Question

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

main()
{
    char tmpChar;
    char *str_1 = "Hello";

    int index;
    int len = strlen(str_1);
    for (int i = 0; i < len/2; i++)
    {
        index = len - 1- i;
        tmpChar = str_1[len - i -1];
        str_1[index] = str_1[i]; <<<<<--------------- core dumps at this point. Not sure why
        str_1[i] = tmpChar;
        printf("str_1[%d] = %c\n", i, str_1[i]);
    }
    str_1[len] = '\0';
    printf("str_1 = %s\n", str_1);
}

Answer 1

The ISO C99 standard has this to say about string literals (section 6.4.5/6):

It is unspecified whether these arrays are distinct provided their elements have the
appropriate values. If the program attempts to modify such an array, the behavior is
undefined.

That’s because, typically, all the string literals are lumped into a single area which may be marked read-only, and they’re sometimes merged to save space. In other words, the two string literals:

char *p1 = "Googolplex";
char *p2 = "plex";

may be stored thus:

  +--- p1           p2 ---+
  |                       |
  V                       V
+---+---+---+---+---+---+---+---+---+---+----+
| G | o | o | g | o | l | p | l | e | x | \0 |
+---+---+---+---+---+---+---+---+---+---+----+

That means: don’t try to modify them. It may work under certain situations but it’s not something that you should rely on if you value portability even a little.

Change:

char *str_1 = "Hello";

to:

char str_1[] = "Hello";

since this is effectively the same as:

char str_1[6];
strcpy (str_1, "Hello");