Hello,
I'm trying to understand how pipes work when terminating a forked process. It
seems when sending kill to the forked process, connected pipes don't always
break. At least a subsequent write might work. Is it possible to make it
reliably fail anyway or is it necessary to close the file descriptors from the
calling process?
#include <u.h>
#include <libc.h>
#include <stdio.h>
void sendnote(int cpid) {
int fd;
char fn[20], buf[20];
sprintf(fn, "/proc/%d/note", cpid);
if ((fd = open(fn, OWRITE)) <= 0) {
printf("open %s\n", fd);
}
if (write(fd, "kill", 5) == 0) {
printf("write error\n");
}
close(fd);
return;
}
int main() {
int cpid, infd[2], outfd[2], n;
char outbuf[20], inbuf[20];
char *args[] = {"/bin/cat", NULL};
pipe(infd);
pipe(outfd);
if ((cpid = fork()) != 0) {
close(infd[1]);
close(outfd[1]);
n = write(infd[0], "test", 4);
printf("check process: wrote %d bytes\n", n);
sendnote(cpid);
// close(infd[0]); // <--- uncomment to make write fail
// close(outfd[0]);
n = write(infd[0], "test2", 5);
printf("write: wrote %d bytes\n", n);
n = read(outfd[0], outbuf, 20);
printf("outbuf=%s (n=%d)\n", outbuf, n);
wait();
return 0;
}
dup(infd[1], 0);
close(infd[0]);
close(infd[1]);
dup(outfd[1], 1);
close(outfd[0]);
close(outfd[1]);
exec("/bin/cat", args);
return 0;
}
Will output:
check process: wrote 4 bytes
write: wrote 5 bytes
outbuf=test (n=4)
6.out 82435: main
Closing the FDs after sending the note:
check process: wrote 4 bytes
write: wrote -1 bytes
outbuf= (n=-1)
6.out 82423: main
On the other hand instead of closing just adding sleep(1) after sendnote()
produces:
check process: wrote 4 bytes
6.out 82595: sys: write on closed pipe pc=0x202603
(That's more or less also reproducing a part of Go's os/exec TestContextCancel)
Greetings, Philip
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