c++ string replace_all

To replace all occurrences of a substring in a C string with a new substring, you can use the following code:

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

void replace_all(char *str, const char *old_sub, const char *new_sub) {
    char *pos = str;
    int old_len = strlen(old_sub);
    int new_len = strlen(new_sub);
    while ((pos = strstr(pos, old_sub)) != NULL) {
        memmove(pos + new_len, pos + old_len, strlen(pos + old_len) + 1);
        memcpy(pos, new_sub, new_len);
        pos += new_len;
    }
}

int main() {
    char str[] = "Hello, world! Hello, world! Hello, world!";
    replace_all(str, "Hello", "Hi");
    printf("%s\n", str); // Output: "Hi, world! Hi, world! Hi, world!"
    return 0;
}

The `replace_all()` function takes a C string `str` and two other C strings `old_sub` and `new_sub`. It searches for all occurrences of `old_sub` in `str` using `strstr()` and replaces them with `new_sub` using `memmove()` and `memcpy()`. The function modifies `str` in place.