NAME

fopen, freopen, fdopen − open a stream

SYNOPSIS

#include <stdio.h>

FILE ∗fopen (filename, type)
char ∗filename, ∗type;

FILE ∗freopen (filename, type, stream)
char ∗filename, ∗type;
FILE ∗stream;

FILE ∗fdopen (fildes, type)
int fildes;
char ∗type;

DESCRIPTION

fopen opens the file named by filename and associates a stream with it.  fopen returns a pointer to the FILE structure associated with the stream.

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:

ropen for reading
wtruncate or create for writing
aappend; open for writing at end of file, or create for writing
r+open for update (reading and writing)
w+truncate or create for update
a+append; open or create for update at end-of-file

freopen substitutes the named file in place of the open stream. The original stream is closed, regardless of whether the open ultimately succeeds.  freopen returns a pointer to the FILE structure associated with stream.

freopen is typically used to attach the preopened streams associated with stdin, stdout and stderr to other files. 

fdopen associates a stream with a file descriptor obtained from open, dup, creat, or pipe(2), which will open files but not return pointers to a FILE structure stream which are necessary input for many of the section 3S library routines.  The type of 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 end-of-file.

When a file is opened for append (i.e., when type is ‘a’ or ‘a+’), it is impossible to overwrite information already in the file.  fseek may be used to reposition the file pointer to any position in the file, but when output is written to the file the current file pointer is disregarded.  All output is written at the end of the file and causes the file pointer to be repositioned at the end of the output.  If two separate processes open the same file for append, each process may write freely to the file without fear of destroying output being written by the other.  The output from the two processes will be intermixed in the file in the order in which it is written. 

SEE ALSO

open(2), fclose(3S). 

DIAGNOSTICS

fopen and freopen return a NULL pointer on failure. 

Version 3.6  —  December 20, 1987