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Manual page for FOPEN(3V)

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(fd, type)
int fd;
char *type;

DESCRIPTION

fopen() opens the file named by filename and associates a stream with it. If the open succeeds, 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 EOF

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 fd. File descriptors are obtained from calls like open.2v dup.2v creat.2v or pipe.2v 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 access permissions 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.3s or rewind(), and input may not be directly followed by output without an intervening fseek(), rewind(), or an input operation which encounters EOF.

SYSTEM V DESCRIPTION

When a file is opened for append (that is, 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.

RETURN VALUES

On success, fopen(), freopen(), and fdopen() return a pointer to FILE which identifies the opened stream. On failure, they return NULL.

BUGS

In order to support the same number of open files that the system does, fopen() must allocate additional memory for data structures using calloc() after 64 files have been opened. This confuses some programs which use their own memory allocators.