what could limit bytes read in from read() - sockets

Question

I am reading bytes off a socket initialised like this:
fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
However when I read from this socket
char buf[ETH_FRAME_LEN]
len = read(fd, buf, sizeof(buf));
len shows only 1500 bytes were read. I checked with wireshark and the packet returned is 5854. The total length field under IP says 5840 (so + 14 bytes for ethernet header = 5854). I tried using a larger buffer (6000) but still only 1500 bytes were being read off the wire.
I tried requesting a smaller file from the server (1504 bytes), but I get the same results. As it is a raw socket, the data read in includes the ethernet headers, so it is not reading the last 4 bytes into the buffer.
What could be the cause of this? I'm not aware of any argument to socket() that could cause this.

Answer 1

What happens if you try calling read again? Is the next chunk of the message quickly returned?
From the read man page (my emphasis)
read() attempts to read up to count bytes
If you want to read a certain number of bytes, you should be prepared to call read in a loop until you receive your target total cumulatively over the calls.

Answer 2

What is happening is that you're getting exactly one Ethernet MTU's worth of payload per call to read().

Answer 3

read() returns:
On success, the number of bytes read is returned (zero indicates end of
file), and the file position is advanced by this number. It is not an
error if this number is smaller than the number of bytes requested;
this may happen for example because fewer bytes are actually available
right now (maybe because we were close to end-of-file, or because we
are reading from a pipe, or from a terminal), or because read() was
interrupted by a signal. On error, -1 is returned, and errno is set
appropriately. In this case it is left unspecified whether the file
position (if any) changes.
You can try to use recv() with MSG_WAITALL instead of pure read():
This flag requests that the operation block until the full
request is satisfied. However, the call may still return less
data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different
type than that returned.
len = recv(fd, buf, sizeof(buf), MSG_WAITALL);
Another way is to read or recv in a loop like:
ssize_t Recv(int fd, void* buf, ssize_t n)
{
ssize_t read = 0;
ssize_t r;
while(read != n)
{
r = recv(fd, ((char*)buf)+read, n-read, 0);
if(r == -1)
return (read) ? read : -1;
if(r == 0)
return 0;
read += r;
}
return read;
}