与えられた文字列ポインタから整数または文字を書き出す関数をCで定義するには、以下のコードを使う：

```c
void write_int_or_char(const char* str, int index) {
    char c = str[index]; // get the character at the specified index
    if (isdigit(c)) {
        int num = atoi(&c); // convert the character to an integer
        printf("%d", num); // print the integer
    } else {
        printf("%c", c); // print the character
    }
}
```
To define a function in C that writes an integer or a character from a given string pointer, you can use the following code:

```c
void write_int_or_char(const char* str, int index) {
    char c = str[index]; // get the character at the specified index
    if (isdigit(c)) {
        int num = atoi(&c); // convert the character to an integer
        printf("%d", num); // print the integer
    } else {
        printf("%c", c); // print the character
    }
}
```