Segmentation fault occuring when trying reverse a string

Question

So I have this function which is supposed to reverse a string and then return it

char *my_revstr(char *str)
{
    int i = 0;
    int j = 0;
    int k = 0;

    while(str[j] != '\0') {
        j++;
    }
    j--;
    while(i < j) {
        k = str[i];
        str[i] = str[j];
        str[j] = k;
        i++;
        j--;
    }
    return (str);
}

But whenever I try to run it I have segmentation fault, and I'll be honest I don't very know why. I hope someone can help me fix this ^^.

Answer 1

I am sure that you have a segmentation fault due to passing to the function a string literal something like

my_revstr( "Hello" );

You may not change a string literal. Any attempt to change a string literal results in undefined behavior.

You should write like

char s[] = "Hello";
my_revstr( s );

Pay attention to that the variables i and j should have the type size_t because the type int can be not enough large to store sizes of strings.

The function can be defined for example the following way as it is shown in the demonstrative program below,

#include <stdio.h>

char * my_revstr( char *s )
{
    size_t n = 0;

    while ( s[n] != '\0' ) ++n;

    if ( n != 0 )
    {
        for ( size_t i = 0; i < --n; ++i )
        {
            char c = s[n];
            s[n] = s[i];
            s[i] = c;
        }
    }

    return s;
}

int main(void) 
{
    char s[] = "Hello";
    
    puts( s );
    puts( my_revstr( s ) );

    return 0;
}

The program output is

Hello
olleH

The function can be defined also for example the following way

char * my_revstr( char *s )
{
    size_t n = 0;

    while ( s[n] != '\0' ) ++n;

    for ( size_t i = 0; i < n / 2; ++i )
    {
        char c = s[i];
        s[i] = s[n - i - 1];
        s[n - i - 1] = c;
    }

    return s;
}

Or you could implement the function using pointers. For example

char * my_revstr( char *s )
{
    char *first = s, *last = s;

    while ( *last ) ++last;

    if ( first != last )
    {
        for ( ; first < --last; ++first )
        {
            char c = *first;
            *first = *last;
            *last = c;
        }
    }

    return s;
}