Reading in values using pointers

Question

I have the following code, and i'm wondering how I can store each line of a text document into an array of pointers. I think im close but i'm getting a few errors.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{

    FILE * fp;
    char buffer[50];
    int totalSize;
    totalSize = 6;
    int size = 0;
    char * array;

    fp = fopen(location,"r");


    while (fgets(buffer,150,fp) != NULL)
    {
        array = malloc(sizeof(strlen(buffer))+1);
        strcpy(array[size],buffer);
        size++;
    }

    for (int x = 0; x < size ; x++)
    {
        printf("%s",array[x]);
    } 

    free(array);
    return 0;
}

the `sizeof(strlen(buffer))` is a very interesting construct, which at the first glance I am failing to interpret. Anyway, this is obviously not doing what you intend. `strlen` alone will be enough here. — Eugene Sh., Feb 18 '15 at 19:58
`strcpy(array[size],buffer);` - your compiler should be throwing a warning about this line, heed it! `strcpy` expects a `char*`, and you're passing it a `char`. You probably want to take the address of it ;). — slugonamission, Feb 18 '15 at 19:58
And `malloc`-ing stuff in a loop, but freeing it once should ignite some red lights in your head. — Eugene Sh., Feb 18 '15 at 19:59
You declare buffer to be an array of 50 characters, yet in your fgets statement you are reading up to 150 characters into that buffer (i.e. a buffer overflow). Also, each time through the loop you will be assigning a new memory location to array leading to a lot of dangling pointers. — thurizas, Feb 18 '15 at 20:01

Jean-Baptiste Yunès · Answer 1 · 2015-02-18T20:34:24.337

1

Your array should be an array of pointers. You have lines (so an array of lines), each line being a C-string (a pointer to a NUL terminated char array). So you need to declare your variable as char **array = NULL.

Now, each time you read a line you must alloc a new entry in your array of lines, this way array=realloc(array,(size+1)*sizeof(char *)) then read a buffer, allocate memory to store the string read with array[size]=malloc(strlen(buffer)+1) and then copy strcpy(array[size],buffer) then increment the size++.

You must free everything accordingly (free all pointers and then free the array). Also, take care about sizes, buffer is 50 but you tried to read 150... Be consistent.

edited Feb 18 '15 at 20:34

answered Feb 18 '15 at 20:03

Jean-Baptiste Yunès

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Thanks, everything you wrote worked, except when it reads the first line, for some reason it is giving me stuff like this: ?K?p? instead of the actual line. First i do the realloc, then i malloc, copy it and increment the size – BeginnerC96 Feb 18 '15 at 20:31
Extending 'array' by a single element each iteration calls to mind the tale of Shlemiel and the painter's algorithm ... – jschultz410 Feb 18 '15 at 20:38

Reading in values using pointers

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