File content as of revision 0:5e920a8fb894:

#include "mbed.h"
#include "FATFileSystem.h"
#include "SDBlockDevice.h"
#include <stdio.h>
#include <errno.h>
/* mbed_retarget.h is included after errno.h so symbols are mapped to
 * consistent values for all toolchains */
#include "platform/mbed_retarget.h"

#if defined(TARGET_WIO_3G) || defined(TARGET_WIO_BG96) 
DigitalOut SD_POWER(PA_15, 1);
#endif

SDBlockDevice sd(MBED_CONF_SD_SPI_MOSI, 
                 MBED_CONF_SD_SPI_MISO, 
                 MBED_CONF_SD_SPI_CLK, 
                 MBED_CONF_SD_SPI_CS);
FATFileSystem fs("sd", &sd);

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

void errno_error(void* ret_val){
  if (ret_val == NULL) {
    printf(" Failure. %d \n", errno);
    while (true) __WFI();
  } else
    printf(" done.\n");
}

int main()
{
    int error = 0;
    printf("Welcome to the filesystem example.\n");

    printf("Opening a new file, numbers.txt.");
    FILE* fd = fopen("/sd/numbers.txt", "w+");
    errno_error(fd);

    for (int i = 0; i < 20; i++){
        printf("Writing decimal numbers to a file (%d/20)\r", i);
        fprintf(fd, "%d\n", i);
    }
    printf("Writing decimal numbers to a file (20/20) done.\n");

    printf("Closing file.");
    fclose(fd);
    printf(" done.\n");

    printf("Re-opening file read-only.");
    fd = fopen("/sd/numbers.txt", "r");
    errno_error(fd);

    printf("Dumping file to screen.\n");
    char buff[16] = {0};
    while (!feof(fd)){
        int size = fread(&buff[0], 1, 15, fd);
        fwrite(&buff[0], 1, size, stdout);
    }
    printf("EOF.\n");

    printf("Closing file.");
    fclose(fd);
    printf(" done.\n");

    printf("Opening root directory.");
    DIR* dir = opendir("/sd/");
    errno_error(fd);

    struct dirent* de;
    printf("Printing all filenames:\n");
    while((de = readdir(dir)) != NULL){
        printf("  %s\n", &(de->d_name)[0]);
    }

    printf("Closeing root directory. ");
    error = closedir(dir);
    return_error(error);
    printf("Filesystem Demo complete.\n");

    while (true) {}
}