Is the behavior described below ( "read() returns 0 unless stat() detects the
growth of file size" and "read() returns non-0 when stat() detects the growth
of file size but the data actually read is '\0'" ) a feature of cache=loose
option?
Steps to reproduce:
---------- Start of writer1.c ----------
#include <unistd.h>
int main(int argc, char *argv[])
{
while (write(1, ".", 1) == 1)
sleep(1);
return 0;
}
---------- End of writer1.c ----------
---------- Start of reader1.c ----------
#include <unistd.h>
int main(int argc, char *argv[])
{
while (1) {
char c;
if (read(0, &c, 1) == 1) {
if (c)
write(1, &c, 1);
else
write(1, "!", 1);
} else {
sleep(1);
}
}
return 0;
}
---------- End of reader1.c ----------
---------- Start of reader2.c ----------
#include <sys/stat.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
while (1) {
struct stat buf;
char c;
fstat(0, &buf);
if (read(0, &c, 1) == 1) {
if (c)
write(1, &c, 1);
else
write(1, "!", 1);
} else {
sleep(1);
}
}
return 0;
}
---------- End of reader2.c ----------
Run the following commands on the Samba server side.
$ cc -Wall -O3 -o writer1 writer1.c
$ ./writer1 > shared_file
Run the following commands on the Samba client side.
$ cc -Wall -O3 -o reader1 reader1.c
$ strace -tt -o /tmp/log1 ./reader1 < shared_file
.......... (<= Makes no progress by default)
The shared_file shown above is a regular file shared via CIFS mounted with
cache=loose option. (e.g. "mount -t cifs -o cache=loose //server/path /mnt" )
I expect that the reader1 process reads one byte whenever the writer1
process writes one byte. But we can observe via /tmp/log1 that the read()
continues returning 0 until the growth of file size of shared_file is
recognized by stat() using programs like /bin/ls and /usr/bin/stat .
Then, terminate the reader1 process using Ctrl-C and run the following
commands on the Samba client side.
$ cc -Wall -O3 -o reader2 reader2.c
$ strace -tt -o /tmp/log2 ./reader2 < shared_file
......................!!!!!! (<= Makes progress with wrong data)
We can observe via /tmp/log2 that the read() returns 1 because fstat()
detects the growth of file size of shared_file . However, the data
returned by read() is not '.' but '\0'.
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