decrypt and encrypt text with command line arguments - c

Question

I am trying to make a program that will encrypt and decrypt when user enters the string they want to encrypt/decrypt for argv[2] and enters either "encrypt" or "decrypt" for argv[3]. Here is the code I am trying to compile and run as of now
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv)
{
int i;
// char *string;
char *key_ch;
char key_int;
char *string_ = calloc(80, 1);
string = argv[1];
char encrypted_string[strlen(string)];
char decrypted_string[strlen(string)];
//char *key_ch;
//char key_int;
string = argv[1];
key_ch = argv[2];
key_int = atoi(key_ch);
if (argc < 3)
{
printf("Not enough arguments!\n");
exit (1);
}
if (strcmp(argv[3], "encrypt") == 0)
{
i = 0;
while(i <= strlen(string)-1)
{
encrypted_string[i] = string[i] + key_int;
i++;
}
// printf("Encrypted string: ");
i = 0;
while (i <= strlen(string) -1)
{
printf("%c", encrypted_string[i]);
i++;
}
printf("\n");
}
if (strcmp(argv[3], "decrypt") == 0)
{
i = 0;
while(i <= strlen(string) -1)
{
decrypted_string[i] = string[i] - key_int;
i++;
}
// printf("Decrypted String: ");
i = 0;
while (i <= strlen(string) -1)
{
printf("%c", decrypted_string[i]);
i++;
}
printf("\n");
}
return 0;
}
When I try to compile it without the -Wall command it compiles fine but when I run the program I am getting a segmentation fault, when I compile with -Wall I am getting
sam0.c:9:24: warning: 'string' is used uninitialized in this function [-Wuninitialized]
char encrypted_string[strlen(string)];
Can anyone possible shine some light on this error? Thank you
Edit:
Changed my code to your suggestions. I am not getting a compiling error at all even when using "-Wall" however somewhere in my program it is causing me to get a segmentation fault... any ideas? I put quotes around where I changed my code for reference in case I did it wrong.

Answer 1

warning: 'string' is used uninitialized in this function
[-Wuninitialized]
char *string; creates a pointer to char. At this point it is not yet a string, but you are using it as a string argument.
Before using char *string; it must have memory assigned, and should be initialized. Among other methoods, this can be done by:
char *string = calloc(80, 1);//initializes with known values (NULL).
Now string is usable, but has zero length. Values can be assigned via string functions:
strcpy(string, argv[1]);
sprintf(string, "%s", argv[1]);
strcat(string, argv[1]);
... more string functions
When using input from command line, argv, argc, malloc/calloc and string cpy functions can be avoided by using strdup. a value can be assigned like this:
if(argc == 2)
{
char *string = strdup(argv[1]);
if(!string) return -1;
...
EDIT (addressing your OP edit)
You are now using two different variables: string_ and string
char *string_ = calloc(80, 1);
^
string = argv[1];
Make them the same throughout your code and it should build and run.

Answer 2

When a meaningful value for the variable can be determined, try to declare and initialize the variable in one step:
char *string = argv[1];
char encrypted_string[strlen(string)];
char decrypted_string[strlen(string)];
char *key_ch = argv[2];
char key_int = atoi(key_ch);
This will fix the warning: you were trying to get the length of an uninitialized string.
(here assuming C99. char encrypted_string[strlen(string)] is a VLA. If you're restricted to C89 only Rykker's answer works).
Also:
use const to prevent unwanted modifications, e.g.
const char *string = argv[1];
/* ... */
const char *key_ch = argv[2]
string is a constant so you can get its length just one time (do not recalculate the length every time... take a look at Shlemiel the painter's algorithm). So
const char *string = argv[1];
const int length = strlen(string);
char encrypted_string[length];
char decrypted_string[length];
const char *key_ch = argv[2];
char key_int = atoi(key_ch);
check inputs: what happen if
argc < 4
argv[3] isn't in ("encrypt", "decrypt")
key_ch is not a number
key_int is too big
you're printing encrypted_string / decryped_string one character at a time... and it works, but consider that if you want to manipulate them as 'strings' they aren't null terminated and their length isn't correct.
don't repeat yourself. The differences between the encryption and the decryption phases are minimal: you can use just one buffer and change key_int to a negative value to decrypt.

Answer 3

char *string;
char encrypted_string[strlen(string)];
char decrypted_string[strlen(string)];
This results in undefined behavior - you try to get the length of an uninitialized piece of memory (I'm amazed that your program doesn't crash). You may only call strlen after assigning something to string.
The easiest fix (assuming you don't want to start using malloc, strdup, etc.) would be moving the last two lines after string = argv[1];
Also, you need to check argc to make sure enough arguments have been passed!