File IO: Line by Line Comparison In C, Segmentation Fault - c

Question

I'm really stuck on this problem. I'm trying to write a program in C that will take a file, and check to see if every line is in alphabetical order.
So the text file
apple
banana
grape
grape
orange
The program would print to stdout "Lines are in order". If the lines are out of order it would print say the lines are out of order and print the first pair of lines where that occurs.
The main thing I'm having trouble with is just debugging, I keep getting a segmentation fault and I'm not sure why. I don't think that theres an instance where I'm trying to dereference a null pointer and I don't think there's an instance where I try to assign something with more memory than a pointer can handle so I'm not sure what the issue is.
Below is my code, I'm really new to C so if there is anything obvious or fundamental flaws in my code I'd really appreciate being told so.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
int main(char* argv[],int argc){
char* fileN = argv[1];
FILE* file = fopen(fileN,"r");
if (file == NULL){
perror("Error: Couldn't open file");
exit(1);
}else{
char *line = malloc(101*sizeof(char));
fgets(line,101,file);
char *comp = malloc(101*sizeof(char));
while(line){
fgets(comp,101,file);
if (comp){
if(strcmp(line,comp) > 0){
printf("Lines out of order\n");
printf("%s\n",line);
printf("%s\n",comp);
free(line);
free(comp);
fclose(file);
exit(1);
}
}
line = comp;
}
printf("Lines are ordered\n");
free(line);
free(comp);
fclose(file);
exit(0);
}
}

Answer 1

Thanks for the help everyone, here is a version of the program that works.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
int main(int argc,char* argv[]){
char* fileN = argv[1];
FILE* file = fopen(fileN,"r");
if (file == NULL){
perror("Error: Couldn't open file");
exit(1);
}else{
char line[101], comp[101];
fgets(line,101,file);
int bool = 1;
while(line && bool){
if (fgets(comp,101,file) != NULL){
if(strcmp(line,comp) > 0){
printf("Lines out of order\n");
printf("%s",line);
printf("%s\n",comp);
fclose(file);
exit(1);
}
strncpy(line,comp,100);
}else{
bool = 0;
}
}
printf("Lines are ordered\n");
fclose(file);
exit(0);
}
}
My mistakes were A) forgetting how to properly copy one string over to one another and B) not realizing that fgets would return null if it fails, that's what I needed to use to make sure my loops actually ends.

Answer 2

Tell me, if you need comments.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAX_LINE 100
int main(int argc, char* argv[]) {
char a[MAX_LINE];
char b[MAX_LINE];
FILE* fd = fopen(argv[1], "r");
if(!fd) {
puts("Error: Couldn't open file");
exit(1);
}
fgets(a, MAX_LINE, fd);
while(fgets(b, MAX_LINE, fd)) {
if(strcmp(a, b) > 0) {
puts("Lines out of order");
printf("a = %s", a);
printf("b = %s", b);
fclose(fd);
exit(1);
}
strcpy(a, b);
}
puts("Lines are ordered");
fclose(fd);
return 0;
}