code to change file extension (up for review)

Question

I am using the following piece of C code to change file extension.

{

 #define EVAL_MAX_LEN (300)

 int nLen;
 char szOut [EVAL_MAX_LEN] = {0};
 char szPath [EVAL_MAX_LEN] = "/db/file/face.bmp";

 // Get string length !!!
 nLen = strlen (szPath);

 if ((nLen > 0) && (nLen < EVAL_MAX_LEN)) {

      while (nLen) {

           // Check for extension character !!!
           if (szPath [nLen] == '.') {

                szPath [nLen] = '\0';
                break;
           }

           nLen --;

      }//while (nLen ...

 // Create output file name and with new extension
 sprintf (szOut, "%s.txt", szPath);

 }// if ((nLen > 0) && (nLen < EVAL_MAX_LEN ...

}

Any suggestion for a better and more elegant code is welcome. I also know that in Windows we can use _splitpath to do this in an elegant manner. But right now, I am using this code in Linux.

Answer 1

There can be more than one period. You should use PathFindExtension() and PathRenameExtension().

Answer 2

I would consider using strnlen() and base EVAL_MAX_LEN from the system's PATH_MAX (or whatever is applicable) limit (check its return value prior to proceeding).

That will avoid even trying to write questionable content to szOut, or having to check for the # of characters not printed to szOut if the string is too big (though I'd still suggest favoring snprintf() over sprintf()).

Answer 3

You should use snprintf() instead of sprintf() to avoid a potential buffer overflow. You can also use strrchr(3) to find the last period, instead of coding your own loop. However, two things to be careful of:

1. What if the input file has no extension, i.e. there is no dot in the path name?
2. What if the input file has no extension, but one of the folders in the path name has a dot in it? For example: "/home/joeuser/dotted.folder/thefile"

Make sure you handle both of those cases appropriately.

Answer 4

How about this one?

#define EVAL_MAX_LEN (300)
int nLen;
char szOut [EVAL_MAX_LEN] = {0};
char szPath [EVAL_MAX_LEN] = "/db/file/face.bmp";
char *res = NULL;
// Get string length !!!
nLen = sizeof (szPath);
memset(szOut,0,sizeof(szOut));
if ((nLen > 0) && (nLen <= EVAL_MAX_LEN)) {
    res = strrchr( szPath, '.' );
    if(res != NULL)
        *res = '\0';
}
// Create output file name and with new extension
sprintf (szOut, "%s.txt", szPath);
// if ((nLen

Answer 5

// find file name
char *pFile = strrchr(szPath, '/');
pFile = pFile == NULL ? szPath : pFile+1;
// change extension
char *pExt = strrchr(pFile, '.');
if (pExt != NULL)
    strcpy(pExt, ".txt");
else
    strcat(pFile, ".txt");

Answer 6

You need to use dirname and basename to split the directory and filename and then split filename to get last "." and then merge dirname and '/' and basename to get renamed extension.

Pseduodcode where result is the output..

 char *dirc, *basec, *bname, *dname,*result;
 char *path = "/etc/passwd";
 char *extn = NULL;
  dirc = strdup(path);
  basec = strdup(path);
  dname = dirname(dirc);
  bname = basename(basec);
  extn = strrchr( bname, '.' );
  if(extn != NULL)  *extn = '\0';
  sprintf(result, "%s/%s.txt"", dname, bname);
  free(dirc);
  free(basec);

Answer 7

How about writing some test cases? If the above is a subroutine named change_ext_to_txt(char* szPath), you'll want to test:

change_ext_to_txt("/foo/bar.baz"); 
change_ext_to_txt("/foo/bar"); 
change_ext_to_txt("/foo/bar.baz/bal.bat"); 
change_ext_to_txt("/foo/bar.baz/bal");

and make sure the right thing comes out of all of them. Hint: all the solutions presented so far (including yours!) fail on at least one of the above.