ping
Eric Wong <normalperson@xxxxxxxx> wrote:
> The following program illustrates the difference between TCP and Unix
> stream sockets doing sendfile. Since TCP implements zero-copy, the new
> modifications to the file tranferred is seen upon reading despite
> the modifications happening after sendfile was last called.
>
> Unix stream sockets do not implement zero-copy (as of Linux 3.15),
> so readers continue to see the contents of the file at the time it
> was sent, not as they are at the time of reading.
>
> ----------------- sendfile-mod.c ---------------
> #define _GNU_SOURCE
> #include <sys/ioctl.h>
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <sys/sendfile.h>
> #include <arpa/inet.h>
> #include <stdio.h>
> #include <errno.h>
> #include <string.h>
> #include <unistd.h>
> #include <assert.h>
> #include <fcntl.h>
>
> static void tcp_socketpair(int sv[2])
> {
> struct sockaddr_in addr;
> socklen_t addrlen = sizeof(addr);
> int l = socket(PF_INET, SOCK_STREAM, 0);
> int c = socket(PF_INET, SOCK_STREAM, 0);
> int a;
> int val = 1;
>
> addr.sin_family = AF_INET;
> addr.sin_addr.s_addr = INADDR_ANY;
> addr.sin_port = 0;
> assert(0 == bind(l, (struct sockaddr*)&addr, addrlen));
> assert(0 == listen(l, 1024));
> assert(0 == getsockname(l, (struct sockaddr *)&addr, &addrlen));
> assert(0 == connect(c, (struct sockaddr *)&addr, addrlen));
> a = accept4(l, NULL, NULL, SOCK_NONBLOCK);
> assert(a >= 0);
> close(l);
> assert(0 == ioctl(c, FIONBIO, &val));
> sv[0] = a;
> sv[1] = c;
> }
>
> int main(int argc, char *argv[])
> {
> int pair[2];
> FILE *tmp = tmpfile();
> int tfd;
> char buf[16384];
> ssize_t w, r;
> size_t i;
> const size_t n = 2048;
> off_t off = 0;
> char expect[4096];
> int flags = SOCK_STREAM|SOCK_NONBLOCK;
>
> tfd = fileno(tmp);
> assert(tfd >= 0);
>
> /* prepare the tempfile */
> memset(buf, 'a', sizeof(buf));
> for (i = 0; i < n; i++)
> assert(sizeof(buf) == write(tfd, buf, sizeof(buf)));
>
> if (argc == 2 && strcmp(argv[1], "unix") == 0)
> assert(0 == socketpair(AF_UNIX, flags, 0, pair));
> else if (argc == 2 && strcmp(argv[1], "pipe") == 0)
> assert(0 == pipe2(pair, O_NONBLOCK));
> else
> tcp_socketpair(pair);
>
> /* fill up the socket buffer */
> for (;;) {
> w = sendfile(pair[1], tfd, &off, n);
> if (w > 0)
> continue;
> if (w < 0 && errno == EAGAIN)
> break;
> assert(0 && "unhandled error" && w && errno);
> }
> printf("wrote off=%lld\n", (long long)off);
>
> /* rewrite the tempfile */
> memset(buf, 'A', sizeof(buf));
> assert(0 == lseek(tfd, 0, SEEK_SET));
> for (i = 0; i < n; i++)
> assert(sizeof(buf) == write(tfd, buf, sizeof(buf)));
>
> /* we should be reading 'a's, not 'A's */
> memset(expect, 'a', sizeof(expect));
> do {
> r = read(pair[0], buf, sizeof(expect));
>
> /* TCP fails here since it is zero copy (on Linux 3.15.5) */
> if (r > 0)
> assert(memcmp(buf, expect, r) == 0);
> } while (r > 0);
>
> return 0;
> }
>
> Signed-off-by: Eric Wong <normalperson@xxxxxxxx>
> ---
> One of my applications turned out to be buggy due to my failure to
> realize this in time. Hopefully this note prevents others from
> suffering the same fate.
>
> I think splice.2 and tee.2 will need similar notes
> (not sure if my current wording here is good enough,
> I am not comfortable writing documentation :x)
>
> man2/sendfile.2 | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/man2/sendfile.2 b/man2/sendfile.2
> index 6e9ec42..fcc2724 100644
> --- a/man2/sendfile.2
> +++ b/man2/sendfile.2
> @@ -190,6 +190,15 @@ fails with
> or
> .BR ENOSYS .
>
> +If
> +.I out_fd
> +refers to a socket or pipe with zero-copy support, callers must ensure the
> +transferred portions of the file referred to by
> +.I in_fd
> +remain unmodified until the reader on the other end of
> +.I out_fd
> +has consumed the transferred data.
> +
> The Linux-specific
> .BR splice (2)
> call supports transferring data between arbitrary files
> --
> EW
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