File pointers led to core dump. Probably something stupid - c

Question

Write the program myuniq.c that contains a function void process_file(FILE* f) that reads all input from the given file one line at the time while keeping two consecutive lines in memory, and prints each line to the standard output if it is not equal to the previously read line.
^^This is the assignment i'm working on. My code below is:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void process_file(FILE* f);
int main()
{
FILE *fil = fopen("text.txt","r");
process_file(fil);
return 0;
}
void process_file(FILE* f)
{
FILE *fi = f;
char *firstLine = fgets(firstLine, 999, f);
char *secondLine = fgets(secondLine, 999, f);
while (feof(fi))
{
if (firstLine == secondLine)
{
puts(secondLine);
}
else
{
puts(firstLine);
puts(secondLine);
}
firstLine++;
secondLine++;
}
}
It compiled fine...but on every run it says core dumped. I can't see where I went wrong? Any ideas?

Answer 1

You don't check the return value of fopen, you don't allocate any memory for the strings into which you read, you don't continue reading input from the file, you don't correctly check for the end of input.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MY_MAX_LINE 999
void process_file(FILE* f)
{
char firstLine[MY_MAX_LINE + 1];
char secondLine[MY_MAX_LINE + 1];
while (1)
{
if (!fgets(firstLine, sizeof(firstLine), f))
break;
puts(firstLine);
if (!fgets(secondLine, sizeof(secondLine), f))
break;
if (strncmp(firstLine, secondLine, sizeof(firstLine)))
puts(secondLine);
}
if (!feof(f))
perror("Problem reading from file"), exit(1);
}
int main(int argc, char **argv)
{
FILE *f = fopen("text.txt", "r");
if (!f)
perror("text.txt"), exit(1);
process_file(f);
fclose(f);
return 0;
}