Rerouting stdin and stdout from C

Why use freopen()? The C89 specification has the answer in one of the endnotes for the section on <stdio.h>:

116. The primary use of the freopen function is to change the file associated with a standard
text stream (stderr,
stdin, or stdout), as those identifiers need not be
modifiable lvalues to which the value
returned by the fopen function
may be assigned.

freopen is commonly misused, e.g. stdin = freopen("newin", "r", stdin);. This is no more portable than fclose(stdin); stdin = fopen("newin", "r");. Both expressions attempt to assign to stdin, which is not guaranteed to be assignable.

The right way to use freopen is to omit the assignment: freopen("newin", "r", stdin);