SPI on the raspberry pi with http://www.airspayce.com/mikem/bcm2835/index.html

spi.c

// spi.c
//
// Example program for bcm2835 library
// Shows how to interface with SPI to transfer a byte to and from an SPI device
//
// After installing bcm2835, you can build this 
// with something like:
// gcc -o spi spi.c -l bcm2835
// sudo ./spi
//
// Or you can test it before installing with:
// gcc -o spi -I ../../src ../../src/bcm2835.c spi.c
// sudo ./spi
//
#include <bcm2835.h>
#include <stdio.h>
int main(int argc, char **argv)
{
    if (!bcm2835_init())
    {
      printf("bcm2835_init failed. Are you running as root??\n");
      return 1;
    }
    if (!bcm2835_spi_begin())
    {
      printf("bcm2835_spi_begin failed. Are you running as root??\n");
      return 1;
    }
    bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST);      // The default
    bcm2835_spi_setDataMode(BCM2835_SPI_MODE0);                   // The default
    bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_65536); // The default
    bcm2835_spi_chipSelect(BCM2835_SPI_CS0);                      // The default
    bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW);      // the default

    uint8_t send_data = 3;

    while(1) {
    // Send a byte to the slave and simultaneously read a byte back from the slave
    // If you tie MISO to MOSI, you should read back what was sent
      bcm2835_delay(1000);
      send_data++;
      uint8_t read_data = bcm2835_spi_transfer(send_data);
      printf("Sent to SPI: %u. Read back from SPI: %u.\n", send_data, read_data);
    }
    bcm2835_spi_end();
    bcm2835_close();
    return 0;
}