Getting bus error while adding ascii values to array characters. Why? (C)

Question

#include <stdio.h>

char    *ft_strupcase(char *str);

char    *ft_strupcase(char *str)
{
    int i;

    i = 0;
    while (str[i])
    {
        if (str[i] >= 'a' && str[i] <= 'z')
        {
             str[i] -= 32;
        }
        i++;
    }
    return (str);
}

int main(void)
{
    char *test = ft_strupcase("fdfFEhk");
    for (int k = 0; test[k] != '\0'; k++)
    {
        printf("%c", test[k]);
    }
    return (0);
}

The expected result is to print the string passed to the function, all in capital letters. Instead, I get a bus error. Why and how can I fix this?

You can't do it with a string literal. See here https://stackoverflow.com/questions/1614723/why-is-this-string-reversal-c-code-causing-a-segmentation-fault — Federico klez Culloca, Jul 04 '17 at 07:52
If you executes this program with a debugger you will discover that the instruction `str[i] -= 32;` will raise a segmentation fault at the first while loop cycle. This is 'cause the string `"fdfFEhk"` is a CONSTANT. Under the operating systems such as Linux or Windows this is an error. If you use this code under DOS it should run. — Sir Jo Black, Jul 04 '17 at 08:16
It is the [same problem](https://stackoverflow.com/questions/44856233/why-is-this-c-code-getting-a-bus-error-no-external-functions-allowed). Why don't you read those answer? — BLUEPIXY, Jul 04 '17 at 13:38

Sourav Ghosh · Answer 1 · 2017-07-04T09:00:31.873

3

In your ft_strupcase() function, you're trying to modify the content of a string literal. This causes undefined behavior.

To quote the standard, C11, chapter §6.4.5/P7, String literals

[...] If the program attempts to modify such an array, the behavior is undefined.

The argument you received is a string literal and you are not allowed to change the content. To avoid, you have to either

Pass a modifiable memory as the actual argument.
Inside the function, create a block (need to take care of the lifetime, either static storage or allocated via memory allocator functions), copy the content, perform the modify operations and return the address.

edited Jul 04 '17 at 09:00

answered Jul 04 '17 at 07:52

Sourav Ghosh

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If you executes this program with a debugger you will discover that the instruction str[i] -= 32; will raise a segmentation fault at the first while loop cycle. This is 'cause the string "fdfFEhk" is a CONSTANT. Under the operating systems such as Linux or Windows this is an error. If you use this code under DOS it should run. – Sir Jo Black Jul 04 '17 at 08:18
1

@SergioFormiggini that's why it is undefined behaviour. Sometimes works, sometimes not. – unalignedmemoryaccess Jul 04 '17 at 08:21
1

@SergioFormiggini-- [The Standard explicitly states that it is undefined behavior to attempt to modify the contents of the array representing a string literal.](http://port70.net/~nsz/c/c11/n1570.html#6.4.5p7) – ad absurdum Jul 04 '17 at 08:27
@tilz0R Thanks for the follow up, I've added the reference into my answer. :) – Sourav Ghosh Jul 04 '17 at 09:00

score 1 · Answer 2 · answered Jul 04 '17 at 07:54

1

This is undefined behaviour as you are modifying string literal using ft_strupcase function

Rewrite your code to:

char source[] = "fdfFEhk";
char *test = ft_strupcase(source);

Now your string is not string literal anymore and is copied to RAM everytime source variable is initialized. Therefore, you have defined behaviour.

answered Jul 04 '17 at 07:54

unalignedmemoryaccess

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In the code written by @EatABagel the problem is that the string he uses is a costant, not an UB (IMHO). – Sir Jo Black Jul 04 '17 at 08:20
@SergioFormiggini Trying to modify a constant is by definition undefined behaviour. – unalignedmemoryaccess Jul 04 '17 at 08:21
Under DOS it was possible. I'm just searching the old DOS compiler to see if it generates error, but is archeology!!! I think that is not undefined, but SO related. BTW is not a good style to modify constants. – Sir Jo Black Jul 04 '17 at 08:29
1

@SergioFormiggini still, undefined behaviour. Period. – unalignedmemoryaccess Jul 04 '17 at 08:29
The error is generated by the CPU that doesn't accept you write on memory area declared read-only. This behaviour doesn't depend by the C-Language. – Sir Jo Black Jul 04 '17 at 08:30
@SergioFormiggini Im done talking here, sorry. – unalignedmemoryaccess Jul 04 '17 at 08:31
@SergioFormiggini-- it _is_ undefined behavior [according to the Standard](http://port70.net/~nsz/c/c11/n1570.html#6.4.5p7), which is the final word on the matter. That it appears to work may be a manifestation of UB; this is not proof that it is not UB. But, Microsoft is notoriously noncomformant. – ad absurdum Jul 04 '17 at 08:34

Getting bus error while adding ascii values to array characters. Why? (C)

2 Answers2