How to compare 2 files lexicographically using C - c

Question

Hey guys, I'm currently trying to implement a function using C that takes in two file names as command line arguments and compare them lexicographically.
The function will return -1 if the contents of the first file are less than the contents of the second file, 1 if the contents of the second file are less than the contents of the first file, and 0 if the files are identical.
Please give me some advice on how I should start with this.
[EDIT]
Hey guys sorry if there's any unclear part in the question, so I'll just post the link to the question here: Original question. Thing is it's an uni assignment so we're expected to do it using only basic C properties, probably only including stdio.h, stdlib.h, and string.h. Sorry for the trouble caused. Also here's the code I already have, my main problem now is that the function doesn't know that file1.txt (refer to the link) has it's first line longer than file2.txt, but is actually lexicographically less:
int filecmp(char firstFile[], char secondFile[])
{
int similarity = 0;
FILE *file1 = fopen(firstFile, "r");
FILE *file2 = fopen(secondFile, "r");
char line1[BUFSIZ];
char line2[BUFSIZ];
while (similarity == 0)
{
if (fgets(line1, sizeof line1, file1) != NULL)
{
if (fgets(line2, sizeof line2, file2) != NULL)
{
int length;
if (strlen(line1) > strlen(line2))
{
length = strlen(line1);
}
else
{
length = strlen(line2);
}
for (int i = 0; i < length; i++)
{
if (line1[i] < line2[i]) similarity = -1;
if (line1[i] > line2[i]) similarity = 1;
}
}
else
{
similarity = 1; //As file2 is empty
}
}
else
{
if (fgets(line2, sizeof line2, file2) != NULL)
{
similarity = -1; // As file1 is empty
}
else break;
}
}
fclose(file1);
fclose(file2);
return similarity;
}
[END EDIT]
Many thanks,
Jonathan Chua

Answer 1

Take a look the source code of the UNIX cmp utility, e.g. here. The relevant file is regular.c. If you can't use mmap, the principle of implementation through fgetc() is the same: keep reading a single character from each of the two files as long as they compare the same. When (if!) you find a difference, return the result of the comparison. The borderline case of one file being proper prefix of the other (e.g. "ABC" "ABCCC") can be resolved by treating EOF as an infinitely small value. This is already neatly solved in C as fgetc() guarantees to return a negative value ONLY on EOF; proper characters are >= 0.

Answer 2

Are you allowed to use strcmp?
If so (untested):
int ret = 0;
while (ret == 0)
{
char line1 [ MAX_LINE_LEN ];
char line2 [ MAX_LINE_LEN ];
if (fgets(line1, MAX_LINE_LEN, file1) != NULL )
{
if (fgets(line2, MAX_LINE_LEN, file2) != NULL )
{
ret = strcmp(line1, line2);
}
else
{
ret = 1;
}
}
else
{
if (fgets(line2, MAX_LINE_LEN, file2) != NULL )
{
ret = -1;
}
else
{
break;
}
}
}
return ret;