These are bits of code we've found for linux that don't necessarily relate exactly to raspbian on the RPi, but may be useful

Network Config

//Code sourced from: http://www.network-builders.com/programatically-change-ip-address-t58439.html
//The RPi doesn't have a folder "/etc/sysconfig/" so this isn't usable as currently implemented
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
#include <arpa/inet.h>



int SetNetworkConfig(char *ifname,in_addr_t ip,in_addr_t mask,in_addr_t gw);
in_addr_t IP2Broadcast(in_addr_t IP, in_addr_t Mask);
in_addr_t IP2Network(in_addr_t IP, in_addr_t Mask);
in_addr_t ValidateMask(in_addr_t Mask);
int GetIP(char *ifname, in_addr_t *ip, in_addr_t *mask);
static int set_default_gw( int sockfd, in_addr_t gip );
void UpdateIP(const char *ifname, in_addr_t ip, in_addr_t mask, in_addr_t gip);
int GetMacAddress(char *ifname, char *addr);
int SetMacAddress(char *ifname, char *addr);


static const char *l_pszConfigFile = "/etc/sysconfig/network/ifcfg-";


//==============================================================================
// function: SetNetworkConfig
// description: updates the network configuration file
//==============================================================================
int SetNetworkConfig(char *ifname, in_addr_t ip, in_addr_t mask, in_addr_t gw)
{
	char szTemp[256];
	in_addr_t bc,nw;

	mask = ValidateMask(mask);
	nw = IP2Network(ip,mask);
	bc = IP2Broadcast(ip,mask);

	sprintf(szTemp,"%s%s", l_pszConfigFile, ifname);
	printf("open -> %s\n",szTemp);
	FILE *file = fopen(szTemp,"wr");
	if (file == NULL)
	{
		perror("SetNetworkConfig");
		return -1;
	}

	fprintf(file, "IPADDR=\"%s\"\n", inet_ntoa(inet_makeaddr(ip,0)));
	fprintf(file, "NETMASK=\"%s\"\n", inet_ntoa(inet_makeaddr(mask,0)));
	fprintf(file, "NETWORK=\"%s\"\n", inet_ntoa(inet_makeaddr(nw,0)));
	fprintf(file, "BROADCAST=\"%s\"\n", inet_ntoa(inet_makeaddr(bc,0)));
	if ( gw != 0 )
		fprintf(file, "GATEWAY=\"%s\"\n", inet_ntoa(inet_makeaddr(gw,0)));

	fclose(file);
	system("/etc/init.d/network restart");

	return 0;
}

//==============================================================================
// function: IP2Broadcast
// description: Takes a subnet mask and IP returns a subnet broadcast address
//==============================================================================
in_addr_t IP2Broadcast(in_addr_t IP, in_addr_t Mask)
{
	in_addr_t bcIP;
	in_addr_t bit;
	in_addr_t notbit;
	int i;

	bit = 0x80000000;
	notbit = 0xFFFFFFFF;
	bcIP = 0;
	for ( i=0; i<31; i++)
	{
		if ((bit & Mask) == 0)
			break;

		bcIP |= (IP & bit);
		notbit &= ~bit;
		bit = bit >> 1;
	}
	//bcIP != notbit;
	//printf("bcIP = %s\n",inet_ntoa(inet_makeaddr(bcIP,0)));
	return bcIP;
}

//==============================================================================
// function: IP2Network
// description: Takes a subnet mask and IP returns a subnet address
//==============================================================================
in_addr_t IP2Network(in_addr_t IP, in_addr_t Mask)
{
	in_addr_t netIP;
	in_addr_t bit;
	int i;

	bit = 0x80000000;
	netIP = 0;
	for ( i=0; i<31; i++)
	{
		if ((bit & Mask) == 0)
			break;
		netIP |= (IP & bit);
		bit = bit >> 1;
	}
	// printf("Network = %s\n",inet_ntoa(inet_makeaddr(netIP,0)));
	return netIP;
}

//==============================================================================
// function: ValidateMask
// description: Takes a subnet mask and returns a valid version
//==============================================================================
in_addr_t ValidateMask(in_addr_t Mask)
{
	in_addr_t newMask;
	in_addr_t bit;
	int i;

	bit = 0x80000000;
	newMask = 0;
	for ( i=0; i<31; i++)
	{
		if ((bit & Mask) == 0)
			break;
		newMask |= bit;
		bit = bit >> 1;
	}
	// printf("Mask = %s\n",inet_ntoa(inet_makeaddr(Mask,0)));
	return Mask;
}

//==============================================================================
// function: GetLocalIP
// description: retrieve current network ip and mask
//==============================================================================
int GetIP(char *ifname, in_addr_t *ip, in_addr_t *mask)
{
	struct ifreq ifr;
	int sock = socket(AF_INET,SOCK_DGRAM,0);

	// Get the interface IP address
	strcpy( ifr.ifr_name, ifname );
	ifr.ifr_addr.sa_family = AF_INET;

	if (ioctl( sock, SIOCGIFADDR, &ifr ) < 0)
		perror( "SIOCGIFADDR" );

	*ip = ((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr.s_addr;
	shutdown(sock,SHUT_RDWR);

	return 0;
}

//==============================================================================
// function: set_default_gw
// description: sets routing table's default gateway to the sock peeraddr
//==============================================================================
static int set_default_gw( int sockfd, in_addr_t gip )
{
	struct sockaddr_in *dst, *gw, *mask;
	struct rtentry route;

	memset(&route,0,sizeof(struct rtentry));

	dst = (struct sockaddr_in *)(&(route.rt_dst));
	gw = (struct sockaddr_in *)(&(route.rt_gateway));
	mask = (struct sockaddr_in *)(&(route.rt_genmask));

	/* Make sure we're talking about IP here */
	dst->sin_family = AF_INET;
	gw->sin_family = AF_INET;
	mask->sin_family = AF_INET;

	/* Set up the data for removing the default route */
	dst->sin_addr.s_addr = 0;
	gw->sin_addr.s_addr = 0;
	mask->sin_addr.s_addr = 0;
	route.rt_flags = RTF_UP | RTF_GATEWAY;

	/* Remove the default route */
	ioctl(sockfd,SIOCDELRT,&route);

	/* Set up the data for adding the default route */
	dst->sin_addr.s_addr = 0;
	gw->sin_addr.s_addr = gip;
	mask->sin_addr.s_addr = 0;
	route.rt_metric = 1;
	route.rt_flags = RTF_UP | RTF_GATEWAY;

	/* Remove this route if it already exists */
	ioctl(sockfd,SIOCDELRT,&route);

	/* Add the default route */
	if( ioctl(sockfd,SIOCADDRT,&route) == -1 )
	{
		fprintf( stderr,"Adding default route: %d", errno);
		return -1;
	}

	//fprintf( stdout,"Added default route successfully." );
	return 0;
}

//==============================================================================
// function: UpdateIP
// description: updates the networks parameters
//==============================================================================
void UpdateIP(const char *ifname, in_addr_t ip, in_addr_t mask, in_addr_t gip)
{
	struct ifreq ifr;
	char *myip;
	int sock = socket(AF_INET,SOCK_DGRAM,0);
	in_addr_t IP;
	struct rtentry rt;
	struct sockaddr_in sa;
	memset(&sa, 0, sizeof(sa));
	memset(&rt, 0, sizeof(rt));

	// Get the interface IP address
	strcpy( ifr.ifr_name, ifname );
	ifr.ifr_addr.sa_family = AF_INET;

	if (ioctl( sock, SIOCGIFADDR, &ifr ) < 0)
		perror( "SIOCGIFADDR" );

	myip = inet_ntoa( ((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr );

	IP = ((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr.s_addr;
	((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr.s_addr = ip;
	if (ioctl( sock, SIOCSIFADDR, &ifr ) < 0)
		perror( "SIOCSIFADDR" );

	set_default_gw(sock, gip);
	shutdown(sock,SHUT_RDWR);
}

//==============================================================================
// function: GetMacAddress
// description: retrieve current network address
//==============================================================================
int GetMacAddress(char *ifname, char *addr)
{
	struct ifreq ifr;
	int sock = socket(AF_INET,SOCK_DGRAM,0);

	// Get the interface IP address
	strcpy( ifr.ifr_name, ifname );
	ifr.ifr_addr.sa_family = AF_INET;

	if (ioctl( sock, SIOCGIFHWADDR, &ifr ) < 0)
	{
		perror( "SIOCGHWADDR" );
		return -1;
	}
	memcpy(addr,ifr.ifr_hwaddr.sa_data,6);
	//uint8_t *hwaddr = (uint8_t*)addr;
	//printf("The hardware address (SIOCGIFHWADDR) of %s is type %d "
	// "%2.2x:%2.2x:%2.2x:%2.2x:%2.2x:%2.2x.\n", ifname,
	// ifr.ifr_hwaddr.sa_family, hwaddr[0], hwaddr[1],
	// hwaddr[2], hwaddr[3], hwaddr[4], hwaddr[5]);
	shutdown(sock,SHUT_RDWR);
	return 0;
}

//==============================================================================
// function: SetMacAddress
// description: Set network address
//==============================================================================
int SetMacAddress(char *ifname, char *addr)
{
	struct ifreq ifr;
	int sock = socket(AF_INET,SOCK_DGRAM,0);

	// Set the interface IP address
	strcpy( ifr.ifr_name, ifname );
	ifr.ifr_addr.sa_family = AF_UNIX;
	memcpy(ifr.ifr_hwaddr.sa_data,addr,6);

	if (ioctl( sock, SIOCSIFHWADDR, &ifr ) < 0)
	{
		perror( "SIOCSIFHWADDR" );
		return -1;
	}
	shutdown(sock,SHUT_RDWR);
	return 0;
}