How do I fix realloc(): invalid next size in C - c

Question

I have a Problem when I try to load a file into memory as an array.
I am trying to load a file into an array and print it out again but I would like to allow the memory to grow as the file length can be arbitrary.
When I run my Program locally on my Mac it seems to Work fine but when I try it on my Ubuntu VM, I get the following error
realloc(): invalid next size
Aborted (core dumped)
My Code is as follows
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char **loadfile(char *filename, int *len);
int main(int argc, char *argv[])
{
if (argc == 1)
{
printf("Usage add file\n");
return 1;
}
int length = 0;
char **words = loadfile(argv[1],&length);
printf("%d\n", length);
for (int i = 0; i < length; i++) {
printf("%s\n",words[i]);
}
printf("Done\n");
return 0;
}
char **loadfile(char *filename, int *len)
{
const int STEPSIZE = 10;
FILE *f = fopen(filename,"r");
if (!f) {
fprintf(stderr, "Can't open file\n");
return NULL;
}
int arrlen = STEPSIZE;
char **lines = (char **)malloc(STEPSIZE);
char buf[100];
int i = 0;
int counter = 2;
while (fgets(buf,100,f))
{
if (i == arrlen)
{
counter++;
arrlen += STEPSIZE;
char **newlines = (char **)realloc(lines,counter * STEPSIZE);
if(!newlines)
{
printf("Out of memory\n");
//return 2;
}
lines = newlines;
}
buf[strlen(buf)-1] = '\0';
int slen = strlen(buf);
char *str = (char *)malloc(slen + 1 *sizeof(char ));
strcpy(str, buf);
lines[i] = str;
i++;
}
*len =i;
return lines;
}
and for the life of me I cannot find the problem.
I can only assume the problem is somewhere in this section but I may be wrong:
if (i == arrlen)
{
counter++;
arrlen += STEPSIZE;
char **newlines = (char **)realloc(lines,counter * STEPSIZE);
if(!newlines)
{
printf("Out of memory\n");
//return 2;
}
lines = newlines;
}
Your Help is greatly appreciated

Answer 1

const int STEPSIZE = 10;
char **lines = (char **)malloc(STEPSIZE);
char **newlines = (char **)realloc(lines,counter * STEPSIZE);
You don't want to allocate 10 bytes, but memory for 10 char * elements. Thus some subsequent access to lines[i] = str; is invalid.
What you meant to do is:
char **lines = malloc(sizeof(*lines) * STEPSIZE);
char **newlines = realloc(lines, sizeof(*newlines) * counter * STEPSIZE);
Alternatively you can use sizeof(char*).
Also:
char *str = (char *)malloc(slen + 1 *sizeof(char ));
although it is correct and will work, because sizeof(char) is 1, but it's more clearly the intention was:
char *str = malloc((slen + 1) * sizeof(char));
Also, it's good to think if you should cast the result of malloc.

Answer 2

You forgot that malloc and realloc take an amount of bytes, not an amount of 'cells' in your array.
The program works as expected if you replace your malloc(STEPSIZE) by malloc(STEPSIZE * sizeof(char*)) and realloc(lines, counter * STEPSIZE) by realloc(lines, (counter * STEPSIZE) * sizeof(char*))