In C, you can extract a substring from a string using the `strncpy()` function from the `string.h` library. Here is an example:

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

int main() {
    char str1[100] = "Hello, world!";
    char str2[100];

    strncpy(str2, str1 + 7, 5); // Extract a substring from str1

    printf("%s", str2); // Print the extracted substring

    return 0;
}
```

In this example, we extract a substring from `str1` starting at index 7 (which is the space character after "Hello,") and with a length of 5 (which includes "world" and the null terminator character). The resulting substring is stored in `str2`. Finally, the extracted substring is printed using the `printf()` function.

Note that `strncpy()` does not automatically add a null terminator character at the end of the extracted substring. If you want to use the extracted substring as a string, you must add a null terminator character manually. In this example, we did not need to add a null terminator character because `str2` was initialized to an empty string, which already has a null terminator character at the end.

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