Getting segmentation fault(core dumped) - c

Question

My program has to encrypt/decrypt the textfile but I'm getting segmentation fault(core dumped) when I do this:
./program 9999 input.txt output.txt
The program takes every character from the input file and converts it based on the passed key. It compiles fine when I compile in CodeBlocks and does not give any errors. Could smb tell me what's wrong with the code? Thanks!
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//Checks if the input arguments are less than 2 (starting from 0)
//A user should enter 3 arguments so as not to reach this method
int badProgram(const char *const program){
printf("The program is missing some of the files!");
return -1;
}
//Encrypts the passed inputFile and
//Produces its output to the passed outputFile
//Key is also passed as the seeding number
int encryptFile(FILE *input, FILE *output){
char c;
char p;
int r = 0;
char p1 = 0;
char c1 = 0;
while((p = fgetc(input)) != EOF){
r = rand() % 97;
//change all displayable characters [0...96]
if(p == 't'){
p1 = 0;
}
else if(p == '\n'){
p1 = 1;
}
else{
p1 = p - 30;
}
c1 = p1 ^ r;//bitwise xor
if(c1 == 0){
c = 't';
}
else if(c1 == 1){
c = '\n';
}
else{
c = c1 + 30;
}
//Write
fprintf(output, "%c", c);
}
}
int main(int argc, char *argv[])
{
//Check the number of the entered arguments
if(argc < 2){
return badProgram(argv[0]);
}
else{
FILE *input;
FILE *output;
//Seed a number into Random Generator
int key = *argv[0];
srand(key);
input = fopen(argv[1], "r");
output = fopen(argv[2], "w");
encryptFile(input, output);
}
return 0;
}
The **input.txt** looks like this:
Hello, World!
Bye!

Answer 1

Couple of things that are wrong with your code:
int key = *argv[0]; is most likely not doing what you think it does. What it actually does is the following:
assign an ASCII value of the first character of the [0] argument (program name) to an int variable
It is likely that what you intended to do there is:
int key = atoi(argv[1]); // this converts "9999" into an int 9999
input = fopen(argv[1], "r"); opens a file named (in your case) "9999" for reading and fails. You never check for the error so this is causing a crash the moment you are trying to use the input FILE pointer. The fix is to use the argv[2]
Similarly you should be using argv[3] for the output file
Your encryptFile function must return a value as it is declared int (don't know why you want to return a value from it as you never use it)
Once you fix the above issues your program no longer crashes
Update
A bit of explanation for the above issues and general info:
The argv lists all the input parameters as strings (char*) where the first ([0]) argument is the executable name and is not your first argument "after" the program name
One should always check the results of file operations as they are quite likely to fail during "normal" program operation
C/C++ doesn't "automatically" convert a string into an int (or a double, for that matter) but provides a whole set of functions to deal with numbers' parsing. Some examples of those functions are: 'atoi', 'atol', 'atof'