Reverse a pointer to an array

Question

i have a simple pointer to an array and i want to reverse that array, this is the code that i wrote:

void str_reverse(char *s) {
    int l, r;
    l = 0;
    r = str_len(s) - 1;
    char temp;
    printf("length %i ", r);
    while (l <= r) {
        temp = *(s + l);
        printf("\n%c [%i] | %c [%i]\n", temp, l, *(s + r), r);

        *(s + l) = *(s + r);
        *(s + r) = temp;
        r--;
        l++;
    }
}

As you can see i use this fuction:

int str_len(char *p) {
    int l = 0;
    while (*(p + l) != '\0') {
//        printf("%c %i", *(p + l),l);
        l++;
    }
    return l;
}

I order to get the length of my array, this is my little main

int main() {
    char *s = "Hallo world!";

    str_reverse(s);

    return 0;
}

And this is the output:

length 10 
H [0] | d [10]
Bus error: 10

Now i tried to only print the values, so in the while loop of str_reverse i commented this two lines:

*(s + l) = *(s + r);
*(s + r) = temp;

And this is the output:

H [0] | ! [11]

a [1] | d [10]

l [2] | l [9]

l [3] | r [8]

o [4] | o [7]

  [5] | w [6]

That is good! But how can i store them? Thank you in advance

This happens when you call Pointer =>> Array. A Pointer is not an Array and an Array is not a Pointer. — Michi
– Michi, Commented Jan 29, 2016 at 16:37
A pointer points to "something", but an Array don't. Inside the Array "something". Will be copied. That's all...so if you want to copy "something" inside a Pointer, then you need to dinamicaly allocate some memory an use that pointer to point there. — Michi
– Michi, Commented Jan 29, 2016 at 16:40

dbush · Accepted Answer · 2016-01-29 16:21:26Z

2

The problem is that in main, s points to a string literal. These literals are typically stored in read only memory, so they can't be modified.

Change s to an array initialized with a string literal and it will work.

char s[] = "Hallo world!";

Using dynamically allocated memory will also work:

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

...

char *s = strdup("Hallo world!");
str_reverse(s);
free(s);

edited Jan 29, 2016 at 16:21

answered Jan 29, 2016 at 16:08

dbush

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5 Comments

dbush Over a year ago

@Francesco You can do it with a pointer, but it can't point to read only memory like char *s = "Hallo world!";. It's fine if it points to an array or to dynamically allocated memory.

dbush Over a year ago

@Francesco And what are those warnings?

Francesco Over a year ago

string.c:56:15: warning: implicitly declaring library function 'strdup' with type 'char *(const char *)' char *s = strdup("Hallo world!"); ^ string.c:56:15: note: include the header <string.h> or explicitly provide a declaration for 'strdup' string.c:87:5: warning: implicit declaration of function 'free' is invalid in C99 [-Wimplicit-function-declaration] free(s); ^ 2 warnings generated.

dbush Over a year ago

@Francesco Then you need to include string.h and stdlib.h. See my edit.

Michi Over a year ago

Strdup is not a C standard library.

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