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)
char ∗type;

DESCRIPTION

fopen opens the file named by filename and associates a stream with it.  fopen returns a pointer to be used to identify the stream in subsequent operations. 

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:

"r" open for reading

"w" truncate or create for writing

"a" append: 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. It returns the original value of stream. The original stream is closed, regardless of whether the open ultimately succeeds.

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.  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 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), fseek(3S), fopen(3S)

DIAGNOSTICS

fopen and freopen return a NULL pointer on failure. 

BUGS

In order to support the same number of open files as does the system, fopen must allocate additional memory for data structures using calloc after 20 files have been opened.  This confuses some programs which use their own memory allocators. 

Sun Release 3.2  —  Last change: 17 July 1986