C Basic Head Command - c

Question

I'm trying to recreate the head, and tail commands from linux for my programming class.
We just started using C so I'm new to the idea of allocating memory and pointers.
I'm wondering why this doesn't work.
#include <stdio.h>
#include <stdlib.h>
int main(int argc,char **argv){
/* Checks if correct amount of arguements */
if(argc != 2 || argc != 4){
printf("Usage: %s head <file> \n Or: head <file> -n <number of characters>", argv[0]);
exit(-1);
}
if(strcmp(argv[1], "-n" != 0)){
char fileName[strlen(argv[1])] = argv[1];
}
}
//Compile error on char fileName[strlen(argv[1])] = argv[1];
Any additional insight would also be helpful.

Answer 1

I think it's better to write:
char fileName[strlen(argv[1])+1];
strcpy(fileName, argv[1]);
or (if you don't whant to make a copy of string) :
char* fileName = argv[1];

Answer 2

First things first, your usage doesn't match your argument checking. According to the usage, you must use one of:
head <filename>
head <filename> -n <count>
In other words, argv[1] is always the filename, argv[2] is the one that needs to be set to -n if there are more than two arguments.
Secondly, unless you want to use VLAs (variable length arrays), you should probably just set up a pointer to the filename argument with something like:
char *fileName = argv[1];
You don't need to change it at all (you'll just be passing it to fopen, presumably), so it's a waste trying to make another copy.
In addition, your if statement is wrong as an or, it should be an and. It's guaranteed that argc will either not be 2 or not be 4, since it can't be both at the same time.
I would start with something like:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static int usage (void) {
printf ("Usage: head <file>\n");
printf (" or: head <file> -n <number of characters>\n");
return -1;
}
int main (int argc,char *argv[]) {
char *fileName;
int lineCount;
// Checks if correct arguments
if ((argc != 2) && (argc != 4)) return usage();
if ((argc == 4) && (strcmp(argv[2], "-n" != 0)) return usage();
// Get file spec and line count
fileName = argv[1];
lineCount = (argc == 2) ? 10 : atoi (argv[3]); // or strtol for purists
if (linecount < 0) lineCount = 0;
// Now go ahead and implement the logic for head.
}