Socket Programming

How to find MAC address of a linux network interface using SIOCGIFHWADDR ioctl

Jan 28th

Posted by Neo in C/C++

1 comment

The following C programming example retrieves all network interfaces in a Linux system and their IP addresses and MAC addresses. The name of network interface and IP address are retrieved using SIOCGIFCONF ioctl while the hardware address of each individual interface is found using SIOCGIFHWADDR ioctl.

/* iflist.c */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>

int get_iface_list(struct ifconf *ifconf)
{
  int sock, rval;

  sock = socket(AF_INET,SOCK_DGRAM,0);
  if(sock < 0)
  {
    perror("socket");
    return (-1);
  }

  if((rval = ioctl(sock, SIOCGIFCONF , (char*) ifconf  )) < 0 )
  perror("ioctl(SIOGIFCONF)");

  close(sock);

  return rval; More >

How to make a BSD socket non-blocking – EWOULDBLOCK

Oct 5th

Posted by Neo in C/C++

No comments

Sometimes it is needed to make a socket non-blocking so that I/O requests on the socket don’t block. For a non-blocking socket, when I/O requests like read() and write() return a negative value, the errno should be checked against -EWOULDBLOCK code to find if the I/O request would have blocked.

The following utility function sets non-blocking flag of a socket.

int socketSetNonBlocking(int sockfd)
{
  unsigned int flags;

  flags = fcntl(sockfd. F_GETFL, 0);
  if(flags < 0) return -1;
  if(fcntl(sockfd, F_SETFL, flags|O_NONBLOCK) < 0) return -1;
  return 0;
}

Similarly the following function makes a non-blocking socket a blocking socket.

int socketSetBlocking(int sockfd)
{
  unsigned int More >
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Why my application can’t open more than 1024 or 4096 sockets in Linux

Dec 5th

Posted by Neo in C/C++

No comments

Typically socket “open” call doesn’t fail unless you have missed out closing the previous connections using “close” system call. Typically Linux/Unix sets a maximum limit for the number of open FDs. That means you can’t keep FDs in open state more than certain number. These settings can’t be changed as a normal user of the system.

These limits can be seen using “ulimit -a” command.

[neo@techpulp ~]# ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 31729
max locked memory       (kbytes, -l) 32
max memory More >
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