str_replace at the low level

char *str_replace(const char *find, const char *replace, const char *source, char *buffer) {
    int i = 0, p = 0, c = 0;
    size_t find_len    = strlen(find);
    size_t replace_len = strlen(replace);
    size_t source_len  = strlen(source);
    while ((c = strpos(find, source, i++)) >= 0) {
        strncpy(buffer+strlen(buffer), source+p, c-p);
        strncpy(buffer+strlen(buffer), replace, replace_len);
        p = c + find_len;
    }
    strncpy(buffer+strlen(buffer), source+p, source_len - p);
    buffer[strlen(buffer)] = '\0';
    return buffer;
}

#define BUF_SIZE 255

int main(void) {
    char text[255] = " finding all spaces in this string is a fun, i guess? ";
    char buf[BUF_SIZE];
    memset(buf, 0, BUF_SIZE);
   
    str_replace(" ", "==", text, buf);

    printf("buf = \"%s\"\n", buf);
   
    return 0;
}