malloc() into a recursive function that modifies a string - c

Question

THE CODE:
The aim of this code is to take a string of 0 1 and * and print all the combinations of strings obtained replacing * with 0 or 1.
Ex.
input : 0*1 => output: 001 011
the idea is to build a recursive function (for practice purposes):
void rec_print (char *mod_str)
which counts the occurrences of *, the offset of the first * encountered while looping the string (I have used ternary just to practice them)
for(int i=0; mod_str[i]; i++) {
n_star = (mod_str[i] == '*') ? n_star+1 : n_star;
if (offset==-1) {
offset = (mod_str[i] == '*') ? i : -1;
}
}
The base case occurs when there is only one *, in which case replaces * with 0, prints the string, replace the 0 with a 1, and finally prints the string:
if (n_star==1) {
mod_str[offset] = "0";
printf("\n%s", mod_str);
mod_str[offset] = "1";
printf("\n%s", mod_str);
}
otherwise modifies the first * of the string to a 0 then call itself, then modify it to a 1 when it comes back, and finally call itself:
else {
// replace the first encountered * to 0 and recall itself
mod_str[offset] = "0";
rec_print(mod_str);
// replace the previous 0 to 1 and recall itself
mod_str[offset] = "1";
rec_print(mod_str);
}
THE ISSUE:
as you will notice here the problem is that I am trying to modify a string which memory is read-only (yes, I have a huge "Python bias" here), normally I would use a malloc() to resolve this but I can't figure out how to use it inside a function to modify a string.
I am well aware recursion is not the best solution to this exercise but I need to satisfy my curiosity about this.
I thank everyone for the time spent here and apologize for my eventual English mistakes in advance.

Answer 1

The base case occurs when there is only one *, in which case replaces * with 0, prints the string, replace the 0 with a 1, and finally prints the string:
This base case isn't really "base." For example, you could receive the string "001" as input, which contains zero stars. You should modify your base case to handle only zero stars, and rely on your recursive case to simplify the case with one star to zero stars.
as you will notice here the problem is that I am trying to modify a string which memory is read-only
If the memory is read-only, you should clarify that fact to the compiler! Then the compiler can check your code to make sure it never writes to a read-only variable.
Example:
void rec_print (const char *mod_str)
as you will notice here the problem is that I am trying to modify a string which memory is read-only (yes, I have a huge "Python bias" here), normally I would use a malloc() to resolve this but I can't figure out how to use it inside a function to modify a string.
You need to copy the string to a place where you can modify it. Here's an example:
void rec_print (const char *src_str) { // note name change
// Allocate memory
char *mod_str = malloc((strlen(src_str) + 1) * sizeof(char));
// This function copies src_str to mod_str
strcpy(mod_str, src_str);
// Rest of function
...
// Clean up memory used
free(mod_str);
}
Here we allocate enough memory to hold the input string. Then, we copy the input string to the space we allocated. We can do all modifications on this copy. At the end we free the allocation.