MY CODE

#include "mbed.h"
#include "FATFileSystem.h"
#include "SDBlockDevice.h"
#include <errno.h>
#include "EthernetInterface.h"

EthernetInterface net;
SDBlockDevice sd1(MBED_CONF_APP_SPI_MOSI, MBED_CONF_APP_SPI_MISO, MBED_CONF_APP_SPI_CLK, 
    MBED_CONF_APP_SPI_CS);

void return_error(int ret_val){
  if (ret_val)
    printf("Failure. %d\r\n", ret_val);
  else
    printf("done.\r\n");
}

void errno_error(void* ret_val){
  if (ret_val == NULL)
    printf(" Failure. %d \r\n", errno);
  else
    printf(" done.\r\n");
}
 
int main() {
    net.connect();
    
    FATFileSystem fs1("sd1", &sd1); 
    printf("Welcome to the filesystem example.\r\n");

    printf("Opening a new file, stampvalue.txt.");
    FILE* fd1;
    fd1 = fopen("/sd1/stampvalue.txt", "w+");
    errno_error(fd1);

    for (int i = 0; i < 20; i++){
        printf("Writing decimal stampvalue to a file (%d/20)\r", i);
        fprintf(fd1, "%d\n", i);
    }
    printf("Writing decimal stampvalue to a file (20/20) done.\r\n");
    printf("Closing file.\r\n");
    fclose(fd1);
    printf(" done.\r\n");
        
    net.disconnect();

    while(1) {  }
    
}