Getting my values from a C pointer array

Question

Really struggling here and cannot figure out how to get my value's from an array.

I first declare this array which I want to hold a set of numbers. IDK why the size is 64, I simply am frustrated and gave it a size.

   char *numberList1[64];

I then read in a file using some code I found.

  FILE * fp;
   char * line = NULL;
   size_t len = 0;
   ssize_t read;
   char string[100];
   int counter = 0;

   printf("\n\nEnter name of File 1: ");
   scanf("%s", string);

   fp = fopen(string, "r");
   if (fp == NULL) {
       printf("invalid filename of %s",string);
       exit(EXIT_FAILURE);
    }
   while ((read = getline(&line, &len, fp)) != -1) {
       numberList1[counter] = line;
       counter++;
   }

Now if were to say something like printf("%s",numberList1[counter]); within the while, I would get all my numbers back.

However when I say the following, I only get the last item printed out however times there are numbers. (vectorLength).

   int j;
   //vectorLength is the user-entered # of lines/numbers in the file.
   for (j = 0; j < vectorLength; j++) {
       printf("Writing out: %s \n", numberList1[j] );
       //write(pipe1[WRITE], currentNum, bitSize+1);  
   }

i.e. If I had numbers, 1, 2, 3, 4 I would get: 4 4 4 4

What am I doing wrong??? I have tried to find guides on understanding pointers and arrays in C but I cannot figure it out...

so sorry! I ask the user for it off the bat, it basically is how many numbers/lines are in the file. Hence, how many values should be in the array — Austin
– Austin, Commented Oct 2, 2014 at 2:12

WhozCraig · Accepted Answer · 2014-10-02 02:58:58Z

You're not preparing the parameters sent to getline() properly. According to the documentation:

If *lineptr is set to NULL and *n is set 0 before the call, then getline() will allocate a buffer for storing the line. This buffer should be freed by the user program even if getline() failed.

You want a new line buffer for each line, and you at-least-seem to want getline() to make that for you, so, after each successful read, and once you save the returned buffer pointer allocated by getline() in your array, clear the line pointer back to NULL and the length parameter back to zero:

char * line = NULL;
size_t len = 0;
while ((read = getline(&line, &len, fp)) != -1 && counter < 64) 
{
   numberList1[counter++] = line;
   line = NULL; // <<==== ADDED
   len = 0;     // <<==== ADDED
}

free(line); // note: this is here because, though getline() failed, the
            // documentation for the api mandates its presence.

Brett Wolfington · Accepted Answer · 2014-10-02 10:43:03Z

1

I believe I've found the root of the problem.

In the code below, you are reading each line into memory at the location pointed to by line. You are then storing the pointer to the memory into each index in the array. The problem is that you are overwriting the memory at the location pointed to by line each time you read in a line. As a result, only the last-read value is stored at that location. Since each index in the array points to the same location, each iteration of the final loop in your code will display the same value.

char * line = NULL;
while ((read = getline(&line, &len, fp)) != -1) {
   numberList1[counter] = line; // You are storing the pointer here, NOT the value
   counter++;
}

Instead, allocate memory for the array (getline does this automatically) at each iteration of the loop and copy that pointer into the array.

while ((read = getline(&line, &len, fp)) != -1) {
    numberList1[counter] = line;
    line = NULL;
    counter++;
}

Make sure to free all memory allocated during this process.

edited Oct 2, 2014 at 10:43

answered Oct 2, 2014 at 2:26

Brett Wolfington

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7 Comments

Austin Over a year ago

Confused on some things. 1. what are line and len? I assume one is the number of actual characters in the string per line, but idk the other? 2. in the original code it has char * line is that the same as line = (char *)? 3. What's with all the len + 1's?

Brett Wolfington Over a year ago

These are the same variables as in your code. When the getline function returns, line should contain the maximum size of the string to read. getline will return the actual number of bytes read, and if len is not large enough to contain the string, line will be resized to contain it (assuming sufficient memory is available), and len will be updated to the size of memory that was allocated.

Austin Over a year ago

So if I already know the size of each line, in advance from user input, can I just set that up by default? i.e. If I know each line will have a 8-bit number..

Brett Wolfington Over a year ago

Also, I would recommend doing some research on memory allocation, particularly using malloc(). the line line = (char *)malloc(len + 1) allocates len + 1 bytes of memory (sufficient to hold a null-pointer at the end of the string), cast to a character pointer.

Brett Wolfington Over a year ago

And yes, if each line has an 8-bit number, that would indicate that the line size is 3 characters (0-255) or less (plus a newline, and possibly a carriage return). As a result, to hold the number, plus the newline and carriage return, I would set len to 5 if you are on Windows and 4 if you are on Linux (Windows includes a carriage return at the end of each line, while Linux only uses newlines).

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