./my_fifo";
ts.rfid = ts.wfid = -1;
(void)unlink(ts.name);
(void)mkfifo(ts.name, 0666);
pthread_create(&r, 0, reader, (void *)&ts);
pthread_create(&w, 0, writer, (void *)&ts);
pthread_join(r, &value);
pthread_join(w, &value);
if (ts.rfid != -1)
(void)close(ts.rfid);
if (ts.wfid != -1)
(void)close(ts.wfid);
return 0;
}
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