/usr/ucb/cc [flag . . . ] file . . .
FILE *fopen(const char *filename, const char *type);
FILE *freopen(const char *filename, const char *type, FILE *stream);
FILE *fdopen(int fildes, const char *type);
filename points to a character string that contains the name of the file to be opened.
type is a character string having one of the following values:
freopen opens the file named by filename and associates the stream pointed to by stream with it. The type argument is used just as in fopen. The original stream is closed, regardless of whether the open ultimately succeeds. If the open succeeds, freopen returns the original value of stream.
freopen is typically used to attach the preopened streams associated with stdin, stdout, and stderr to other files.
fdopen associates a stream with the file descriptor fildes. File descriptors are obtained from calls like open, dup, creat, or pipe(2), which open files but do not return streams. Streams are necessary input for many of the Section 3S library routines. The type of the stream must agree with the mode of the open file.
When a file is opened for update, both input and output may be done on the resulting stream. However, output may not be directly followed by input without an intervening fseek or rewind, and input may not be directly followed by output without an intervening fseek, rewind, or an input operation which encounters EOF.