Copying a string from a pointer to a string

Question

I'm developing some code which reads file names from an sd-card (using FatFs) and displays them to the screen. Here's a snipet of what I have working, this prints out the files on the card as expected -

FRESULT result;
        char *path = '/'; //look in root of sd card
        result = f_opendir(&directory, path);   //open directory
        if(result==FR_OK){
            for(;;){
                result = f_readdir(&directory, &fileInfo); //read directory
                if(result==FR_OK){
                    if(fileInfo.fname[0]==0){ //end of dir reached
                        //LCD_UsrLog("End of directory.\n");
                        break;
                    }
                    if(fileInfo.fname[0]=='.')continue; //ignore '.' files

                    TCHAR *fn_ptr; //file name, why a pointer?
                    fn_ptr=&fileInfo.fname; //get file name            
                    LCD_UsrLog("%s\n",fn_ptr);
                    for(delay=0;delay<0x0FFFFF;delay++){ShortDelay();} //delay to display

                }//end result==fr_ok
            }//end for
        }//end result==fr_ok

Where

typedef char TCHAR

and

typedef struct {
DWORD   fsize;          /* File size */
WORD    fdate;          /* Last modified date */
WORD    ftime;          /* Last modified time */
BYTE    fattrib;        /* Attribute */
TCHAR   fname[13];      /* Short file name (8.3 format) */

} FILINFO;

I need to copy the names of the files into an array for processing however I've tried a few ways but can't seem to get the array working. I have tried creating an arbitrarily large array of TCHARs and dereferencing the file name pointer but this prints garbage.

FRESULT result;
        char *path = '/'; //look in root of sd card
        TCHAR fileList[50];
        u32 index=0;
        result = f_opendir(&directory, path);   //open directory
        if(result==FR_OK){
            for(;;){
                result = f_readdir(&directory, &fileInfo); //read directory
                if(result==FR_OK){
                    if(fileInfo.fname[0]==0){ //end of dir reached
                        //LCD_UsrLog("End of directory.\n");
                        break;
                    }
                    if(fileInfo.fname[0]=='.')continue; //ignore '.' files

                    TCHAR *fn_ptr; //file name, why a pointer?
                    fn_ptr=&fileInfo.fname; //get file name            

                    fileList[index]=*fn_ptr;
                    LCD_UsrLog("%s\n",fileList[index]);
                    for(delay=0;delay<0x0FFFFF;delay++){ShortDelay();} //delay to display
                    index++;
                }//end result==fr_ok
            }//end for
        }//end result==fr_ok

I suspect this is a simple mistake regarding pointers or the proper usage of an array of chars but it has been 4+ years since I've last touched C and I'm lost!

Any help would be greatly appreciated.

Péter Török · Accepted Answer · 2011-08-23 09:59:11Z

1

First problem: currently your file list is an array of chars, while it should be an array of strings. So declare it as

TCHAR* fileList[50];

then allocate the strings of proper length for each filename (not forgetting the extra space for the terminating 0 char). You also need to explicitly copy the filenames into your name list, because the contents of fileInfo get overwritten in each loop cycle, so simply storing the pointers would result in your list containing the name of the last file 50 times.

All in all, you need something like this:

                if(fileInfo.fname[0]=='.')continue; //ignore '.' files

                fileList[index] = malloc(strlen(fileInfo.fname) + 1);
                strcpy(fileList[index], fileInfo.fname);

                LCD_UsrLog("%s\n",fileList[index]);

(Disclaimer: No guarantee that this works as it is, I have no chance to test it, but hopefully this gives you the idea).

Alternatively, if you know the upper limit of the filename length, you can declare an array of filenames with fixed length, and get rid of dynamic allocation. But then you should use strncpy instead of strcpy to be on the safe side, to prevent buffer overflows. And this also requires the terminating 0 character to be appended, again to be on the safe side:

TCHAR fileList[50][MAX_FILENAME_LENGTH + 1];

...
strncpy(fileList[index], fileInfo.fname, strlen(fileInfo.fname));
fileList[index][MAX_FILENAME_LENGTH] = '\0';

edited Aug 23, 2011 at 9:59

answered Aug 23, 2011 at 9:45

Péter Török

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4 Comments

David Over a year ago

This is perfect. Thank you very much for the rapid response. Now I'll have to work out how to change the fileList array to dynamically change it's size (meaning I'll probably be back with another question soon!)

David Over a year ago

Peter, is there any advantage in using the second approach - the multi dimensional array? This is making printing the file names much more complex and I can't see any benefit (as yet).

Péter Török Over a year ago

@David, the benefit is that the memory is allocated on the stack, thus will be automatically freed once it goes out of scope. Using dynamic allocation, you also need to manually free the memory, otherwise you get a memory leak.

David Over a year ago

I see. In that case it is probably best to use the multidimensional array of pointers! Thanks

vromanov · Accepted Answer · 2011-08-23 10:03:32Z

0

You must use next array definition

TCHAR fileList[50][13];

...

                if(fileInfo.fname[0]=='.')continue; //ignore '.' files

                strncpy(fileList[index], sizeof(fileList[index]), fileInfo.fname);
                LCD_UsrLog("%s\n",fileList[index]);

or for dynamic memory. Don't forget to free memory!

TCHAR* fileList[50];

...

                if(fileInfo.fname[0]=='.')continue; //ignore '.' files

                fileList[index]=strdup(fileInfo.fname);
                LCD_UsrLog("%s\n",fileList[index]);

PS:

edited Aug 23, 2011 at 10:03

answered Aug 23, 2011 at 9:57

vromanov

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