What is the error in my c code?

Question

#include <stdio.h>
#define IsLowerCase(c) (((c)>='a') && ((c)<='z'))
#define ToUpperCase(s) (IsLowerCase(s)? (s)-'a' + 'A': (s))

int main(void){

    char *string = "abcdefghijklmnopqrstuvwxyz";

    while(*string != '\0'){
        *string = ToUpperCase(*string);
        ++string;
    }

}

warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]

You're trying to modify a string constant, which isn't allowed. Use an array instead. — Tom Karzes, Oct 29 '17 at 16:36

score 1 · Accepted Answer · answered Oct 29 '17 at 16:39

1

Your char *string should be const char *string (but it is immutable); or better char string[] = .... and then declare another variable char *s = string;, and loop and step s instead of string

answered Oct 29 '17 at 16:39

WWW

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In this case, casting `const char*` to `char*` and modifying the array later on results in UB. – Adam Kotwasinski Oct 29 '17 at 16:47
What’s long form for UB? – WWW Oct 29 '17 at 17:22
Undefined behaviour - https://stackoverflow.com/questions/2766731/what-exactly-do-ib-and-ub-mean – Adam Kotwasinski Oct 29 '17 at 17:22

Nik · Answer 2 · 2017-10-29T17:34:56.070

0

char *string = "abcdefghijklmnopqrstuvwxyz"; is a literal (cannot be modified), and if you try to modify it, you will be getting undefined behaviour. If you want a modifiable string, allocate memory or use a char[] array.

This will do the trick char string[] = "abcdefghijklmnopqrstuvwxyz";. Now it should work with the rest of your code.

You can get more information on your issue in this thread.

edited Oct 29 '17 at 17:34

answered Oct 29 '17 at 16:42

Nik

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It is undefined behavior regardless of operating system, compiler, or hardware because the C Standard says so. It will cause a crash with most compiler-hardware combinations because the compiler will put the string in a read-only memory segment. – aschepler Oct 29 '17 at 17:29

score 0 · Answer 3 · answered Oct 29 '17 at 17:32

the problem started here

char *string = "abcdefghijklmnopqrstuvwxyz";

You declared "string" as pointer to character type but initialized it with "abcdefghijklmnopqrstuvwxyz" ( this is const string and will be invalid ) . In order to correct it you need to do two things.

declare a variable ,say str, of type string i.e. "string str;" and the initialize it with a string , i.e
```
"string str = "abcdefghijklmnopqrstuvwxyz";
```

declare your pointer "string" with type string * ,i.e string *string.

and intialize it with the address of str, i.e

string *string = &str;

so your final code will took like this

    #include <stdio.h>
    #include<string.h> //to use the string data type
#define IsLowerCase(c) (((c)>='a') && ((c)<='z'))
#define ToUpperCase(s) (IsLowerCase(s)? (s)-'a' + 'A': (s))

int main(void){
string str = "abcdefghijklmnopqrstuvwxyz";
string  *string = &str;
while(*string != '\0'){
*string = ToUpperCase(*string);
++string;
}

}

What is the error in my c code?

3 Answers3