Unexpected Segfault - What am I doing wrong - c

Question

I've been trying to blow the cobwebs off my C programming skills, and I've been getting an error I can't seem to figure out. This program reads in a list of integers separated by newlines. This bit happens in read_integer_file... I have no issues going through the input there. It's when I pass the data back to main via out that I have the problem.
#include <stdlib.h>
#include <stdio.h>
int read_integer_file(char* filename, int* out)
{
FILE* file;
file = fopen(filename, "r");
/* check if the file open was successful */
if(file == NULL)
{
return 0;
}
int num_lines = 0;
/* first check how many lines there are in the file */
while(!feof(file))
{
fscanf(file, "%i\n");
num_lines++;
}
/* seek to the beginning of the file*/
rewind(file);
out = malloc(sizeof(int)*num_lines);
if(out == NULL)
return 0;
int inp = 0;
int i = 0;
while(!feof(file))
{
fscanf(file, "%i\n", &inp);
out[i] = inp;
printf("%i\n", out[i]); /* <---- Prints fine here! */
i++;
}
return num_lines;
}
int main(int argc, char** argv)
{
if(argc < 2)
{
printf("Not enough arguments!");
return -1;
}
/* get the input filename from the command line */
char* array_filename = argv[1];
int* numbers = NULL;
int number_count = read_integer_file(array_filename, numbers);
for(int i = 0; i < number_count; i++)
{
/* Segfault HERE */
printf("%i\n", numbers[i]);
}
}

Answer 1

You have not allocated any memory for numbers. Currently it is pointing to no where. When it gets back to the calling function it is still pointed to nowhere. Pass a pointer to a pointer to the function to allocate it within the function.
int read_integer_file(char* filename, int** out)
{
...
*out = malloc(sizeof(int)*num_lines);
...
int number_count = read_integer_file(array_filename, &numbers);

Answer 2

This is a version of your code working.. Keep in mind also that fscanf just skip the \n the way you wrote it so it's like writing fscanf(file, "%d");
And if you don't put a variable to handle what it reads the compiler may not see it but you'll probably get an error..
So here is the code :
#include <stdlib.h>
#include <stdio.h>
int read_integer_file(char* filename, int **out)
{
FILE* file;
file = fopen(filename, "r");
/* check if the file open was successful */
if(file == NULL)
{
return 0;
}
int num_lines = 0;
int garbi;
char garbc;
/* first check how many lines there are in the file */
while(!feof(file))
{
fscanf(file, "%d", &garbi);
fscanf(file, "%c", &garbc);
if (garbc=='\n') ++num_lines;
}
/* seek to the beginning of the file*/
rewind(file);
int *nbr = malloc(sizeof(int)*num_lines);
if(nbr == NULL)
return 0;
int i = 0;
while(!feof(file))
{
fscanf(file, "%d", &nbr[i++]);
fscanf(file, "%c", &garbc);
}
*out=nbr;
return num_lines;
}
int main(int argc, char** argv)
{
if(argc < 2)
{
printf("Not enough arguments!");
return -1;
}
/* get the input filename from the command line */
char* array_filename = argv[1];
int *numbers = NULL;
int number_count = read_integer_file(array_filename, &numbers);
int i;
for(i = 0; i < number_count; ++i)
printf("%d\n", numbers[i]);
return 0;
}