Storing an array of strings and looping until end of file

Question

I'm coding a program in which one of the tasks I need to complete is to store each line of a text file (the name of which is provided via command line) as a separate string for future manipulation.

I have two issues with my program.

The first is the problem of storing the string inside an array. When I assign an index of the array with the string, everything works fine. But as soon as I free() the string for allocation of another string, both strings get deleted.

userText[numStrings - 1] = currentString; 
/* Both userText at index and currentString hold the same value at this point */
free(currentString);
/* Both userText at index and currentString are free'd */

It could be a simple thing that I'm not understanding, I'm still quite new to C.

The second problem I have, is that I have no clue how to loop until the end of the file. I know that feof() exists, but that's kind of pointless since it will only return true until AFTER the end of the file, so I will be looping once more.

Here's the code: note it won't run until you set some condition in the last do/while loop.

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

int main(int argc, char** argv){

    char** userText = NULL;
    char* currentString = NULL;
    int currentStringSize = 0;
    int numStrings = 0;


    FILE* fp = fopen(argv[1],"r");


    do{

        numStrings++;
        currentStringSize = 0;
        do{
            currentStringSize++;
            currentString = (char*)realloc(currentString, currentStringSize * sizeof(char));
            fscanf(fp, "%c", &currentString[currentStringSize - 1]);

        }while(!(currentString[currentStringSize - 1] == '\n'));
        currentString[currentStringSize - 1] = '\0';

        userText = (char**) realloc(userText, numStrings * sizeof(char*));

        for (int i = 0; i < numStrings; i++){
            userText[i] = (char*) realloc(userText[i], currentStringSize * sizeof(char));
        }

        userText[numStrings - 1] = currentString;
        free(currentString);
        currentString = NULL;
        } while (//the end of the file *insert code here*);


    for (int i = 0; i < numStrings; i++){
        free(userText[i]);
    }
    free(userText);
    fclose(fp);

    return 0;
}

Thank you for you help.

Answer 1

These lines are very problematic:

for (int i = 0; i < numStrings; i++){
    userText[i] = (char*) realloc(userText[i], currentStringSize * sizeof(char));
}

userText[numStrings - 1] = currentString;
free(currentString);

First you allocate memory for userText[i], overwriting the pointers already existing in userText.

Then you simply overwrite the last pointer you allocated, making you lose the allocation you just did.

Lastly you free the memory pointed to by userText[numStrings - 1] (both that pointer and currentString are pointing to the same memory).

The solution to all these problems is simple: Just do

userText[numStrings - 1] = currentString;

That's it! That's all you need.

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And as mentioned in a comment, you do need to make currentString a null pointer, before going back to the top of the loop and your calls to realloc.