C / C++ / MFC

static volatile uint32_t *gpio;
int fd ;
// Try Obtain handle to physical memory
if ((fd = open ("/dev/mem", O_RDWR | O_SYNC) ) < 0) {
    if ((fd = open ("/dev/gpiomem", O_RDWR | O_SYNC) ) < 0){    
       printf("Unable to open physical memory handle: %s\n", strerror(errno));      
       return -1;
    }
}

//map a page of memory to gpio at offset 0x3F200000 which is where GPIO goodness starts on a Pi3
gpio = (uint32_t *)mmap(0, getpagesize(), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0x3F200000);

if ((int32_t)gpio < 0){
  printf("Mmap failed: %s\n", strerror(errno));
  return -1;
}

/* Now you have gpio use and abuse it */

uint32_t BaseAddressFromDTFile (void) 
{
   uint32_t base_addr = 0;
   FILE *fp;
   if ((fp = fopen(BMC2835_RPI2_DT_FILENAME, "rb"))) {
     uint32_t base_size;
     unsigned char buf[4];
     fseek(fp, BMC2835_RPI2_DT_PERI_BASE_ADDRESS_OFFSET, SEEK_SET);
     if (fread(buf, 1, sizeof(buf), fp) == sizeof(buf))
     base_addr = (uint32_t *) (buf[0] << 24 | buf[1] << 16 | buf[2] << 8 | buf[3] << 0);
     fseek(fp, BMC2835_RPI2_DT_PERI_SIZE_OFFSET, SEEK_SET);
     if (fread(buf, 1, sizeof(buf), fp) == sizeof(buf))
         base_size = (buf[0] << 24 | buf[1] << 16 | buf[2] << 8 | buf[3] << 0);
     fclose(fp);
   }
   return (base_addr);
}

// use memory file descriptor
/* Base of the peripherals block is mapped to VM */
bcm2835_peripherals = (uint32_t*) mapmem("gpio", bcm2835_peripherals_size,
memfd, (uint32_t) bcm2835_peripherals_base);

#include <stdbool.h>
#include <fcntl.h>				//Needed for SPI port
#include <sys/ioctl.h>			//Needed for SPI port
#include <linux/spi/spidev.h>	//Needed for SPI port
#include <unistd.h>			    //Needed for SPI port

/* This function will open the SPI port (either 0 or 1) with the given parameters */
/* If it fails it will return -1 otherwise it will return handle to opened device */

int OpenSPI( int SPI_Port, int* SPI_WriteMode, int* SPI_ReadMode, int* SPI_WriteLength, int* SPI_ReadLength, int* SPI_WriteSpeed, int* SPI_ReadSpeed)
{
   int status;
   int spi_handle = -1;       // This will be our handle to the SPI

   /* OPEN A HANDLE TO THE SPI DEVICE */
   if (SPI_Port) == 0){
      spi_handle = open ("/dev/spidev0.0", O_RDWR);
   } else if (SPI_Port == 1) {
      spi_handle = open ("/dev/spidev0.1", O_RDWR)
   } else {
      printf("Invalid SPI port number specified\n");
      return -1;
   }

   if (spi_handle < 0) {
      printf("Unable to open physical memory handle: %s\n", strerror(errno));      
      return -1;
   }


   /* SET SPI WRITE MODE */
   status = ioctl(spi_handle, SPI_IOC_WR_MODE, SPI_WriteMode);
   if(status < 0) {
      printf("Could not set SPIMode (WR)...ioctl fail");
      return -1;
   }

   /* SET SPI READ MODE */
   status = ioctl(spi_handle, SPI_IOC_RD_MODE, SPI_ReadMode);
   if(status < 0) {
      printf("Could not set SPIMode (RD)...ioctl fail");
      return(-1);
   }

   /* SET SPI WRITE BITS PER WORD */
   status = ioctl(spi_handle, SPI_IOC_WR_BITS_PER_WORD, SPI_WriteLength);
   if (status < 0) {
      printf("Could not set SPI bitsPerWord (WR)...ioctl fail");
      return -1;
   }

   /* SET SPI READ BITS PER WORD LETS MAKE IT 8 */
   status = ioctl(spi_handle, SPI_IOC_RD_BITS_PER_WORD, SPI_ReadLength);
   if (status < 0) {
      printf("Could not set SPI bitsPerWord(RD)...ioctl fail");
      return -1;
   }


   /* SET SPI WRITE BITS SPEED */
   status = ioctl(spi_handle, SPI_IOC_WR_MAX_SPEED_HZ, SPI_WriteSpeed);
   if (status < 0) {
      printf("Could not set SPI speed (WR)...ioctl fail");
      return -1;
   }

   /* SET SPI READ BITS SPEED */
   status = ioctl(spi_handle, SPI_IOC_RD_MAX_SPEED_HZ, SPI_ReadSpeed);
   if (status < 0) {
      printf("Could not set SPI speed (RD)...ioctl fail");
      return -1;
   }

   /* Everytthing set successfully return the handle */
   return (spi_handle);
}

/* Okay we need to write data to the SPI lets make that procedure */
int SpiWrite (int spi_device, int SPI_WriteSpeed, int SPI_WriteLength, unsigned char *data, int length)
{
    struct spi_ioc_transfer spi[length];
    int i = 0;
    int retVal = -1;

   //one spi transfer for each byte
   for (i = 0 ; i < length ; i++)
   {
	memset(&spi[i], 0, sizeof (spi[i]));
	spi[i].tx_buf  = (unsigned long)(data + i); // transmit from "data"
	spi[i].rx_buf  = 0;
	spi[i].len     = length;
        spi[i].delay_usecs   = 0 ;
        spi[i].speed_hz      = SPI_WriteSpeed;
        spi[i].bits_per_word = SPI_WriteLength;
        spi[i].cs_change = 0;
   }
   retVal = ioctl(spi_device, SPI_IOC_MESSAGE(length), &spi);
   if (retVal < 0){
      printf("Error - Problem transmitting spi data..ioctl");
      return(-1);
   }
   return retVal;
}


/* Okay we need to read data to the SPI lets make that procedure */
int SpiRead (int spi_device, int SPI_ReadSpeed, int SPI_ReadLength, unsigned char *data, int length)
{
   struct spi_ioc_transfer spi[length];
   int i = 0;
   int retVal = -1;

   // one spi transfer for each byte
   for (i = 0 ; i < length ; i++) 
   {
      memset(&spi[i], 0, sizeof (spi[i]));
      spi[i].tx_buf = 0;
      spi[i].rx_buf = (unsigned long)(data + i) ; // receive into "data"
      spi[i].len  = length;  // Amount of data to recieve
      spi[i].delay_usecs = 0 ;
      spi[i].speed_hz = SPI_ReadSpeed;
      spi[i].bits_per_word = SPI_ReadLength;
      spi[i].cs_change = 0;
   }

   retVal = ioctl(spi_device, SPI_IOC_MESSAGE(length), &spi) ;
   if(retVal < 0) {
      printf("Error - Problem transmitting spi data..ioctl");
      return -1;
    }
   return retVal;
}

int main (void){
   static int WriteSpeed = 1000000;  // lets set write speed to 1Mhz
   static int ReadSpeed  = 1000000;  // Lets set read speed to 1Mhz
   static int WriteLength = 8;       // Lets set write length to 8 bits
   static int ReadLength  = 8;       // Lets set read length to 8 bits

//----- There are a number of SPI MODES that are available -----
//SPI_MODE_0 (0,0) 	CPOL = 0, CPHA = 0, Clock idle low, data is clocked in on rising edge, output data (change) on falling edge
//SPI_MODE_1 (0,1) 	CPOL = 0, CPHA = 1, Clock idle low, data is clocked in on falling edge, output data (change) on rising edge
//SPI_MODE_2 (1,0) 	CPOL = 1, CPHA = 0, Clock idle high, data is clocked in on falling edge, output data (change) on rising edge
//SPI_MODE_3 (1,1) 	CPOL = 1, CPHA = 1, Clock idle high, data is clocked in on rising, edge output data (change) on falling edge

   static int WriteMode = SPI_MODE_0 ;       // Lets write MODE 0
   static int ReadMode  = SPI_MODE_0;        // Lets read MODDE 0

   static int SPI_handle;                    // This is our SPI handle that will be return and we hold

    /* Lets assume we want SPI port 0 lets open it */
    SPI_handle = OpenSPI(0, &WriteMode, &ReadMode, &WriteLength, &ReadLength, &WriteSpeed, &ReadSpeed);

    /* Check it opened */
   if (SPI_handle >= 0){
        
        /* let write something A,B,C,D */
        unsigned char Data[4] = { 0x41, 0x42, 0x43, 0x44};
        SpiWrite(SPI_handle, WriteSpeed, WriteLength, &Data[0], 4);


        /* we are finished so close the handle */
        close(SPI_handle); 

   }
}

#include <stdint.h>
#define STATUS_PORT 0x3F000000;
int main (void){
    uint32_t* p = (uint32_t*) STATUS_PORT;
    do {} while (*p != 1); 
}

main:
  mov r3, #1056964608
  ldr r3, [r3]
  cmp r3, #1
  beq .L2
.L3:
  b .L3
.L2:
  mov r0, #0
  bx lr

#include <stdint.h>
#define STATUS_PORT 0x3F000000;
int main (void){
    volatile uint32_t* p = (uint32_t*) STATUS_PORT; // We Add volatile to the pointer
    do {} while (*p != 1); 
}

main:
  mov r2, #1056964608
.L2:
  ldr r3, [r2]
  cmp r3, #1
  bne .L2
  mov r0, #0
  bx lr

#include <stdint.h>
#define STATUS_PORT 0x3F000000;

struct mystruct {
	uint32_t port1;
    uint32_t port2;
};

int main(void) {
	struct mystruct* p = (struct mystruct*)STATUS_PORT;
	do {} while (p->port1 != 1);
	do {} while (p->port2 != 2);
}

main:
  mov r3, #1056964608
  ldr r2, [r3]
  cmp r2, #1
  beq .L2
.L3:
  b .L3
.L2:
  ldr r3, [r3, #4]
  cmp r3, #2
  beq .L4
.L5:
  b .L5
.L4:
  mov r0, #0
  bx lr

#include <stdint.h>
#define STATUS_PORT 0x3F000000;

struct mystruct {
	uint32_t port1;
    uint32_t port2;
};

int main(void) {
	volatile struct mystruct* p = (struct mystruct*)STATUS_PORT; // We add volatile to the pointer again
	do {} while (p->port1 != 1);
	do {} while (p->port2 != 2);
}

main:
  mov r2, #1056964608
.L2:
  ldr r3, [r2]
  cmp r3, #1
  bne .L2
  mov r2, #1056964608
.L3:
  ldr r3, [r2, #4]
  cmp r3, #2
  bne .L3
  mov r0, #0
  bx lr