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.

SEE ALSO

open(2V), fclose(3S), fseek(3S), fopen(3V)

DIAGNOSTICS

fopen and freopen return a NULL pointer on failure. 

BUGS

In order to support the same number of open files as the system does, 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: 24 April 1986