recv with non-blocking socket - c

Question

I am trying to implement non-blocking for socket recv and the problem is that I got an error -1 when there in no data but I expect to get EAGAIN error.
Socket is set definitely to non-blocking state, I checked flags = fcntl(s, F_GETFL, 0) for O_NONBLOCK flag.
Thanks a lot in advance!
#include <arpa/inet.h>
#include <linux/if_packet.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
#include <netinet/ether.h>
#include <unistd.h>
#include <fcntl.h>
#include <asm-generic/errno-base.h>
#include <assert.h>
#define ETH_FRAME_LEN_MY 1400
void print(void *buf, int length)
{
int i;
for (i = 0; i < length; ++i)
putchar(((char *)buf)[i]);
printf("\n");
}
int main(){
int flags, s, r, err;
struct sockaddr_ll socket_addr;
char ifName[IFNAMSIZ] = "eth0";
struct ifreq if_idx;
struct ifreq if_mac;
s = socket(PF_PACKET, SOCK_RAW, htons(0x88b6));
if (s == -1) { perror("socket"); }
flags = fcntl(s,F_GETFL,0);
assert(flags != -1);
fcntl(s, F_SETFL, flags | O_NONBLOCK);
flags = fcntl(s, F_GETFL, 0);
if ((flags & O_NONBLOCK) == O_NONBLOCK) {
printf("it's nonblocking");
}
else {
printf("it's blocking.");
}
/* Get the index of the interface to send on */
memset(&if_idx, 0, sizeof(struct ifreq));
strncpy(if_idx.ifr_name, ifName, IFNAMSIZ-1);
if (ioctl(s, SIOCGIFINDEX, &if_idx) < 0)
{perror("SIOCGIFINDEX");}
memset(&socket_addr, 0, sizeof(socket_addr));
socket_addr.sll_ifindex = if_idx.ifr_ifindex;
socket_addr.sll_protocol = htons(0x88b5);
socket_addr.sll_family = PF_PACKET;
socket_addr.sll_pkttype = PACKET_OUTGOING;
r = bind(s, (struct sockaddr*)&socket_addr,
sizeof(socket_addr));
if ( r < 0) { perror("bind"); }
void* buffer = (void*)malloc(ETH_FRAME_LEN_MY); /*Buffer for ethernet frame*/
int length = 0; /*length of the received frame*/
while(1){
printf("1\n");
length = recv(s, buffer, ETH_FRAME_LEN_MY, 0);
printf("2\n");
if (length < 0)
{
if (length == -EAGAIN)
printf("non-blocking succeeded\n");
else printf("error code %i\n", length);
}
//printf ("buffer %s\n", buffer);
print(buffer, length);
}
return 0;
}

Answer 1

I got an error -1 when there in no data but I expect to get EAGAIN error.
-1 tells you there is an error. errno tells you what the error was.

Answer 2

You need to check errno, not the length value to get the reason why the socket failed to return data. Also, EWOULDBLOCK is the other error code you should check for in addition to EAGAIN. Change your while loop as follows:
while(1)
{
int err;
printf("1\n");
length = recv(s, buffer, ETH_FRAME_LEN_MY, 0);
err = errno; // save off errno, because because the printf statement might reset it
printf("2\n");
if (length < 0)
{
if ((err == EAGAIN) || (err == EWOULDBLOCK))
{
printf("non-blocking operation returned EAGAIN or EWOULDBLOCK\n");
}
else
{
printf("recv returned unrecoverable error(errno=%d)\n", err);
break;
}
}
//printf ("buffer %s\n", buffer);
print(buffer, length);
}
Of course this infinite loop will get into a CPU wasting busy cycle while it's waiting for data. There are a variety of ways to be notified of data arriving on a socket without having to call recv() in a spin loop. This includes select and poll calls.