SPI interface with Nios II

I updated the code with your suggestions but still there are errors.

CODE:

/*
* "Small Hello World" example.
*
* This example prints 'Hello from Nios II' to the STDOUT stream. It runs on
* the Nios II 'standard', 'full_featured', 'fast', and 'low_cost' example
* designs. It requires a STDOUT device in your system's hardware.
*
* The purpose of this example is to demonstrate the smallest possible Hello
* World application, using the Nios II HAL library. The memory footprint
* of this hosted application is ~332 bytes by default using the standard
* reference design. For a more fully featured Hello World application
* example, see the example titled "Hello World".
*
* The memory footprint of this example has been reduced by making the
* following changes to the normal "Hello World" example.
* Check in the Nios II Software Developers Manual for a more complete
* description.
*
* In the SW Application project (small_hello_world):
*
* - In the C/C++ Build page
*
* - Set the Optimization Level to -Os
*
* In System Library project (small_hello_world_syslib):
* - In the C/C++ Build page
*
* - Set the Optimization Level to -Os
*
* - Define the preprocessor option ALT_NO_INSTRUCTION_EMULATION
* This removes software exception handling, which means that you cannot
* run code compiled for Nios II cpu with a hardware multiplier on a core
* without a the multiply unit. Check the Nios II Software Developers
* Manual for more details.
*
* - In the System Library page:
* - Set Periodic system timer and Timestamp timer to none
* This prevents the automatic inclusion of the timer driver.
*
* - Set Max file descriptors to 4
* This reduces the size of the file handle pool.
*
* - Check Main function does not exit
* - Uncheck Clean exit (flush buffers)
* This removes the unneeded call to exit when main returns, since it
* won't.
*
* - Check Don't use C++
* This builds without the C++ support code.
*
* - Check Small C library
* This uses a reduced functionality C library, which lacks
* support for buffering, file IO, floating point and getch(), etc.
* Check the Nios II Software Developers Manual for a complete list.
*
* - Check Reduced device drivers
* This uses reduced functionality drivers if they're available. For the
* standard design this means you get polled UART and JTAG UART drivers,
* no support for the LCD driver and you lose the ability to program
* CFI compliant flash devices.
*
* - Check Access device drivers directly
* This bypasses the device file system to access device drivers directly.
* This eliminates the space required for the device file system services.
* It also provides a HAL version of libc services that access the drivers
* directly, further reducing space. Only a limited number of libc
* functions are available in this configuration.
*
* - Use ALT versions of stdio routines:
*
* Function Description
* =============== =====================================
* alt_printf Only supports %s, %x, and %c ( < 1 Kbyte)
* alt_putstr Smaller overhead than puts with direct drivers
* Note this function doesn't add a newline.
* alt_putchar Smaller overhead than putchar with direct drivers
* alt_getchar Smaller overhead than getchar with direct drivers
*
*/

#include "alt_types.h"
#include "sys/alt_stdio.h"
#include "io.h"
#include "system.h"
#include "sys/alt_cache.h"
#include "altera_avalon_spi.h"
#include "altera_avalon_spi_regs.h"
#include "sys/alt_irq.h"
//This is the ISR that runs when the SPI Slave receives data
static void spi_rx_isr(void* isr_context){
alt_printf("ISR %x \n" ,IORD_ALTERA_AVALON_SPI_RXDATA(SPI_0_BASE));

//This resets the IRQ flag. Otherwise the IRQ will continuously run.

IOWR_ALTERA_AVALON_SPI_STATUS(SPI_0_BASE, 0x0);
}

int main()
{
alt_printf("Hello from Nios II!\n");
int return_code,ret;
char spi_command_string_tx[10] = "$HELLOABC*";
char spi_command_string_rx[10] = "$HELLOABC*";

//This registers the Slave IRQ with NIOS

ret = alt_ic_isr_register(SPI_0_IRQ_INTERRUPT_CONTROLLER_ID,SPI_0_IRQ,spi_rx_isr,(void *)spi_command_string_tx,0x0);
alt_printf("IRQ register return %x \n", ret);

//You need to enable the IRQ in the IP core control register as well.

IOWR_ALTERA_AVALON_SPI_CONTROL(SPI_SLAVE_BASE,ALTERA_AVALON_SPI_CONTROL_SSO_MSK | ALTERA_AVALON_SPI_CONTROL_IRRDY_MSK);
//Just calling the ISR to see if the function is OK.
spi_rx_isr(NULL);
return_code = alt_avalon_spi_command(SPI_0_BASE,0 ,1, spi_command_string_tx, 0, spi_command_string_rx,0);
return_code = alt_avalon_spi_command(SPI_0_BASE,0 ,1, &spi_command_string_tx[1],0, spi_command_string_rx,0);
return_code = alt_avalon_spi_command(SPI_0_BASE,0 ,1, &spi_command_string_tx[2],0, spi_command_string_rx,0);
return_code = alt_avalon_spi_command(SPI_0_BASE,0 ,1, &spi_command_string_tx[3],0, spi_command_string_rx,0);

if(return_code < 0)
alt_printf("ERROR SPI TX RET = %x \n" , return_code);
alt_printf("Transmit done. RET = %x spi_rx %x\n",return_code,spi_command_string_rx[0]);
//RX is done via interrupts.
alt_printf("Rx done \n");
return 0;
}