I'm trying to experiment with using splice (man 2 splice) to copy data from a UDP socket directly to a file. Unfortunately the first call to splice() returns EINVAL.
The man page states:
EINVAL Target file system doesn't support splicing; target file is opened in
append mode; neither of the descriptors refers to a pipe; or offset
given for nonseekable device.
However, I believe none of those conditions apply. I'm using Fedora 15 (kernel 2.6.40-4) so I believe splice() is supported on all filesystems. The target file should be irrelevant in the first call to splice, but for completeness I'm opening it via open(path, O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR). Both calls use a pipe and neither call uses an offset besides NULL.
Here's my sample code:
int sz = splice(sock_fd, 0, mPipeFds[1], 0, 8192, SPLICE_F_MORE);
if (-1 == sz)
{
int err = errno;
LOG4CXX_ERROR(spLogger, "splice from: " << strerror(err));
return 0;
}
sz = splice(mPipeFds[0], 0, file_fd, 0, sz, SPLICE_F_MORE);
if (-1 == sz)
{
int err = errno;
LOG4CXX_ERROR(spLogger, "splice to: " << strerror(err));
}
return 0;
sock_fd is initialized by the following psuedocode:
int sock_fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
fcntl(sock_fd, F_SETFL, flags | O_NONBLOCK);
bind(sock_fd, ...);
Possibly related is that this code snippet is running inside a libevent loop. libevent is using epoll() to determine if the UDP socket is hot.
Found my answer. tl;dr - UDP isn't supported on the inbound side.
After enough Googling I stumbled upon a forum discussion and some test code which prints out a table of in/out fd types and their support:
$ ./a.out
in\out pipe reg chr unix tcp udp
pipe yes yes yes yes yes yes
reg yes no no no no no
chr yes no no no no no
unix no no no no no no
tcp yes no no no no no
udp no no no no no no