Replacing chars in Pointer String results in Segfault

I've written a function which I want to replace all the spaces in a string with '-'. I get a Segmentation Fault when I run it. However, using gdb with a break point set in the function and stepping through it, it works fine.

Code:

---

#include <stdio.h>
   
int replace (char *cat);
   
main()
{
    char *s1 = "The cat sat ";
    printf("The string before replacement is: \n");
    printf("%s\n", s1);
   
    int n = replace(s1);
    printf("Number of replaced spaces: %d\n", n);
    printf("The string after replacement is: \n");
    printf("%s\n", s1);
}
   
int replace (char *cat)
{
    int length = strlen(cat);
    int r = 0;
    int i;
   
    for (i=0; i < length; i++)
    {
        printf("cat[%d]: %c\n", i, cat[i]);
        if (cat[i] == ' ')
        {
            cat[i]='-';  // results in Seg Fault (set break point at beginning of line)
            r++;
        }
    }
   
    return r;
}

---

Thanks in advance!

Thanks for your help!
Does this mean that any time you set a char* to a string constant, it is not writable? Because in the case of my replace function, it has an argument which is a char* and this is writable.

I think to transform it "writeable" you'll need to allocate memory for it.