Modifying string elements within a function gives a segmentation fault.

[placid parcel]

So, I'm trying to do a caesar cipher for CS50, and I'm modifying the contents of an array within a function. I have a for loop that goes through the string, I take a single element of the string, and I'm setting it to something, and the moment I do, I get a segmentation fault. Could anyone explain to me what's going on?

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

void shift(string sentence, int number);

int main(void)
{
    string sentence = "abcde"; // dummy array
    int number = 1; // we are shifting it by one, so a becomes b etc
    shift(sentence, number);
    printf("%s\n", sentence);
}

void shift(string sentence, int number)
{
    for (int i = 0, n = strlen(sentence); i < n; i++) // we loop through it from zero to the length of the string
    {
        if (isalpha(sentence[i]) != 0) // if it is not a letter, we skip it
        {
            sentence[i] = (int) sentence[i] + number; // I get a segmentation fault here
        }
    }
    return;
}```

[tribal pasture]

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[worn mango]

;compile

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

void shift(char *sentence, int number);

int main(void)
{
    char sentence[] = "abcde"; // dummy array
    int number = 1; // we are shifting it by one, so a becomes b etc
    shift(sentence, number);
    printf("%s\n", sentence);
}

void shift(char *sentence, int number)
{
    for (int i = 0, n = strlen(sentence); i < n; i++) // we loop through it from zero to the length of the string
    {
        if (isalpha(sentence[i]) != 0) // if it is not a letter, we skip it
        {
            sentence[i] = (int) sentence[i] + number; // I get a segmentation fault here
        }
    }
    return;
}

[desert juniper]

Program Output

bcdef

Halalaluyafail3#6046 | 53ms | c | x86-64 gcc 12.2 | godbolt.org

[worn mango]

looks like string is just a typedef for char*, so string sentence = "abcde"; is creating a pointer to an immutable string, which you then try and modify

sentence needs to be an array of characters that can be mutated

[placid parcel]

uh, okay? I thought about replacing the string with a normal char array before, but I'm not sure how to get input from the user then. There's a function in the CS50 library that gets a string from the user, and I guess I could just copy over everything from it?

I've no idea what char* means, even the reference just says "think of this as a string", but I don't want to waste your time, under what term should I google it?

nvm, found something on stack overflow on it

ah, I get it now. why the hell is this a thing?

[worn mango]

i see a function get_string in the cs50 docs online
is that what you're referring to?

[placid parcel]

yea

[worn mango]

if so, it looks like it allocates its memory each time instead of returning a string literal

[placid parcel]

uh, as I understand it, it doesn't create an array, but a pointer to an array that's read-only

where the actual data is

[worn mango]

from the description, it creates a new array each time with space sufficient for the whole string (probably using malloc)

it looks like they intended it to not be read only, considering that it returns a pointer to mutable data

but that API is very strange

[placid parcel]

well if that's what they intended, then they done goofed up

anyway, I'm gonna convert the string into a char[], I'll close this thread if it works out

[worn mango]

looking at the source code for get_string it looks like it does really create a new allocated array of characters each time

and it cleans them up at exit itself for some reason

[placid parcel]

alright, eventually just recoded it so that instead of first processing the entire string and then printing it at once, the function only processes a single character at once, prints it instantly, and it's in a loop so it goes through the whole string

thanks for taking your time

"solved