Final answer:
The freopen() function is used to change an open file stream pointer to point to another file, closing the first file and opening a new one with the specified filename and mode.
Step-by-step explanation:
To change an open file stream pointer to point to another file, you use the freopen() function. This C standard library function is designed to first close the file associated with the given stream and then open a new file specified by the filename and mode arguments. It's especially useful when you want to redirect the stream to a different file without altering the file descriptor or creating a new stream.
For example, to change the stream stdin to read from a file instead of standard input, you would use the following:
freopen("newinput.txt", "r", stdin);
In contrast, fopen() is used to open a new file stream. rewind() resets the file position indicator for the stream to the beginning, and fsetpos() sets the file position indicator to a specific value based on a variable of type fpos_t.