Allocating an array of strings when needed in C

Question

I'm trying to read a file and allocating a string in the array when needed but It didn't work correctly.

This is an example of a file.txt:

and this is my simplfied program :

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define LENGTH 256

char *readLine(FILE *f, size_t *len)
{
    char *line = NULL;
    int idxToDel;

    ssize_t nread;
    int mem = 0;

    if (f == NULL)
    {
        return NULL;
    }

    if (len != NULL)
    {
        mem = *len;
    }
    else
    {
        len = (size_t *)malloc(sizeof(size_t));
        *len = 0;
    }

    if ((nread = getline(&line, len, f)) != -1)
    {
        idxToDel = strlen(line) - 1;
        memmove(&line[idxToDel], &line[idxToDel + 1], strlen(line) - idxToDel);
        *len = strlen(line);
        return line;
    }
    else
    {
        if (nread == -1)
        {
            *len = mem;
        }

        free(line);

        return NULL;
    }
}

char **readFile(const char *filename, size_t *fileLen)
{
    char *result;
    int idx = 0;
    char **array = malloc(1 * sizeof(*array));

    if (filename == NULL || fileLen == NULL)
    {
        return NULL;
    }

    FILE *f = fopen(filename, "r");
    void *newptr;
    if (f == NULL)
    {
        return NULL;
    }

    while (1)
    {
        result = readLine(f, fileLen);
        if (result == NULL)
            break;
        else
        {
            *(array + idx) = malloc(LENGTH * sizeof(char *));
            strncpy(*(array + idx), result, strlen(result) + 1);

            idx++;
            array = realloc(array, idx * sizeof(char **));
        }
    }

    if (*(array + idx) == NULL)
    {
        *fileLen = 0;
    }

    *fileLen = idx;

    return array;
}

int main()
{
    char **numbers;
    size_t len = 0;

    numbers = readFile("file.txt", &len);
    
    return 0;
}

As an error I get

realloc(): invalid next size
Aborted (core dumped)

What am I doing wrong here ? When debugging the program , the index idx stops at the value 4 and exits afterwards.

Why are you allocating mem for the size_t len: len = (size_t *)malloc(sizeof(size_t)); in the readLine function? — user15801843
– user15801843, Commented Jun 4, 2021 at 12:54
As an aside, if ((nread = getline(&line, len, f)) != -1){...}else{ if (nread == -1)... might be better written as nread = getline(&line, len, f); if(nread != -1){...}else{ ... because it's easier to read if you don't assign and test a value on the same line - and there's no need to test it twice. — Tim Randall
– Tim Randall, Commented Jun 4, 2021 at 13:00
I think there is a general logic error, cos you realloc the (same) array in the loop (for every line). Maybe it would be easier to use your buffer, strncpy or memcpy every line till end and use len for the final length. When finished realloc the buffer array based on the final len.. — user15801843
– user15801843, Commented Jun 4, 2021 at 13:27

Stephan Schlecht · Accepted Answer · 2021-06-04 19:28:23Z

1

Your realloc assigns one element too few.

 array = realloc(array, idx * sizeof(char **));

For example, on the first pass, idx has the value 1, so the memory allocated for array is not increased. On the second pass, however, the line

*(array + idx) = malloc(LENGTH * sizeof(char *));

is accessed beyond the allocated area. This leads to an undefined behavior, which in your case eventually results in the error described.

You can fix the problem by allocating the correct amount of memory (idx + 1). The line should then look like this:

array = realloc(array, (idx + 1) * sizeof(char **));

answered Jun 4, 2021 at 19:28

Stephan Schlecht

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