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
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
