Realloc on an array of structs, address boundary error when indexing - c

Question

I have some code where I'm trying to read lines in from a file and store some information from each line in a struct. Since I don't know how long the file will be, I'm dynamically adjusting the array of structs using realloc.
My issue is that my code seems to work fine for the first 3 (technically 6) lines, and then I receive SIGSEGV (address boundary error). gdb says that this happens when trying to index the array (array[i]->string = (char*) _tmp).
typedef struct {
char* string;
int len;
} buffer;
int read_into_array(char *filename, buffer** array) {
int n;
size_t size;
char* buf = NULL;
FILE *file = fopen(filename, "r");
int i = 0;
while (1) {
buffer *tmp = (buffer*)realloc(*array, sizeof(buffer) * (i + 1));
if (!tmp)
printf("Failed realloc\n");
*array = tmp;
// First line is ignored, second line is taken as data.
getline(&buf, &size, file);
n = getline(&buf, &size, file);
if (n > 0) {
void* _tmp = malloc(sizeof(char) * n);
if (!_tmp)
printf("Failed malloc\n");
array[i]->string = (char*) _tmp;
array[i]->len = n-1;
strncpy(array[i]->string, buf, n-1);
}
i++;
if (feof(file)) {
printf("saw end of file, leaving.\n");
break;
}
}
return i;
}
int main(int argc, char* argv[]) {
char *filename = argv[1];
buffer *array = (buffer*) calloc(1, sizeof(buffer));
int num = read_into_array(filename, &array);
}
Apologies for the somewhat poor formatting, I've been trying to figure this out for a while.
Since it seems to work for the first few lines, my assumption is that I'm going wrong somewhere in the realloc calculation. My other guess is that I'm somehow using/reading the file incorrectly.
Thanks for any help. For posterity, the file looks something like this https://hastebin.com/vinidiyita.sm (the real file is thousands of lines long).

Answer 1

when you do *array=tmp you're allocating memory for array[0]
then you're using array[i] that should be a pointer to a buffer, but points to garbage or 0
You're confusing two ways to use data.
The first is by using arrays - there's the non-dynamic:
buffer array[x] = {0};
int num = read_into_array(filename, &array);
then you can use array[i]
and there's the dynamic type:
buffer **array = calloc(initial_len*sizeof(buffer *));
int num = read_into_array(filename, array, initial_len);
read_into_array(char *filename, buffer **&array, int initial_len)
{
int len = initial_len;
...
while()
{
...
if(i>len)
{
array = realloc(array, sizeof(buffer*) * (i + 1));
len = i;
}
array[i] = calloc(sizeof(buffer));
}
}