Segmentation fault in Substitution - Problem Set 2

Question

I have finally finished writing my code for the Substitution question in Problem Set 2, but Check50 tells me that my code has a segmentation fault when there is a lack of key. My code is as follows:

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, string argv[])
{
    int n = strlen(argv[1]);
    if (argc == 2 && n == 26)
    {
        for (int i = 0; i < n; i++)
        {
            if (!(isalpha(argv[1][i])))
            {
                printf("Please enter a valid substitution key in the command-line. Your key must contain each and every letter of the alphabet only once in no particular order, but nothing more.\n");
                return 1;
            }
            else
            {
                for (int j = 0; j < n; j++)
                {
                    if (i != j && argv[1][i] == argv[1][j])
                    {
                            printf("Please enter a valid substitution key in the command-line. Your key must contain each and every letter of the alphabet only once in no particular order, but nothing more.\n");
                            return 1;
                    }
                }
            }
        }
    }
    else
    {
        printf("Please enter a valid substitution key in the command-line. Your key must contain each and every letter of the alphabet only once in no particular order, but nothing more.\n");
        return 1;
    }
    string ptext = get_string("plaintext: ");
    printf("ciphertext: ");
    for (int k = 0, o = strlen(ptext); k < o; k++)
    {
        if (islower(ptext[k]))
        {
            printf("%c", tolower(argv[1][ptext[k] - 'a']));
        }
        else if (isupper(ptext[k]))
        {
            printf("%c", toupper(argv[1][ptext[k] - 'A']));
        }
        else
        {
            printf("%c", ptext[k]);
        }
    }
    printf("\n");
    return 0;
}

Indeed, when I try to execute my program without a key, it does result in a segmentation fault, as shown here:

~/pset1/substitution/ $ make substitution
clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    substitution.c  -lcrypt -lcs50 -lm -o substitution
~/pset1/substitution/ $ ./substitution
Segmentation fault

I thought that

else
    {
        printf("Please enter a valid substitution key in the command-line. Your key must contain each and every letter of the alphabet only once in no particular order, but nothing more.\n");
        return 1;
    }

would have already considered the case when arcc == 1, that is, when there is no key? Any intuitions and reasons as to why my code has a segmentation fault and where that fault might lie would be greatly appreciated :)

Answer 1

Think about what this code is doing;

int main(int argc, string argv[])
{
    int n = strlen(argv[1]);

If there's no argument, then argv[1] doesn't exist. Trying to access it generates a seg fault. That's why it's necessary to check the value of argc before trying to use any argv element.

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