We will start with a generic tuning subroutine that uses the frontend and SEC, as well as the demux devices. The example is given for QPSK tuners, but can easily be adjusted for QAM.

 #include <sys/ioctl.h>
 #include <stdio.h>
 #include <stdint.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <time.h>
 #include <unistd.h>

 #include <linux/dvb/dmx.h>
 #include <linux/dvb/frontend.h>
 #include <linux/dvb/sec.h>
 #include <sys/poll.h>

 #define DMX "/dev/dvb/adapter0/demux1"
 #define FRONT "/dev/dvb/adapter0/frontend1"
 #define SEC "/dev/dvb/adapter0/sec1"

 /⋆ routine for checking if we have a signal and other status information⋆/
 int FEReadStatus(int fd, fe_status_t ⋆stat)
 {
	 int ans;

	 if ( (ans = ioctl(fd,FE_READ_STATUS,stat) < 0)){
		 perror("FE READ STATUS: ");
		 return -1;
	 }

	 if (⋆stat & FE_HAS_POWER)
		 printf("FE HAS POWER\n");

	 if (⋆stat & FE_HAS_SIGNAL)
		 printf("FE HAS SIGNAL\n");

	 if (⋆stat & FE_SPECTRUM_INV)
		 printf("SPEKTRUM INV\n");

	 return 0;
 }


 /⋆ tune qpsk ⋆/
 /⋆ freq:             frequency of transponder                      ⋆/
 /⋆ vpid, apid, tpid: PIDs of video, audio and teletext TS packets  ⋆/
 /⋆ diseqc:           DiSEqC address of the used LNB                ⋆/
 /⋆ pol:              Polarisation                                  ⋆/
 /⋆ srate:            Symbol Rate                                   ⋆/
 /⋆ fec.              FEC                                           ⋆/
 /⋆ lnb_lof1:         local frequency of lower LNB band             ⋆/
 /⋆ lnb_lof2:         local frequency of upper LNB band             ⋆/
 /⋆ lnb_slof:         switch frequency of LNB                       ⋆/

 int set_qpsk_channel(int freq, int vpid, int apid, int tpid,
		 int diseqc, int pol, int srate, int fec, int lnb_lof1,
		 int lnb_lof2, int lnb_slof)
 {
	 struct secCommand scmd;
	 struct secCmdSequence scmds;
	 struct dmx_pes_filter_params pesFilterParams;
	 FrontendParameters frp;
	 struct pollfd pfd[1];
	 FrontendEvent event;
	 int demux1, demux2, demux3, front;

	 frequency = (uint32_t) freq;
	 symbolrate = (uint32_t) srate;

	 if((front = open(FRONT,O_RDWR)) < 0){
		 perror("FRONTEND DEVICE: ");
		 return -1;
	 }

	 if((sec = open(SEC,O_RDWR)) < 0){
		 perror("SEC DEVICE: ");
		 return -1;
	 }

	 if (demux1 < 0){
		 if ((demux1=open(DMX, O_RDWR|O_NONBLOCK))
		     < 0){
			 perror("DEMUX DEVICE: ");
			 return -1;
		 }
	 }

	 if (demux2 < 0){
		 if ((demux2=open(DMX, O_RDWR|O_NONBLOCK))
		     < 0){
			 perror("DEMUX DEVICE: ");
			 return -1;
		 }
	 }

	 if (demux3 < 0){
		 if ((demux3=open(DMX, O_RDWR|O_NONBLOCK))
		     < 0){
			 perror("DEMUX DEVICE: ");
			 return -1;
		 }
	 }

	 if (freq < lnb_slof) {
		 frp.Frequency = (freq - lnb_lof1);
		 scmds.continuousTone = SEC_TONE_OFF;
	 } else {
		 frp.Frequency = (freq - lnb_lof2);
		 scmds.continuousTone = SEC_TONE_ON;
	 }
	 frp.Inversion = INVERSION_AUTO;
	 if (pol) scmds.voltage = SEC_VOLTAGE_18;
	 else scmds.voltage = SEC_VOLTAGE_13;

	 scmd.type=0;
	 scmd.u.diseqc.addr=0x10;
	 scmd.u.diseqc.cmd=0x38;
	 scmd.u.diseqc.numParams=1;
	 scmd.u.diseqc.params[0] = 0xF0 | ((diseqc ⋆ 4) & 0x0F) |
		 (scmds.continuousTone == SEC_TONE_ON ? 1 : 0) |
		 (scmds.voltage==SEC_VOLTAGE_18 ? 2 : 0);

	 scmds.miniCommand=SEC_MINI_NONE;
	 scmds.numCommands=1;
	 scmds.commands=&scmd;
	 if (ioctl(sec, SEC_SEND_SEQUENCE, &scmds) < 0){
		 perror("SEC SEND: ");
		 return -1;
	 }

	 if (ioctl(sec, SEC_SEND_SEQUENCE, &scmds) < 0){
		 perror("SEC SEND: ");
		 return -1;
	 }

	 frp.u.qpsk.SymbolRate = srate;
	 frp.u.qpsk.FEC_inner = fec;

	 if (ioctl(front, FE_SET_FRONTEND, &frp) < 0){
		 perror("QPSK TUNE: ");
		 return -1;
	 }

	 pfd[0].fd = front;
	 pfd[0].events = POLLIN;

	 if (poll(pfd,1,3000)){
		 if (pfd[0].revents & POLLIN){
			 printf("Getting QPSK event\n");
			 if ( ioctl(front, FE_GET_EVENT, &event)

			      == -EOVERFLOW){
				 perror("qpsk get event");
				 return -1;
			 }
			 printf("Received ");
			 switch(event.type){
			 case FE_UNEXPECTED_EV:
				 printf("unexpected event\n");
				 return -1;
			 case FE_FAILURE_EV:
				 printf("failure event\n");
				 return -1;

			 case FE_COMPLETION_EV:
				 printf("completion event\n");
			 }
		 }
	 }


	 pesFilterParams.pid     = vpid;
	 pesFilterParams.input   = DMX_IN_FRONTEND;
	 pesFilterParams.output  = DMX_OUT_DECODER;
	 pesFilterParams.pes_type = DMX_PES_VIDEO;
	 pesFilterParams.flags   = DMX_IMMEDIATE_START;
	 if (ioctl(demux1, DMX_SET_PES_FILTER, &pesFilterParams) < 0){
		 perror("set_vpid");
		 return -1;
	 }

	 pesFilterParams.pid     = apid;
	 pesFilterParams.input   = DMX_IN_FRONTEND;
	 pesFilterParams.output  = DMX_OUT_DECODER;
	 pesFilterParams.pes_type = DMX_PES_AUDIO;
	 pesFilterParams.flags   = DMX_IMMEDIATE_START;
	 if (ioctl(demux2, DMX_SET_PES_FILTER, &pesFilterParams) < 0){
		 perror("set_apid");
		 return -1;
	 }

	 pesFilterParams.pid     = tpid;
	 pesFilterParams.input   = DMX_IN_FRONTEND;
	 pesFilterParams.output  = DMX_OUT_DECODER;
	 pesFilterParams.pes_type = DMX_PES_TELETEXT;
	 pesFilterParams.flags   = DMX_IMMEDIATE_START;
	 if (ioctl(demux3, DMX_SET_PES_FILTER, &pesFilterParams) < 0){
		 perror("set_tpid");
		 return -1;
	 }

	 return has_signal(fds);
 }

The program assumes that you are using a universal LNB and a standard DiSEqC switch with up to 4 addresses. Of course, you could build in some more checking if tuning was successful and maybe try to repeat the tuning process. Depending on the external hardware, i.e. LNB and DiSEqC switch, and weather conditions this may be necessary.