I understand I can redirect to null in comand line or script. However, the message is being displayed during library call to pcap_open_live() in the libpcap library.
Anyways, I have found the printk statement in the kernel file /net/core/dev.c., function dev_set_promiscuity(). So, I just commented it out and rebuilt the kernel for my embedded system.

I created a thread to continually sniff certain network packets at the datalink layer every few seconds using libpcap. eth0 is being opened in promiscuous mode. I do not want to leave the pcap object open, so I am calling pcap_open_live() and pcap_close() each time in the loop. This results in

entered promiscuous mode

left promiscuous mode

#include <errno.h>
#include <linux/ethtool.h>
#include <linux/sockios.h>
#include <net/if.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>

int main ( int argc, char **argv )
{
struct ifreq ifr;
int sock = socket ( AF_INET, SOCK_DGRAM, 0 );
struct ethtool_cmd edata;
char ifname[32];
int result;

strcpy ( ifname, "eth0" );
strncpy ( ifr.ifr_name, ifname, sizeof ( ifr.ifr_name ));
ifr.ifr_addr.sa_family = AF_INET;

result = ioctl ( sock, SIOCGIFFLAGS, &ifr );
if ( result == 0 ) {
// successfully got the flags
if ( ifr.ifr_flags | IFF_UP ) {
printf ( "Ethernet interface is up\n" );
}
else {
printf ( "Ethernet interface is down\n" );
}
}
else {
printf ( "%s: Error getting ethernet flags. %s (%d)\n",
__FUNCTION__,
strerror ( errno ),
errno );
}

ifr.ifr_data = (char *) &edata;
edata.cmd = ETHTOOL_GSET;
result = ioctl ( sock, SIOCETHTOOL, &ifr );
if ( result == 0 ) {
switch ( edata.speed ) {
case SPEED_10:
printf ( "Speed is 10Mbps\n" );
break;
case SPEED_100:
printf ( "Speed is 100Mbps\n" );
break;
case SPEED_1000:
printf ( "Speed is 1Gbps\n" );
break;
case SPEED_10000:
printf ( "Speed is 10Gbps\n" );
break;
default:
printf ( "Speed (%d) is unknown\n", edata.speed );
break;
}

if ( edata.duplex == DUPLEX_HALF ){
printf ( "Half duplex\n" );
}
else {
printf ( "Full duplex\n" );
}
}
else {
printf ( "%s: Error getting ethernet info. %s (%d)\n",
__FUNCTION__,
strerror ( errno ),
errno );
}
return 0;
}

In C code, I'm trying to retrieve the emac link status, speed, and duplex. I made a system call to "cat /proc/net/emac_link" and parsed the output. But this takes WAY too much time (around 75ms). Is there a faster (and better) way to get this information, say, through a driver IOCTL?