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Name

fopen, freopen - open a stream

Synopsis

/usr/ucb/cc [ flag ... ] file ...

#include <stdio.h>

FILE *fopen(file, mode)
const char *file, *mode;

FILE *freopen(file, mode, iop)
const char *file, *mode;
register FILE *iop;

Description

fopen() opens the file named by file and associates a stream with it. If the open succeeds, fopen() returns a pointer to be used to identify the stream in subsequent operations.

file points to a character string that contains the name of the file to be opened.

mode 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 file and associates the stream pointed to by iop with it. The mode 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 iop.

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

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(3S) , and input may not be directly followed by output without an intervening fseek(3S) or rewind(3S) . An input operation which encounters EOF will fail.

Return Values

fopen() and freopen() return a NULL pointer on failure.

See Also

open(2) , fclose(3S) , fopen(3S) , freopen(3S) , fseek(3S) , malloc(3C) , rewind(3S)

Notes

Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-thread applications is unsupported.

In order to support the same number of open files that the system does, fopen() must allocate additional memory for data structures using malloc(3C) after 64 files have been opened. This confuses some programs which use their own memory allocators.

The interfaces of fopen() and freopen() differ from the Standard I/O Functions fopen(3S) and freopen(3S) . The Standard I/O Functions distinguish binary from text files with an additional use of ’b’ as part of the mode. This enables portability of fopen(3S) and freopen(3S) beyond SunOS 4.X systems.

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