C - 为什么将一个元素分配给动态分配数组的第一个元素总是什么都不返回

Question

I am developing an application where I need to read some files off an SD Card. Everything runs on an STM32 microcontroller with FatFS. I just open the root, and iterate over each element in that directory and check whether it's a file or a folder, I save the information how many files I have. Then I allocate given amout of memory for said files (no of files x buffer for the name). Next, I read the directory again, like in the first step. This time I copy the name of each file to the aforementioned place in memory. My problem is, everytime I copy anything to the 2D array, the firs element contains literally nothing, and file names start from the second index. What am I doing wrong?

//Returns 0 on error or no files
uint8_t files_get_card_file_count(void)
{
    auto uint8_t file_count = 0;
    auto FRESULT result;
    auto DIR directory;
    auto FILINFO info;

    //Read card contents
    if(f_opendir(&directory, "/")) return 0;

    //Check if there is a card mounted and functioning
    if(disk_status(0) == (STA_NOINIT || STA_NODISK)) return 0;

    for (;;)
    {
        result = f_readdir(&directory,&info);              // Read a directory item
        if (result != FR_OK || info.fname[0] == 0) break;  // Break on error or end of dir
        if (!(info.fattrib & AM_DIR)) ++file_count;
    }
    return file_count;
}

//Make sure we dont allocate more than MAX_FILE_COUNT blocks
char** files_mem_allocate(uint8_t file_count)
{
    auto uint8_t i;

    char **array;
    array = calloc(file_count, sizeof(char *));

    if(array == NULL) return 0;

    for(i=0; i < file_count; i++)
    {
        array[i] = calloc(MAX_FILENAME_LENGTH, sizeof(char));
        if(array[i] == NULL) return 0;
    }

    return array;
}

uint8_t files_get_names(char** array, uint8_t file_no)
{
    auto uint8_t i;
    auto FRESULT result;
    auto DIR directory;
    auto FILINFO info;

    //Read card contents
    if(f_opendir(&directory, "/")) return 1;

    //Check if there is a card mounted and functioning
    if(disk_status(0) == (STA_NOINIT || STA_NODISK)) return 1;

    for(i=0;i<file_no;i++)
    {
        result = f_readdir(&directory,&info);              //Read a directory item
        if (result != FR_OK || info.fname[0] == 0) break;  // Break on error or end of dir
        if (!(info.fattrib & AM_DIR))
        {
            //It is a file.
            //Mind the string safety?
            //Check bounds!
            strcpy(array[i], info.fname);
        }
    }
    return 0;
}

Then if I do: lcd_str_XY(0,1, file_names_array[0]); There is nothing in this place in memory lcd_str_XY(0,1, file_names_array[1]); Prints the first file's name

Answer 1

files_get_names doesn't take into account anymore that f_opendir also returns directory. In fact, from your description it is likely that it returns a directory as the first entry. Yet files_get_names still increases the index and goes on.

Fix it like so:

uint8_t files_get_names(char** array, uint8_t file_no)
{
    auto uint8_t i;
    auto FRESULT result;
    auto DIR directory;
    auto FILINFO info;

    //Read card contents
    if(f_opendir(&directory, "/")) return 1;

    //Check if there is a card mounted and functioning
    if(disk_status(0) == (STA_NOINIT || STA_NODISK)) return 1;

    i = 0;
    for(;;)
    {
        result = f_readdir(&directory,&info);              //Read a directory item
        if (result != FR_OK || info.fname[0] == 0) break;  // Break on error or end of dir
        if (!(info.fattrib & AM_DIR))
        {
            //It is a file.
            //Mind the string safety?
            //Check bounds!
            strcpy(array[i], info.fname);
            i++;
        }
    }
    return 0;
}