C using a file as an argument - c

Question

The problem I am having is that whenever I run the code I get the Null value for myfile before I even get to pass the file name as the argument into the program and i'm not sure why any help is much appreciated.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
FILE* myFile; // file pointer
myFile = fopen(argv[1] , "r"); //open file
if(myFile==NULL)
{
printf("Can't Open FIle.\n"); // if file doesn't exist then exit
exit(0);
}
int A[10000]={0};
int B[10000]={0};
double C[10000]={0};
int N,M;
int i=0;
fscanf (myFile, "%d", &N); //input N from file
printf("%d\n",N);
if(N>100)
{
exit(0); // if N>100 then exit
}
while (!feof (myFile)) // loop until file pointer reaches to the end of file
{
fscanf (myFile, "%d", &A[i]); //input source
fscanf (myFile, "%d", &B[i]); // input destination
fscanf (myFile, "%lf", &C[i]); // input time
i++;
}
fclose (myFile); //close file
M=i; // number of lines = M
for (i = 0; i < M; i++)
{
if(A[i]==0) //end of output
break;
else
{
printf("%d %d %lf:\n",A[i],B[i],C[i]); //print source, destination and time
if(A[i]>=1&&A[i]<=N)
{
if(B[i]>=1&&B[i]<=N)
{
if(A[i]==B[i])
{
printf("Error:Source city is the same as destination city.\n"); //same source and destination error:condition
}
else
{
if(C[i]<0)
{
printf("Error:Invalid Time.\n"); //invalid time
}
else
{
//
}
}
}
else
{
printf("Error: Invalid destination city.\n"); //invalid destination condition
}
}
else
{
printf("Error: Invalid source city.\n"); //invalid source condition
}
}
}
return 0;
}

Answer 1

If you don't pass a file name as an argument to the program, the contents of argv[1] are undefined.
Since anything could be there in argv[1] when the program attempts to open the file, the most probable thing is that there is no file with that name, and hence fopen() returns NULL.
You should not try to access command line arguments without checking that they were supplied, for that check argc, if you just expect 1 argument, then a simple check would be
char filename[256]; /* the size should be reasonable
* can be PATH_MAX or MAX_PATH,
* depending on whether it's Windows
* or not
*/
if (argc < 2)
{
size_t length;
fprintf(stdout, "Error: %s program expects one argument\n", argv[0]);
fprintf(stdout, "Please input the file name: ");
if (fgets(filename, sizeof(filename), stdin) == NULL)
{
fprintf(stdout, "Error: unexpected error\n");
return -1;
}
length = strlen(filename);
if (length == 0)
{
fprintf(stdout, "Error: the provided file name is invalid.\n");
return -1;
}
if (filename[length - 1] == '\n')
filename[length - 1] = 0;
}
else
strcpy(filename, argv[1]);