C code about the pointer array

Question

My goal is to read a file, and save each elements in this file into a new array..

rewind(fp); ii = 0; while (!feof(fp)) {
    ii ++;
    fscanf(fp, "%s\n", filename_i);
    fp_i = fopen(filename_i, "r");
    if (fp_i == NULL) {
        fprintf(stderr, "can't open input file %s \n", filename_i);
        exit(1);
    }
    filename_ii[ii] = filename_i;
    printf("%s, %d\n", filename_ii[ii],ii);
    fclose(fp_i);

}


printf("a %s %d\n",filename_ii[9],DataSize[2]);
printf("a %s %d\n",filename_ii[1],DataSize[2]);

In while() function, my output is each elements, but I don't know why the last two printf() returns the same results, i.e, it seems like both filename_ii[1] and filename_ii[9] point the last element in the file. Does anyone have ideas about what's wrong in my code？ Thank you~

Paul R · Accepted Answer · 2013-10-04 14:06:36Z

1

You need to use strcpy to copy a string. Change:

filename_ii[ii] = filename_i;             // this just assigns a pointer -
                                          // it doesn't actually copy a string

to:

strcpy(filename_ii[ii], filename_i);      // copy the *contents* of `filename_i`
                                          // to `filename_ii[ii]`

This assumes of course that the filename_ii array has been correctly initialised and is not just an array of dangling char * pointers (not possible to tell from the code as currently posted in the question).

Note that if filename_ii is just an array of uninitialised char * pointers then you can use strdup to handle the memory allocation and copying all in one convenient function call. In which case you would change the line above to:

filename_ii[ii] = strdup(filename_i);     // allocate memory to `filename_ii[ii]` and
                                          // copy the *contents* of `filename_i`
                                          // to `filename_ii[ii]`

edited Oct 4, 2013 at 14:06

answered Oct 4, 2013 at 13:27

Paul R

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4 Comments

user2760099 Over a year ago

After changing to 'strcpy()', it still doesn't work out. I think maybe I didn't initialize the 'filename_ii' correctly. I used 'char *filename_ii[20]'.

Paul R Over a year ago

As I said above: This assumes of course that the filename_ii array has been correctly initialised and is not just an array of dangling char * pointers. I've added to my answer above to cover this now.

user2760099 Over a year ago

After Changing to strdup(), it works out. Thank you very much! @Paul R

Paul R Over a year ago

OK - don't forget to free() the memory when you're done with it.

harald · Accepted Answer · 2013-10-04 14:14:26Z

1

Stop using feof()/fscanf() like that, it's super-brittle and needlessly hard to get right.

Instead:

char line[1024]; /* or whatever makes you feel comfortable */
while(fgets(line, sizeof line, fp) != NULL)
{
  size_t len = strlen(line);
  if(len == 1) /* Ignore blank lines. */
    continue;
  if(line[len - 1] == '\n')
    line[--len] = '\0'; /* Remove linefeed. */
  if(access(line, R_OK) == 0)
    strcpy(filename_ii[ii++], line);
}

This:

Uses fgets() to read in a whole line.
Uses access() to check if the file can open. Note that this kind of checking is always prone to race-conditions.
Uses strcpy() to copy the filename, assuming filename_ii[] is a properly set up array.

edited Oct 4, 2013 at 14:14

harald

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answered Oct 4, 2013 at 13:37

unwind

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4 Comments

user2760099 Over a year ago

Why I get this error "read-only variable is not assignable" for ' line[--len] == '\0';' @unwind

user2760099 Over a year ago

How to set up 'filename_ii[]' properly? I still haven't work out.

harald Over a year ago

@user2760099, see updated code. Should be an assignment (=) not an equality test (==) in that line.

harald Over a year ago

Ah, drop the const when declaring len.

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