I'm a little confused about the behavior of select().
It seems to behave differently in Cygwin than on other
systems. This is specifically regarding the exception
fd_set when piping program output to another program.
Here's an example (which might have a problem or two
but should demonstrate sufficiently):
<code_sample>
#include <sys/types.h>
#include <sys/time.h>
#include <fcntl.h>
#include <stdio.h>
#define TIMEOUT 1
int openFile(char *);
int
main(int argc, char **argv)
{
fd_set readfds, writefds, exceptfds;
int n, fd, so, se, f;
struct timeval tv;
fd = openFile("select.c");
FD_ZERO(&readfds);
FD_ZERO(&writefds);
FD_ZERO(&exceptfds);
FD_SET(fd, &readfds);
FD_SET(fd, &exceptfds);
FD_SET(1, &writefds);
FD_SET(2, &writefds);
FD_SET(1, &exceptfds);
FD_SET(2, &exceptfds);
tv.tv_sec = TIMEOUT;
tv.tv_usec = 0;
n = select(3, &readfds, &writefds, &exceptfds,
&tv);
switch (n) {
case -1:
perror("select");
exit(1);
case 0:
printf("\nTimeout expired. Type
something!\n");
break;
default:
f = FD_ISSET(fd, &readfds);
so = FD_ISSET(1, &readfds),
se = FD_ISSET(2, &readfds);
printf("READ - file: %d, stdout: %d, stderr: %d\n",
f, so, se);
f = FD_ISSET(fd, &writefds),
so = FD_ISSET(1, &writefds),
se = FD_ISSET(2, &writefds);
printf("WRITE - file: %d, stdout: %d, stderr: %d\n",
f, so, se);
f = FD_ISSET(fd, &exceptfds),
so = FD_ISSET(1, &exceptfds),
se = FD_ISSET(2, &exceptfds);
printf("EXCEPT - file: %d, stdout: %d, stderr:
%d\n", f, so, se);
break;
}
}
int openFile(char *file)
{ int fd;
fd = -1;
if ((fd = open(file, O_RDONLY)) < 0) {
perror(file);
exit(1);
}
return fd;
}
</code_sample>
<output_on_cygwin>
$ ./a.exe
READ - file: 0, stdout: 0, stderr: 0
WRITE - file: 0, stdout: 2, stderr: 4
EXCEPT - file: 0, stdout: 0, stderr: 0
$ ./a.exe |cat
READ - file: 0, stdout: 0, stderr: 0
WRITE - file: 0, stdout: 2, stderr: 4
EXCEPT - file: 0, stdout: 2, stderr: 0
</output_on_cygwin>
<output_on_linux>
cef@yawmp:~$ ./a.out
READ - file: 0, stdout: 0, stderr: 0
WRITE - file: 0, stdout: 1, stderr: 1
EXCEPT - file: 0, stdout: 0, stderr: 0
cef@yawmp:~$ ./a.out |cat
READ - file: 0, stdout: 0, stderr: 0
WRITE - file: 0, stdout: 1, stderr: 1
EXCEPT - file: 0, stdout: 0, stderr: 0
</output_on_linux>
Notice the "EXCEPT" lines. AFAIK, the behavior on
Cygwin is incorrect, though I'm *far* from being an
expert.
Any help would be greatly appreciated.
Thanks,
Chad Fowler
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